FRINK Linked Data Fragments server

Matches in SemOpenAlex for { <https://semopenalex.org/work/W2112534728> ?p ?o ?g. }

• W2112534728 endingPage "19489" @default.
• W2112534728 startingPage "19482" @default.
• W2112534728 abstract "Protein kinase C (PKC) plays a major role in regulating cell growth, transformation, and gene expression; however, identifying phosphorylation events that mediate these responses has been difficult. We expression-cloned a group of PKC-binding proteins and identified a high molecular weight, heat-soluble protein as the major PKC-binding protein in REF52 fibroblasts (Chapline, C., Mousseau, B., Ramsay, K., Duddy, S., Li, Y., Kiley, S. C., and Jaken, S. (1996) J. Biol. Chem. 271, 6417–6422). In this study, we demonstrate that this PKC-binding protein, clone 72, is also a PKC substrate in vitro and in vivo. Using a combination of phosphopeptide mapping, Edman degradation, and electrospray mass spectrometry, serine residues 283, 300, 507, and 515 were identified as the major in vitro PKC phosphorylation sites in clone 72. Phosphorylation state-selective antibodies were raised against phosphopeptides encompassing each of the four phosphorylation sites. These antibodies were used to determine that phorbol esters stimulate phosphorylation of serines 283, 300, 507, and 515 in cultured cells, indicating that clone 72 is directly phosphorylated by PKC in living cells. Phosphorylated clone 72 preferentially accumulates in membrane protrusions and ruffles, indicating that PKC activation and clone 72 phosphorylation are involved in membrane-cytoskeleton remodeling. These data lend further evidence to the model that PKCs directly interact with, phosphorylate, and modify the functions of a group of substrate proteins, STICKs (substrates that interact withC-kinase). Protein kinase C (PKC) plays a major role in regulating cell growth, transformation, and gene expression; however, identifying phosphorylation events that mediate these responses has been difficult. We expression-cloned a group of PKC-binding proteins and identified a high molecular weight, heat-soluble protein as the major PKC-binding protein in REF52 fibroblasts (Chapline, C., Mousseau, B., Ramsay, K., Duddy, S., Li, Y., Kiley, S. C., and Jaken, S. (1996) J. Biol. Chem. 271, 6417–6422). In this study, we demonstrate that this PKC-binding protein, clone 72, is also a PKC substrate in vitro and in vivo. Using a combination of phosphopeptide mapping, Edman degradation, and electrospray mass spectrometry, serine residues 283, 300, 507, and 515 were identified as the major in vitro PKC phosphorylation sites in clone 72. Phosphorylation state-selective antibodies were raised against phosphopeptides encompassing each of the four phosphorylation sites. These antibodies were used to determine that phorbol esters stimulate phosphorylation of serines 283, 300, 507, and 515 in cultured cells, indicating that clone 72 is directly phosphorylated by PKC in living cells. Phosphorylated clone 72 preferentially accumulates in membrane protrusions and ruffles, indicating that PKC activation and clone 72 phosphorylation are involved in membrane-cytoskeleton remodeling. These data lend further evidence to the model that PKCs directly interact with, phosphorylate, and modify the functions of a group of substrate proteins, STICKs (substrates that interact withC-kinase). Protein kinase C (PKC) 1The abbreviations used are: PKC, protein kinase C; ESMS, electrospray mass spectrometry; LC, liquid chromatography; MARCKS, myristoylated, alanine-rich protein kinase C substrate; MS/MS, tandem mass spectrometry; PS, phosphatidylserine; PDBu, phorbol 12,13-dibutyrate; HPLC, high pressure liquid chromatography; RP-HPLC, reverse phase HPLC; Fmoc,N-(9-fluorenyl)methoxycarbonyl. is a family of phospholipid-dependent protein kinases expressed in all cells and tissues (1Nishizuka Y. FASEB J. 1995; 9: 484-496Crossref PubMed Scopus (2368) Google Scholar). PKC activity has been associated with a variety of growth and pathological defects, indicating that PKCs are involved in regulating fundamental cellular processes. PKC is the major cellular receptor for tumor-promoting phorbol esters, and consequently, PKC activation has been linked to later events in multistage carcinogenesis, i.e. tumor promotion/progression. Phorbol esters and other PKC activators rapidly induce a variety of cellular responses including cell shape changes, cytoskeletal remodeling, decreased cell-cell communication, and increased exocytosis. The diversity of the observed responses has made it complicated to focus on particular phosphorylation events that mediate changes in biological activity. Identifying target proteins and determining how PKC phosphorylation alters their activities are keys to understanding the role of PKC in cell regulation. Several approaches have been used to identify PKC-binding proteins. PICKs (proteins that interact withC-kinase) are PKC substrates identified by yeast two-hybrid screening (2Staudinger J. Lu J. Olson E.N. J. Biol. Chem. 1997; 272: 32019-32024Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). RACKs (receptors foractivated C-kinase) are non-substrate proteins that bind to catalytically active PKC. These may target active PKCs to the vicinity of appropriate substrate proteins (3Mochly-Rosen D. Gordon A.S. FASEB J. 1998; 12: 35-42Crossref PubMed Scopus (510) Google Scholar). Several years ago, we began using a PKC overlay assay to identify PKC-interacting proteins (4Chapline C. Ramsay K. Klauck T. Jaken S. J. Biol. Chem. 1993; 268: 6858-6861Abstract Full Text PDF PubMed Google Scholar, 5Hyatt S.L. Liao L. Chapline C. Jaken S. Biochemistry. 1994; 33: 1223-1228Crossref PubMed Scopus (80) Google Scholar, 6Liao L. Hyatt S.L. Chapline C. Jaken S. Biochemistry. 1994; 33: 1229-1233Crossref PubMed Scopus (70) Google Scholar). We found that many of the binding proteins identified by this method, including MARCKS and the related protein MacMARCKS, were PKC substrates (5Hyatt S.L. Liao L. Chapline C. Jaken S. Biochemistry. 1994; 33: 1223-1228Crossref PubMed Scopus (80) Google Scholar, 7Hyatt S.L. Liao L. Aderem A. Nairn A. Jaken S. Cell Growth Diff. 1994; 5: 495-502PubMed Google Scholar). We therefore named this group of PKC-binding proteins/substrates STICKs (forsubstrates that interact withC-kinase. In cultured cells, phorbol esters stimulate 32PO4 incorporation into STICKs (7Hyatt S.L. Liao L. Aderem A. Nairn A. Jaken S. Cell Growth Diff. 1994; 5: 495-502PubMed Google Scholar,8Dong L. Chapline C. Mousseau B. Fowler L. Ramsay K. Stevens J.L. Jaken S. J. Biol. Chem. 1995; 270: 25534-25540Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar), indicating that PKC activation increases their phosphorylation. However, since activating PKC can also lead to increased activity of other cellular kinases, additional evidence is needed to determine if the increased phosphorylation is directly mediated by PKC. This is accomplished by comparing in vitro and in vivophosphorylation sites in target proteins. In recent studies, we determined that two of the PKC-binding proteins we identified, α-adducin and the novel γ-adducin (8Dong L. Chapline C. Mousseau B. Fowler L. Ramsay K. Stevens J.L. Jaken S. J. Biol. Chem. 1995; 270: 25534-25540Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar), are in vivo PKC substrates by demonstrating that Ser660 is phosphorylated by PKC in vitro and that phorbol esters stimulate Ser660 phosphorylation in living cells (9Fowler L. Dong L.Q. van de Water B. Bowes R. Stevens J.L. Jaken S. Cell Growth Differ. 1998; 9: 177-184PubMed Google Scholar). Increased phosphorylation in transformed renal cells in culture and in renal tumors was also noted (9Fowler L. Dong L.Q. van de Water B. Bowes R. Stevens J.L. Jaken S. Cell Growth Differ. 1998; 9: 177-184PubMed Google Scholar, 10Fowler L. Everitt J. Stevens J.L. Jaken S. Cell Growth Differ. 1998; 9: 405-413PubMed Google Scholar). The major PKC-binding protein/substrate detected by overlay assays in normal REF52 fibroblasts (REF A cells) is a >200-kDa heat-soluble PKC substrate referred to as clone 72 (11Chapline C. Mousseau B. Ramsay K. Duddy S. Li Y. Kiley S.C. Jaken S. J. Biol. Chem. 1996; 271: 6417-6422Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar) and also as SSeCKS (12Lin X. Tombler E. Nelson P.J. Ross M. Gelman I.H. J. Biol. Chem. 1996; 271: 28430-28438Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar, 13Lin S. Nelson P.J. Frankfort B. Tombler E. Johnson R. Gelman I.H. Mol. Cell. Biol. 1995; 15: 2754-2762Crossref PubMed Scopus (82) Google Scholar). Clone 72 is also closely related to gravin, an autoantigen identified in patients with myasthenia gravis (14Nauert J.B. Klauck T.M. Langeberg L.K. Scott J.D. Curr. Biol. 1997; 7: 52-62Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar). Clone 72 protein and message levels are down-modulated in oncogene-transformed cells, indicating that this protein may play an important role in normal growth regulation. In fact, reexpressing clone 72 (SSeCKS) in transformed cells has been shown to attenuate growth and transformation (15Lin X.Y. Gelman I.H. Cancer Res. 1997; 57: 2304-2312PubMed Google Scholar). The goal of the present studies was to determine if clone 72 is an in vivo PKC substrate by comparingin vitro and in vivo sites of phosphorylation. Our results indicate that clone 72 is phosphorylated by PKC at multiple sites in vitro and in vivo. These studies extend our evidence that PKC directly binds to and phosphorylates a group of target proteins (STICKs). These results contribute to the emerging model that STICKs are primary targets for PKC phosphorylation and that phosphorylation of STICKs is directly related to the role of PKC in regulating the complex processes of cell growth, death, and differentiation. Site-directed mutagenesis was used to introduce HindIII and Xma sites at the 5′- and 3′-ends of the binding and phosphorylation domain encompassing amino acid residues 496–531 (Table I). In this process, Val497 in the wild type sequence was converted to Leu. This conservative change did not alter PKC binding, PS binding, or PKC phosphorylation. The HindIII and Xma sites were used to insert double-stranded synthetic oligonucleotides that encode for the deletion mutant sequences 1–4 (Table I). Site-directed mutagenesis was also used to prepare the single and triple amino acid changes in mutants 5–7 (Table I). Mutagenesis was done with double-stranded plasmid DNA using the Chameleon mutagenesis kit (Stratagene). Mutant proteins were expressed from pTrc (Invitrogen) as His-tagged fusion proteins. All sequences were verified by sequence analysis.Table ILysyl peptides identified by LC-ESMS following PKC phosphorylation of C34EPeptide residuesRetention timeObserved massCalculated massDifference (Obs. − Calc.)SequencePhosphorylation sitePeptideClone 72min1 and 2NDaND, peptide not detected.249.1CK3–829.6670.8670.40.4VQSPLK3–928.5798.8798.50.3VQSPLKK10–1732.8837.7837.50.2LFSSSGLK10–1733.2917.5837.580.0LFS SS GLK10–1734.8917.5837.580.0LF pS SSGLKSer12Ser50710–1831.7965.7965.60.1LFSSSGLKK10–1832.21045.8965.680.2LFSSSGLKK10–1833.51045.8965.680.2LFSSSGLKK18–22 or 19–2310.6611.5531.380.2KLpSGK or LpSGKKSer20Ser51519–22ND403.2LSGK19–229.4483.5403.280.3LpSGKSer20Ser51519–23ND531.3LSGKK19–23 or 18–2212.3611.3531.380.0LpSGKK or KLpSGKSer20Ser51524–25ND274.3QK26–27ND203.2GK28–347.2646.5646.30.2RGGGDESynthetic peptide C34E (designed from clone 72 residues; (see “Materials and Methods”) was phosphorylated with PKC, digested with endoproteinase Lys-C, and analyzed by LC-ESMS (Fig. 6) as described under “Materials and Methods.” Lysyl peptides are listed by residues, HPLC retention time, calculated and observed masses, and amino acid sequence. Calculated mass values are based upon monoisotopic residue weights without phosphate. Underlined sequences are supported by MS/MS sequence analysis. Potential phosphorylation sites are in italic type; identified sites are in boldface type.a ND, peptide not detected. Open table in a new tab Synthetic peptide C34E (designed from clone 72 residues; (see “Materials and Methods”) was phosphorylated with PKC, digested with endoproteinase Lys-C, and analyzed by LC-ESMS (Fig. 6) as described under “Materials and Methods.” Lysyl peptides are listed by residues, HPLC retention time, calculated and observed masses, and amino acid sequence. Calculated mass values are based upon monoisotopic residue weights without phosphate. Underlined sequences are supported by MS/MS sequence analysis. Potential phosphorylation sites are in italic type; identified sites are in boldface type. For quantitative measurements, purified, recombinant proteins and peptides were phosphorylated with purified recombinant PKCα (PanVera) at 30 °C for 15 min using standard conditions (4Chapline C. Ramsay K. Klauck T. Jaken S. J. Biol. Chem. 1993; 268: 6858-6861Abstract Full Text PDF PubMed Google Scholar). Phosphorylated proteins were separated on SDS-polyacrylamide gels and detected by autoradiography. For maximum phosphorylation needed for identifying phosphorylated residues, synthetic peptide C34E designed from clone 72 residues (Lys496–Glu529, with an N-terminal cysteine added to facilitate coupling) (3.5 μm) was phosphorylated with PKCα (13.5 nm) for 2–4 h until maximum phosphorylation was reached as monitored by incorporation of radiolabeled phosphate. The phosphorylated synthetic peptide (6.25 μg) was digested with 0.6 μg of endoproteinase Lys-C overnight. The digested peptides were analyzed by liquid chromatography electrospray mass spectrometry (LC-ESMS). Synthetic peptide corresponding to clone 72 residues Arg296–Lys305 (10 μm) was phosphorylated with PKCα (3.0 nm). The undigested peptide was analyzed by LC-ESMS. Automated FMOC solid phase synthesis was used to produce PKC phosphorylation substrate peptides R10K and C34E as well as calibration and tuning reagents for the mass spectrometer (peptides G9I, K13L, L20R, V26E, E10A, K6R, and bA5K). Peptides G9I (GRTGRRNSI), K13L (KTREGNVRVSREL), L20R (LKPVKKKKIKREIKILENLR), V26E (VAMKKIRLEEEGVPSTAIREISLLKE), and C34E (CKVQSPLKKLFSSSGLKKLSGKKQKGKRGGGGDE) were synthesized at the 250-μmol scale on HMP (Wang) resin using a Perkin-Elmer Applied Biosystems model 430A Peptide Synthesizer and the Applied Biosystems FastMoc synthesizer program, cleaved from the solid support with trifluoroacetic acid, and extracted with acetic acid as described previously (16Escribano J. Hernando N. Ghosh S. Crabb J. Coca-Prados M. J. Cell. Physiol. 1994; 160: 511-521Crossref PubMed Scopus (153) Google Scholar). Phosphopeptides E10A (EPQpYEEIPIA-NH2) (where pY represents phosphorylated tyrosine) and K6R (KRpTIRR-NH2) (where pT represents phosphorylated threonine) were also prepared on the model 430A instrument (300-μmol scale) using the FastMoc program, Fmoc-phosphotyrosine (PO3H2)−OH, Fmoc-phosphothreonine (O-benzyl), and Fmoc-4-methylbenzylhydrylamine (MBHA) rink amide resin. PKC substrate peptide R10K (RKKTSFKKSK) (100-μmol scale) and phosphopeptide bA5K (biotin-ApSSYK) (where pS represents phosphorylated serine) (50-μmol scale) were synthesized with a Rainin Symphony/Multiplex Peptide Synthesizer using the synthesizer software (version 3.2) provided by the instrument manufacturer (Protein Technologies, Inc.) and Fmoc-phosphoserine (O-benzyl). Following automatic cleavage from the solid support on the Symphony instrument, peptides were precipitated from trifluoroacetic acid with ice-cold ether, washed two or three times with cold ether, and either vacuum-dried or purified by RP-HPLC. Extended double coupling steps were used for Fmoc-phosphoamino acids on both instruments; two equimolar additions of the Fmoc-phosphoamino acid were made approximately 6–12 h apart with a total coupling reaction time of 18 h. Biotin was coupled with a single coupling reaction of 12 h. Fmoc-phosphoamino acids,d-biotin, Wang resins (with or without preloaded Fmoc amino acids), and Fmoc-MBHA rink amide resin were from AnaSpec. Reagents and chemicals used with the Symphony instrument were from Rainin, and those used with the model 430A instrument were from PE Applied Biosystems and Burdick and Jackson. Synthetic peptides used for mass spectrometry tuning and calibration were purified by RP-HPLC using aqueous trifluoroacetic acid/acetonitrile solvents and a 10μ Vydac C18 column (2.2 × 25 cm). All synthetic peptides were quality control-analyzed by analytical RP-HPLC and amino acid analysis (17Crabb J.W. Gaur V.P. Garwin G.G. Marx S.V. Chapline C. Johnson C.M. Saari J.C. J. Biol. Chem. 1991; 266: 16674-16683Abstract Full Text PDF PubMed Google Scholar) in addition to ESMS. Phenylthiocarbamyl amino acid analysis was performed as described previously using an automated analysis system (Applied Biosystems model 420H/130A/920) (18West K.A. Crabb J.W. Angeletti R. Techniques in Protein Chemistry. 1992: 233-242Google Scholar). ESMS, LC-ESMS, and low energy collision tandem MS (MS/MS) were performed with a Perkin-Elmer Sciex API 300 triple quadrupole mass spectrometer (Thornhill, Canada) fitted with articulated ion spray plenums and atmospheric pressure ionization sources. Initial tuning and calibration was with a standard mixture of polypropylene glycol provided by PE Sciex. Resolution was adjusted to about 50% valley between adjacent isotope peaks in a singly charged cluster, allowing singly charged ions to be identified by apparent spacing between peaks and doubly charged ions to be distinguished from those with higher charge states. The nebulization gas for negative ion monitoring was bottled air (Medical Grade USP, Merriam-Graves, Claremont, NH), and for positive ion monitoring nitrogen was used at 40 p.s.i. Nitrogen was also used as the curtain gas and was supplied from a Dewar flask (XL-45, Taylor-Wharton) of liquid nitrogen (Merriam-Graves). For infusions in positive ion mode, spectra were acquired at an orifice potential of 60 V over a scan range of m/z 300–1500 using 0.25-atomic mass unit steps and a total scan time of 4.8 s. For negative ion monitoring, the ion spray needle voltage was operated at −4.0 kV and the focusing ring at −230 V; for positive ion monitoring, the ion spray needle voltage was 4.3 kV and the focusing ring was at 300 V. Low energy collision MS/MS was performed with the RO2 voltage set between −55 and −70 V, nitrogen as the collision gas, and CAD gas settings between 3 and 4, and 50–70 scans were accumulated. Parent ion transmission was maximized by setting Q1 to transmit about a 2–3m/z window around the selected ion. LC-ESMS with selective ion monitoring for PO3− at m/z 79 was performed in negative ion mode at high orifice potential (−190 V) over a narrow scan range of m/z 78.8–79.2 using 0.1-atomic mass unit steps, a dwell time of 100 ms/step, and a total scan time of 0.5 s. In the same analysis, full scans in positive ion mode were acquired at a lower orifice potential (60 V) over the scan range m/z 300–2000 using 0.25-atomic mass unit steps and a total scan time of 4 s. This was accomplished by looping the negative ion and positive ion experiments together with computer-controlled polarity reversal between each scan. The higher orifice potential in negative ion mode enhances collision-induced decomposition of phosphopeptides, producing the diagnostic PO3− fragment ion, thus allowing selective detection of phosphopeptides (19Huddleston M.J. Annan R.S. Bean M.F. Carr S.A. J. Am. Soc. Mass Spectrom. 1993; 4: 710-717Crossref PubMed Scopus (233) Google Scholar). The lower orifice potential used for full scans in positive ion mode allowed acquisition of mass spectra for all peptides (19Huddleston M.J. Annan R.S. Bean M.F. Carr S.A. J. Am. Soc. Mass Spectrom. 1993; 4: 710-717Crossref PubMed Scopus (233) Google Scholar). Seven synthetic peptides, including three phosphopeptides containing phosphotyrosine, phosphoserine, or phosphothreonine, were used to optimize negative ion detection sensitivity for the m/z 79 fragment ion or to serve as internal controls for PO3− monitoring during LC-ESMS or to verify correct calibration during LC-ESMS. Synthetic peptides used for mass spectrometry include G9I (M = 1015.6), K13L (M = 1542.9), L20R (M = 2475.2), V26E (M = 2940.5), and phosphopeptides E10A (M = 1266.6), K6R (M = 907.5), and b-A5K (M = 860.4). For LC-ESMS, the HPLC eluant was split, with 22% going to the mass spectrometer and the remainder collected in 1-min fractions. RP-HPLC for LC-ESMS was performed at a flow rate of 50 μl/min on a 5μ Vydac C18 microbore column (1 × 250 mm) using an Applied Biosystems model 120A HPLC system equipped with a 75-μl dynamic mixer and aqueous acetonitrile/trifluoroacetic acid solvents (17Crabb J.W. Gaur V.P. Garwin G.G. Marx S.V. Chapline C. Johnson C.M. Saari J.C. J. Biol. Chem. 1991; 266: 16674-16683Abstract Full Text PDF PubMed Google Scholar). Portions of select HPLC fractions were diluted 1:3 in 0.2% formic acid, 50% methanol and infused at 2 μl/min for MS/MS analysis. Four phospho-serine peptides containing the phosphorylation sites identified in in vitro reactions were synthesized with N-terminal cysteines added to facilitate coupling to keyhole limpet hemocyanin and/or solid supports. The peptides were as follows: Asn276–Lys286 included phosphoserine 283; Arg296–Lys305 included phosphoserine 300; Lys503–Lys512 included phosphoserine 507; and Leu511–Arg523 included phosphoserine 515. Antisera were raised in rabbits and purified against the cognate peptides coupled to agarose. REF52 cells were grown, and cell lysates were prepared as described previously (7Hyatt S.L. Liao L. Aderem A. Nairn A. Jaken S. Cell Growth Diff. 1994; 5: 495-502PubMed Google Scholar). Aliquots (25–50 μg of protein) were electrophoresed on SDS-PAGE gels and transferred to nitrocellulose. Blots were probed with immunopurified antisera preparations under standard conditions, and reactive bands were detected with ECL reagents. The phosphorylation state-insensitive antibody was raised to the recombinant fusion protein containing amino acid residues 388–790 (clone 72 original) (11Chapline C. Mousseau B. Ramsay K. Duddy S. Li Y. Kiley S.C. Jaken S. J. Biol. Chem. 1996; 271: 6417-6422Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar). REF52 cells were grown on glass coverslips. Cells were fixed in formaldehyde and permeabilized with methanol as described previously (20Jaken S. Leach K. Klauck T. J. Cell Biol. 1989; 109: 697-704Crossref PubMed Scopus (253) Google Scholar). Cells were incubated with affinity-purified primary antibodies, washed, and then incubated with FITC-conjugated species-specific secondary antibodies. Photomicrographs were taken on a Nikon Optiphot microscope. In previous studies, we prepared eight overlapping clone 72 fragments that span the entire open reading frame. Of the eight recombinant proteins, only fragments 1, 2, and 3 (from the N terminus) were phosphorylated in vitrowith PKC. 2C. Chapline, J. Cottom, Y. Li, J. Scott, and S. Jaken, submitted for publication. To identify sites of phosphorylation, the phosphorylated fragments were digested with proteases, and phosphopeptides were isolated by HPLC and identified by Edman degradation. Those studies identified three phosphopeptides containing putative PKC phosphorylation sites, Thr280–Phe288, Arg296–Asp308, and Arg494–Glu530. We have now extended these studies using mass spectrometry to identify the PKC phosphorylation sites in these peptides. The synthetic peptide C34E (designed from residues Lys496–Glu529 with cysteine added) was phosphorylated with PKC and then digested with endoproteinase Lys-C. Identified peptides with their corresponding retention times and masses are summarized in Table I. Several lysyl peptides (representing partially digested fragments) containing a single serine residue (Ser20 in peptide C34E) corresponding to serine 515 in clone 72 showed an increased mass of 80 Da, indicative of phosphate incorporation. Several lysyl peptides containing serine residues 507–509 also showed an 80-Da increase in mass, indicating that one of these serine residues is also a target. Observed masses of lysyl peptides containing serine 500 correlated with calculated masses; therefore, serine 500 is not a PKC target (Table I). To determine which of serines 507–509 were phosphorylated, lysyl peptide LFSSSGLK (corresponding to residues 505–512 from clone 72) was further analyzed by low energy collision MS/MS (Fig.1). The sequence was determined from the loss in fragment mass starting from the C terminus (y fragments) or N terminus (b fragments). This analysis confirms that the serine corresponding to Ser507 in clone 72 is phosphorylated. Thus, Ser507 and Ser515, but not Ser500 are in vitro PKC phosphorylation sites in fragment 3 (Met387–Ile790). Phosphorylated residues in the other phosphorylation site found in fragment 2 (Gln261–Gly532) of clone 72 were identified by phosphorylating the synthetic peptide corresponding to residues Arg296–Lys305. Phosphorylation resulted in an observed mass increase of 80 Da, indicating that only one of the three serines and threonines is the preferred site (Fig.2, A and B). Low energy collision MS/MS analysis showed that phosphorylation occurred at serine 300 (Fig. 2 C). Several mutants within the fragment 3 phosphorylation site were constructed to analyze the structural requirements for PKC binding and phosphorylation (Table II). Fragment 3 and a subfragment (subfragment 3, Ser370–Glu620) had quantitatively similar PKC binding activity, indicating that the PKC binding motif is contained within this region (Fig.3). Mutating the three tandem serine residues to alanines (S507–509A) did not influence binding, indicating that these residues are not critical for PKC binding. Phenylalanine residues in the MARCKS phosphorylation domain have been shown to be important for hydrophobic interactions with PS at the membrane interface (21Wertz S.L. Savino Y. Cafiso D.S. Biochemistry. 1996; 35: 11104-11112Crossref PubMed Scopus (18) Google Scholar). Replacing phenylalanine 506 with valine (F506V) severely attenuated PKC binding and PS binding (Fig. 3). Thus, interactions of the clone 72 and MARCKS phosphorylation domain peptides with PS may be somewhat analogous. Structural changes in this motif were introduced by replacing leucine 511 with proline (L511P). According to Chou-Fasman and Garnier-Robson rules, this mutation is predicted to decrease α-structure and increase β-structure. This mutation also coordinately decreased PKC and PS binding. These results identify the Lys497–Pro511 region as a major determinant of phospholipid-dependent PKC binding. Neither of these mutations influenced the net positive charge of this domain, indicating that PKC and PS binding are primarily regulated by hydrophobic interactions and/or secondary structure.Table IIClone 72 mutant constructsConstructSequenceSubfragment 3(502)LKKLFSSSGLKKLSGKKQK(521)F506VLKKLVSSSGLKKLSGKKQKL511PLKKLFSSSGPKKLSGKKQKS507–509ALKKLFAAAGLKKLSGKKQKMutants in the clone 72 phosphorylation site within the Lys496–Pro531 fragment were prepared and expressed as His6-tagged fusion proteins. Open table in a new tab Mutants in the clone 72 phosphorylation site within the Lys496–Pro531 fragment were prepared and expressed as His6-tagged fusion proteins. As shown in Fig. 4, phosphorylation of S507–509A relative to the wild type sequence was decreased, which verifies that these residues are among the PKC targets. Surprisingly, the F506V and L511P mutants were relatively better PKC substrates than the wild type protein, despite their decreased PKC and PS binding. Kinetic analysis of L511P mutant phosphorylation demonstrated that this was due to an increased Vmax with no significant effect on Km (Fig. 5). The Vmax value was increased 2.2-fold for the F506V mutant compared with subfragment 3. Differences inKm values were <20%. One potential explanation for this result is that phosphorylatable residues in the wild type sequence are partially masked, and mutations in this region increase accessibility to PKC.Figure 5Kinetic analysis of clone 72 phosphorylation. Subfragment 3 and mutant F506V (0.13–5 μm) were phosphorylated by PKC in the presence of 1 mm EGTA, 1.2 mm calcium chloride, 20 μg/ml phosphatidylserine, and 200 nm PDBu at 30 °C for 15 min. Proteins were resolved by electrophoresis, and phosphoproteins were detected by autoradiography. Phosphate incorporation was quantitated by densitometry of the autorad. Data are duplicate measurements. Lines drawn are best fit determined by linear regression analysis. Similar results were found in two additional experiments.View Large Image Figure ViewerDownload Hi-res image Download (PPT) To determine if the PKC phosphorylation sites identified in in vitro studies were actual sites in vivo, antisera were raised to phosphopeptides containing the putative phosphorylation sites Ser300, Ser507, and Ser515. In addition, an antiserum to Ser283 was prepared, since preliminary in vitro phosphorylation studies identified Thr280–Phe289 as a phosphopeptide. Antisera were affinity-purified against the cognate phosphopeptides. Specificities of the antibodies were tested by comparing their reactivities against the phosphorylated and unphosphorylated forms of the immunizing peptides. As shown in Fig.6, each of the antibodies preferentially recognizes the corresponding phosphopeptide compared with the unphosphorylated peptide. The antibodies did not cross-react with unphosphorylated recombinant clone 72 proteins. Furthermore, each of the antibodies specifically recognizes its cognate peptide and does not cross-react with other Ser(P)-containing peptides. Thus, these antibodies recognize context-specific Ser(P) and not Ser(P) alone. These data indicate that these antibodies are phosphorylation state-selective and can be used to monitor clone 72 phosphorylation at selected residues. To determine if clone 72 is phosphorylated by PKC in vivo, we prepared extracts from REF52 cells that had been treated with the PKC activator, PDBu. As shown in Fig. 7, phorbol esters stimulated phosphorylation at serines 283, 300, 507, and 515. The relative increases in antibody binding were determined by densitometry of immunoblots and were 634 ± 351, 297 ± 185, 1996 ± 875, and 783 ± 300 (means ± S.D., n = 4)for phosphoserines 283, 300, 507, and 515, respectively. The correlation between the phosphorylation sites identified in in vitro and in vivo assays, indicates that clone 72 is an endogenous PKC substrate. To gain insight into the function of clone 72 and the effects of PKC phosphorylation, we compared the localization of clone 72 and phosphorylated clone 72 by immunofluorescence. Each of the clone 72 antibodies used for immunofluorescence preferentially recognized only clone 72 on immunoblots of REF52 cell extracts (Fig.8 A), which demonstrates their suitability for immunolocalization studies. In unstimulated cells, total clone 72 (monitored with a phosphorylation state nonspecific antibody) is distributed throughout the cytoplasm and concentrated in membrane ruffles (Fig. 8 B). In contrast, phosphoserine 300 and 515 antibodies did not detect substantial cytoplasmic staining. With these antibodies, staining was concentrated in membrane protrusions and ruffles. These data indicate that under physiological conditions, PKC activation and clone 72 phosphorylation are preferentially localized to the cell perimeter. The prominent nuclear staining observed with the phosphoserine 515 antibody is not seen with other antibodies and appears to be an artifact of this particular antibody. PDBu caused a redistribution of total clone 72 toward the perinuclear space but did not substantially influence the staining intensity. In contrast, PDBu substantially increased the immunostaining seen with the phosphoserine 300 and 515 antibodies. Staining was concentrated in the perinuclear region of the cytoplasm but was also apparent in membrane protrusions and ruffles. PKC isozymes expressed in these cells (α, δ, ε, and ζ) are also found in these locations. However, the broad distribution patterns of PKCs and clone 72 do not permit the detailed comparison required to assess colocalization. Other approaches will be required to determine which PKCs interact with and phosphorylate clone 72 in vivo. Since PDBu is cell-permeable, it can activate PKCs throughout the cell, and its actions are not limited to the cell surface. To investigate physiological PKC activation and clone 72 phosphorylation, cells grown on coverslips were artificially wounded by scraping the monolayer with a sharp object. Cells were then stained with antibodies to actin and clone 72 (Fig. 9). The double staining clearly demonstrates that phosphorylated clone 72 is concentrated in membrane ruffles and regions in which actin is depolymerized. These results demonstrate a strong correlation among PKC activation, clone 72 phosphorylation, and dynamic actin remodeling. PKC phosphorylates a wide variety of proteins in in vitro reactions, many of which are not relevant physiological substrates in vivo. The relatively limited number of PKC phosphorylation events that occur in vivo indicates that PKC access to substrates is regulated in cells. One mechanism for limiting PKC phosphorylations to appropriate substrate proteins would be through direct, high affinity interactions of PKC with its substrates. We have cloned several PKC-interacting proteins based on their ability to bind PKC in an in vitro assay (4Chapline C. Ramsay K. Klauck T. Jaken S. J. Biol. Chem. 1993; 268: 6858-6861Abstract Full Text PDF PubMed Google Scholar). Binding proteins identified by this method include the PKC substrates MARCKS and the related protein MacMARCKS (22Aderem A. Cell. 1992; 71: 713-716Abstract Full Text PDF PubMed Scopus (431) Google Scholar) and two forms of adducin, α-adducin and the novel γ-adducin (8Dong L. Chapline C. Mousseau B. Fowler L. Ramsay K. Stevens J.L. Jaken S. J. Biol. Chem. 1995; 270: 25534-25540Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar). We named this class of PKC binding proteins/substrates STICKs (for substrates thatinteract with C-kinase. Studying the effects of PKC phosphorylation on STICK functions provides an avenue for understanding the mechanisms by which PKC regulates cell growth, death, and gene expression. In the present study, we demonstrated that clone 72, the major PKC-binding protein in REF52 cells, is phosphorylated by PKC at the same sites in vitroand in vivo. In other studies, we found that overexpressing certain PKCs increases clone 72 phosphorylation, whereas inhibiting PKC with bisindoylmaleimide I decreases clone 72 phosphorylation. 3H. M. Tobin, J. Cotton, Y. Li, J. Scott, and S. Jaken, manuscript in preparation. We conclude that PKC is a clone 72 kinase. The selective localization of phosphorylated clone 72 compared with total clone 72 indicates that phosphorylation occurs at discrete sites and may be either a consequence or an effector of localized actin remodeling involved in formation of membrane protrusions and ruffles. Clone 72 is related to gravin, which was also recently shown to be a PKC-binding protein in vitro (14Nauert J.B. Klauck T.M. Langeberg L.K. Scott J.D. Curr. Biol. 1997; 7: 52-62Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar). Gravin is >80% identical to clone 72 in the PKC binding domains/phosphorylation sites that we have identified in clone 72. Studies with gravin failed to detect any evidence for an in vivo complex formation between gravin and PKC (14Nauert J.B. Klauck T.M. Langeberg L.K. Scott J.D. Curr. Biol. 1997; 7: 52-62Abstract Full Text Full Text PDF PubMed Scopus (223) Google Scholar). However, since gravin is highly homologous to clone 72 in the PKC phosphorylation sites/binding domains, it seems likely that gravin is also a PKC substrate that associates (at least transiently) with PKC in vivo. Although a fragment of gravin was reported to inhibit PKC phosphorylation of an exogenous peptide substrate, since clone 72 and (by analogy) gravin are PKC substrates, it seems likely that emphasis should be placed on the role of clone 72 and gravin as PKC substrates rather than inhibitors. Mutational analysis was used in structure-function studies to define the PKC binding site in the fragment 3 region of clone 72. We found that the F506V and L511P mutants had decreased PS and PKC binding activity yet were improved PKC substrates due to an increase inVmax. These results lend support for the model proposed by Ohno et al. (23Fujise A. Mizuno K. Ueda Y. Osada S. Hirai S. Takayanagi A. Shimizu N. Owada M.K. Nakajima H. Ohno S. J. Biol. Chem. 1994; 269: 31642-31648Abstract Full Text PDF PubMed Google Scholar) to explain why MARCKS is a high affinity (low Km) yet slowly phosphorylated (lowVmax) PKC substrate. It is possible that high affinity interactions of PKC with substrates are accomplished at the expense of the rate of the phosphorylation reaction. For example, the decreased binding of the F506V and L511P mutants would facilitate dissociation of product and increase enzyme turnover. Given the high affinity of PKC for STICKs and the less than optimal phosphorylation efficiency, it is possible that PKC-STICK interactions are important for localizing PKC. In summary, these data provide additional evidence that PKC directly interacts with and phosphorylates a class of substrate proteins named STICKs. Thus, STICKs have two important activities: PKC binding partners and PKC substrates. Phosphorylation of certain STICKs, such as MARCKS and adducins, alters their functions with regard to association with cytoskeletal proteins and membranes (20Jaken S. Leach K. Klauck T. J. Cell Biol. 1989; 109: 697-704Crossref PubMed Scopus (253) Google Scholar, 21Wertz S.L. Savino Y. Cafiso D.S. Biochemistry. 1996; 35: 11104-11112Crossref PubMed Scopus (18) Google Scholar). Thus, STICKs are also effectors of PKC activation. Additional studies of PKC-STICK interactions and phosphorylation-induced changes in STICK functions should provide new insight into the initial events in PKC activation that contribute to downstream effects on cell growth, transformation, and gene expression." @default.
• W2112534728 created "2016-06-24" @default.
• W2112534728 creator A5006262283 @default.
• W2112534728 creator A5032840234 @default.
• W2112534728 creator A5047902229 @default.
• W2112534728 creator A5049236815 @default.
• W2112534728 creator A5068769791 @default.
• W2112534728 creator A5075780209 @default.
• W2112534728 date "1998-07-01" @default.
• W2112534728 modified "2023-10-16" @default.
• W2112534728 title "A Major, Transformation-sensitive PKC-binding Protein Is Also a PKC Substrate Involved in Cytoskeletal Remodeling" @default.
• W2112534728 cites W125558696 @default.
• W2112534728 cites W1487426514 @default.
• W2112534728 cites W1547755899 @default.
• W2112534728 cites W1576671418 @default.
• W2112534728 cites W1587087239 @default.
• W2112534728 cites W1604547377 @default.
• W2112534728 cites W1972223792 @default.
• W2112534728 cites W1976319209 @default.
• W2112534728 cites W2004329874 @default.
• W2112534728 cites W2005645407 @default.
• W2112534728 cites W2008972267 @default.
• W2112534728 cites W2009682799 @default.
• W2112534728 cites W2043804549 @default.
• W2112534728 cites W2065750990 @default.
• W2112534728 cites W2137211481 @default.
• W2112534728 cites W2141450570 @default.
• W2112534728 cites W2169819991 @default.
• W2112534728 cites W2186108493 @default.
• W2112534728 cites W4232940172 @default.
• W2112534728 doi "https://doi.org/10.1074/jbc.273.31.19482" @default.
• W2112534728 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/9677369" @default.
• W2112534728 hasPublicationYear "1998" @default.
• W2112534728 type Work @default.
• W2112534728 sameAs 2112534728 @default.
• W2112534728 citedByCount "87" @default.
• W2112534728 countsByYear W21125347282012 @default.
• W2112534728 countsByYear W21125347282014 @default.
• W2112534728 countsByYear W21125347282016 @default.
• W2112534728 countsByYear W21125347282017 @default.
• W2112534728 crossrefType "journal-article" @default.
• W2112534728 hasAuthorship W2112534728A5006262283 @default.
• W2112534728 hasAuthorship W2112534728A5032840234 @default.
• W2112534728 hasAuthorship W2112534728A5047902229 @default.
• W2112534728 hasAuthorship W2112534728A5049236815 @default.
• W2112534728 hasAuthorship W2112534728A5068769791 @default.
• W2112534728 hasAuthorship W2112534728A5075780209 @default.
• W2112534728 hasBestOaLocation W21125347281 @default.
• W2112534728 hasConcept C104317684 @default.
• W2112534728 hasConcept C12554922 @default.
• W2112534728 hasConcept C142669718 @default.
• W2112534728 hasConcept C1491633281 @default.
• W2112534728 hasConcept C185592680 @default.
• W2112534728 hasConcept C18903297 @default.
• W2112534728 hasConcept C195794163 @default.
• W2112534728 hasConcept C204241405 @default.
• W2112534728 hasConcept C2777289219 @default.
• W2112534728 hasConcept C55493867 @default.
• W2112534728 hasConcept C62478195 @default.
• W2112534728 hasConcept C86803240 @default.
• W2112534728 hasConcept C95444343 @default.
• W2112534728 hasConceptScore W2112534728C104317684 @default.
• W2112534728 hasConceptScore W2112534728C12554922 @default.
• W2112534728 hasConceptScore W2112534728C142669718 @default.
• W2112534728 hasConceptScore W2112534728C1491633281 @default.
• W2112534728 hasConceptScore W2112534728C185592680 @default.
• W2112534728 hasConceptScore W2112534728C18903297 @default.
• W2112534728 hasConceptScore W2112534728C195794163 @default.
• W2112534728 hasConceptScore W2112534728C204241405 @default.
• W2112534728 hasConceptScore W2112534728C2777289219 @default.
• W2112534728 hasConceptScore W2112534728C55493867 @default.
• W2112534728 hasConceptScore W2112534728C62478195 @default.
• W2112534728 hasConceptScore W2112534728C86803240 @default.
• W2112534728 hasConceptScore W2112534728C95444343 @default.
• W2112534728 hasIssue "31" @default.
• W2112534728 hasLocation W21125347281 @default.
• W2112534728 hasOpenAccess W2112534728 @default.
• W2112534728 hasPrimaryLocation W21125347281 @default.
• W2112534728 hasRelatedWork W1997012892 @default.
• W2112534728 hasRelatedWork W1999411716 @default.
• W2112534728 hasRelatedWork W2016929896 @default.
• W2112534728 hasRelatedWork W2024493059 @default.
• W2112534728 hasRelatedWork W2037513142 @default.
• W2112534728 hasRelatedWork W2102934096 @default.
• W2112534728 hasRelatedWork W2166267918 @default.
• W2112534728 hasRelatedWork W2332486747 @default.
• W2112534728 hasRelatedWork W4243304189 @default.
• W2112534728 hasRelatedWork W4317623439 @default.
• W2112534728 hasVolume "273" @default.
• W2112534728 isParatext "false" @default.
• W2112534728 isRetracted "false" @default.
• W2112534728 magId "2112534728" @default.
• W2112534728 workType "article" @default.