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• W1987173208 abstract "In plant cells, the plane of division is anticipated at the onset of mitosis by the presence of a preprophase band (PPB) of microtubules and F-actin at a cortical site that circumscribes the nucleus. During cytokinesis, the microtubule- and F-actin-based phragmoplast facilitates construction of a new cell wall and is guided to the forecast division site. Proper execution of this process is essential for establishing the cellular framework of plant tissues. The microtubule binding protein TANGLED1 (TAN1) of maize is a key player in the determination of division planes [1Smith L.G. Gerttula S.M. Han S. Levy J. TANGLED1: a microtubule binding protein required for the spatial control of cytokinesis in maize.J. Cell Biol. 2001; 152: 231-236Crossref PubMed Scopus (93) Google Scholar, 2Cleary A.L. Smith L.G. The Tangled1 gene is required for spatial control of cytoskeletal arrays associated with cell division during maize leaf development.Plant Cell. 1998; 10: 1875-1888PubMed Google Scholar, 3Smith L.G. Hake S. Sylvester A.W. The tangled1 mutation alters cell division orientations throughout maize leaf development without altering leaf shape.Development. 1996; 122: 481-489PubMed Google Scholar]. Lack of TAN1 leads to misguided phragmoplasts and mispositioned cell walls in maize. In a yeast two-hybrid screen for TAN1-interacting proteins, a pair of related kinesins was identified that shares significant sequence homology with two kinesin-12 genes in Arabidopsis thaliana (A. thaliana): PHRAGMOPLAST ORIENTING KINESIN 1 and 2 (POK1, POK2). POK1 and POK2 are expressed in tissues enriched for dividing cells. The phenotype of pok1;pok2 double mutants strongly resembles that of maize tan1 mutants, characterized by misoriented mitotic cytoskeletal arrays and misplaced cell walls. We propose that POK1 and POK2 participate in the spatial control of cytokinesis, perhaps via an interaction with the A. thaliana TAN1 homolog, ATN. In plant cells, the plane of division is anticipated at the onset of mitosis by the presence of a preprophase band (PPB) of microtubules and F-actin at a cortical site that circumscribes the nucleus. During cytokinesis, the microtubule- and F-actin-based phragmoplast facilitates construction of a new cell wall and is guided to the forecast division site. Proper execution of this process is essential for establishing the cellular framework of plant tissues. The microtubule binding protein TANGLED1 (TAN1) of maize is a key player in the determination of division planes [1Smith L.G. Gerttula S.M. Han S. Levy J. TANGLED1: a microtubule binding protein required for the spatial control of cytokinesis in maize.J. Cell Biol. 2001; 152: 231-236Crossref PubMed Scopus (93) Google Scholar, 2Cleary A.L. Smith L.G. The Tangled1 gene is required for spatial control of cytoskeletal arrays associated with cell division during maize leaf development.Plant Cell. 1998; 10: 1875-1888PubMed Google Scholar, 3Smith L.G. Hake S. Sylvester A.W. The tangled1 mutation alters cell division orientations throughout maize leaf development without altering leaf shape.Development. 1996; 122: 481-489PubMed Google Scholar]. Lack of TAN1 leads to misguided phragmoplasts and mispositioned cell walls in maize. In a yeast two-hybrid screen for TAN1-interacting proteins, a pair of related kinesins was identified that shares significant sequence homology with two kinesin-12 genes in Arabidopsis thaliana (A. thaliana): PHRAGMOPLAST ORIENTING KINESIN 1 and 2 (POK1, POK2). POK1 and POK2 are expressed in tissues enriched for dividing cells. The phenotype of pok1;pok2 double mutants strongly resembles that of maize tan1 mutants, characterized by misoriented mitotic cytoskeletal arrays and misplaced cell walls. We propose that POK1 and POK2 participate in the spatial control of cytokinesis, perhaps via an interaction with the A. thaliana TAN1 homolog, ATN. The plant cytoskeleton undergoes meticulously conducted rearrangements to establish the division plane prior to mitosis, separate chromosomes at mitosis, and form a new cell wall at cytokinesis in the previously established division plane. Throughout prophase, the future division plane is predicted by the position of the cortical preprophase band (PPB) of microtubules and actin filaments [4Smith L.G. Plant cell division: building walls in the right places.Nat. Rev. Mol. Cell Biol. 2001; 2: 33-39Crossref PubMed Scopus (152) Google Scholar]. As cells enter mitosis, the PPB is disassembled while microtubules form the spindle, leaving behind an actin-depleted zone in the cell cortex, which marks the former location of the PPB throughout mitosis and cytokinesis [4Smith L.G. Plant cell division: building walls in the right places.Nat. Rev. Mol. Cell Biol. 2001; 2: 33-39Crossref PubMed Scopus (152) Google Scholar]. Recent work has shown that a plasma membrane-localized A. thaliana kinesin, KCA1, is excluded from the PPB zone during prophase, and like cortical F-actin remains locally depleted at the former PPB site throughout the remainder of the cell cycle [5Vanstraelen M. Van Damme D. De Rycke R. Mylle E. Inze D. Geelen D. Cell cycle-dependent targeting of a kinesin at the plasma membrane demarcates the division site in plant cells.Curr. Biol. 2006; 16: 308-314Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]. After mitosis, cytoplasmic microtubules and F-actin are reorganized to form a phragmoplast, which facilitates formation of the cell plate (the new cell wall) via the fusion of Golgi-derived vesicles [6Otegui M. Staehelin L.A. Cytokinesis in flowering plants: more than one way to divide a cell.Curr. Opin. Plant Biol. 2000; 3: 493-502Crossref PubMed Scopus (105) Google Scholar]. The phragmoplast is initiated in isolation from the parental plasma membrane and cell wall, and subsequently expands to attach the cell plate to the parental wall at the site formerly occupied by the PPB (“the division site”). It has been proposed that PPB-dependent modification(s) of the cell wall, plasma membrane, and/or cell cortex guide phragmoplast expansion toward the division site [7Mineyuki Y. Palevitz B.A. Relationship between preprophase band organization, F-actin and the division site in Allium.J. Cell Sci. 1990; 97: 527-537Google Scholar]. Local depletion of F-actin and KCA1 at the division site are likely to be important for phragmoplast guidance [4Smith L.G. Plant cell division: building walls in the right places.Nat. Rev. Mol. Cell Biol. 2001; 2: 33-39Crossref PubMed Scopus (152) Google Scholar, 5Vanstraelen M. Van Damme D. De Rycke R. Mylle E. Inze D. Geelen D. Cell cycle-dependent targeting of a kinesin at the plasma membrane demarcates the division site in plant cells.Curr. Biol. 2006; 16: 308-314Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar]; both pharmacological and genetic studies suggest important roles for actin and myosin in this process [8Wick S.M. Spatial aspects of cytokinesis in plant cells.Curr. Opin. Cell Biol. 1991; 3: 253-260Crossref PubMed Scopus (95) Google Scholar, 9Molchan T.M. Valster A.H. Hepler P.K. Actomyosin promotes cell plate alignment and late lateral expansion in Tradescantia stamen hair cells.Planta. 2002; 214: 683-693Crossref PubMed Scopus (73) Google Scholar, 10Holweg C. Nick P. Arabidopsis myosin XI mutant is defective in organelle movement and polar auxin transport.Proc. Natl. Acad. Sci. USA. 2004; 101: 10488-10493Crossref PubMed Scopus (65) Google Scholar]. For the most part, however, mechanisms governing phragmoplast guidance remain mysterious. The TAN1 protein of maize plays an important role in the orientation of cell division planes and is one of the few proteins known to be involved in the spatial control of cytokinesis. TAN1 is a novel, highly basic protein that binds to microtubules in vitro and belongs to a family of proteins that are preferentially localized to mitotic microtubule arrays in dividing cells [1Smith L.G. Gerttula S.M. Han S. Levy J. TANGLED1: a microtubule binding protein required for the spatial control of cytokinesis in maize.J. Cell Biol. 2001; 152: 231-236Crossref PubMed Scopus (93) Google Scholar]. In tan1 mutants, PPBs are occasionally misoriented and the majority of phragmoplasts fail to be guided to former PPB sites, pointing to a role for TAN1 in phragmoplast guidance [2Cleary A.L. Smith L.G. The Tangled1 gene is required for spatial control of cytoskeletal arrays associated with cell division during maize leaf development.Plant Cell. 1998; 10: 1875-1888PubMed Google Scholar]. A yeast two-hybrid screen for interaction partners of TAN1 led to the isolation of two truncated, predicted coiled-coil forming maize proteins, which are related to each other and to the carboxy (COOH)-terminal tails of two predicted A. thaliana kinesins, POK1 (At3g17360) and POK2 (At3g19050). One of the maize protein fragments is most similar to POK1 (43% identical over its entire length of 435 amino acids), while the other is most similar to POK2 (30% identical over its entire length of 439 amino acids). RT-PCR analysis revealed that POK1 is comprised of 34 exons, encoding a protein of 2066 amino acids and a molecular weight of 234 kDa, while POK2 is comprised of 36 exons, encoding a protein of 2770 amino acids and a predicted molecular weight of 315 kDa (Figures 1A and 1B, and Figure S2A in the Supplemental Data available with this article online). Protein-protein interaction between the C terminus of POK1 and the A. thaliana TAN1 homolog, ATN (At3g05330), was confirmed in directed yeast two-hybrid experiments (Figure S1), suggesting that ATN and POK1 could act together. In A. thaliana, 61 putative kinesins have been identified based on the presence of the characteristic motor domain [11Reddy A.S. Day I.S. Kinesins in the Arabidopsis genome: a comparative analysis among eukaryotes.BMC Genomics. 2001; 2: 2Crossref PubMed Scopus (150) Google Scholar, 12Schliwa M. Woehlke G. Molecular motors.Nature. 2003; 422: 759-765Crossref PubMed Scopus (749) Google Scholar]. POK1 and POK2 are most closely related to each other and, based on sequence homology in their motor domains, belong to the kinesin-12 class [13Lawrence C.J. Dawe R.K. Christie K.R. Cleveland D.W. Dawson S.C. Endow S.A. Goldstein L.S. Goodson H.V. Hirokawa N. Howard J. et al.A standardized kinesin nomenclature.J. Cell Biol. 2004; 167: 19-22Crossref PubMed Scopus (534) Google Scholar], which includes the well-characterized group of phragmoplast-associated kinesins AtPAKRPs [14Pan R. Lee Y.R. Liu B. Localization of two homologous Arabidopsis kinesin-related proteins in the phragmoplast.Planta. 2004; 220: 156-164Crossref PubMed Scopus (33) Google Scholar] (Figure S2B). Both kinesins are composed of an N-terminal motor domain (Figure S2A) followed by coiled-coil domains (predicted by COILS program (http://www.ch.embnet.org/software/coils/COILS_doc.html) throughout the entire C terminus and putative cargo binding tail domains. We investigated the expression pattern of POK1 and POK2 in transgenic plants expressing a β-glucuronidase (GUS) reporter gene under the control of POK1 and POK2 putative native promoter regions. For POK1 expression analysis, two different constructs were analyzed that gave identical expression patterns, one with 2610 bp (pPOK1:GUS) and the other with 1134 bp (mpPOK1:GUS) upstream of the ATG. POK2 expression was analyzed with a GUS fusion construct containing 2578 bp of upstream sequence. A variety of tissues were analyzed for GUS expression in multiple transgenic lines for all three constructs. Similar GUS expression patterns were found for both POK1 and POK2, which are expressed in tissues enriched in dividing cells, such as root meristems (Figures 2A and 2D), root primordia (Figures 2B and 2E), and leaf primordia/young leaves (Figures 2C and 2F). Although the expression domains for POK1 constructs were more limited than those for POK2, the intensity of GUS staining in pPOK1:GUS and mpPOK1:GUS lines was much weaker overall than in pPOK2:GUS lines (requiring extended incubation times to see even weak signals, unlike pPOK2:GUS lines, where strong staining was observed after just 2 hr of incubation with substrate). Thus, the full extent of endogenous POK1 gene expression may not be revealed by the pPOK1:GUS and mpPOK1:GUS lines. Together with the sequence similarity between POK1 and POK2, the similar GUS staining patterns observed in POK1 and POK2 promoter-GUS lines suggested that these genes might act redundantly. The expression domains of POK1 and POK2 revealed by this analysis also overlap with that of ATN (K. Walker and L.G.S., unpublished data), supporting the possibility of a direct interaction between POK1/2 and ATN in vivo. In order to gain insight into the function of POK1 and POK2, we searched the SALK T-DNA collection database [15Alonso J.M. Stepanova A.N. Leisse T.J. Kim C.J. Chen H. Shinn P. Stevenson D.K. Zimmerman J. Barajas P. Cheuk R. et al.Genome-wide insertional mutagenesis of Arabidopsis thaliana.Science. 2003; 301: 653-657Crossref PubMed Scopus (3916) Google Scholar] (http://signal.salk.edu/cgi-bin/tdnaexpress) and identified T-DNA insertions in both genes. SALK_001081 (pok1-1), SALK_067862 (pok1-2), SALK_013944 (pok2-1), and SALK_066406 (pok2-2) were obtained from the Arabidopsis Biological Resource Center (ABRC, Columbus, OH; all mutations are in the Columbia background). The presence of T-DNA insertions in exons was confirmed by sequencing (Figures 1A and 1B). Genotypes were analyzed with a PCR-based method (see Supplemental Data for primers). Transcript accumulation in flower bud tissues or 6-day-old seedlings, respectively, was analyzed in plants homozygous for each of the four insertions. Northern blot analysis with a POK1 probe revealed a 6.2 kb transcript in wild-type tissues, which is consistent with the intron/exon structure of this gene. No POK1 transcript was detected in pok1-1 or pok1-2 mutants (Figure 1C). Moreover, RT-PCR analysis revealed no detectable POK2 transcripts in pok2-1 or pok2-2 mutants spanning their respective T-DNA insertion sites (Figure 1D). Plants homozygous for any one of these mutant alleles did not show obvious phenotypes. Considering the similarity in both sequence and expression pattern for POK1 and POK2, we generated the double mutant to uncover potentially redundant functions for these genes. POK1 and POK2 are closely linked on chromosome III (approximately 0.6 MB apart). pok1-1 was crossed to pok2-1 and pok1-2 was crossed to pok2-2 to create two different double heterozygotes, and F1 progeny from both crosses were backcrossed to Columbia wild-type plants. 1/106 F2 plants genotyped for the pok1-1;pok2-1 allele combination and 1/157 genotyped for the pok1-2;pok2-2 combination contained a recombinant chromosome carrying both mutations. These double heterozygous plants were allowed to self-fertilize, and progeny were genotyped. For both allele combinations, double homozygotes showed similar defects at a variety of developmental stages, while double heterozygotes were indistinguishable from wild-type plants, indicating that the mutations are recessive. Due to linkage of pok1 and pok2, the pok1;pok2 double mutations segregated as a single locus with approximately 1 out of 4 plants homozygous for both mutations (e.g., 38 homozygous pok1-1;pok2-1 individuals, n = 173). pok1;pok2 double mutant seedlings differ from wild-type in having smaller cotyledons as well as shorter, wider roots and hypocotyls (Figures 3A–3D and Figures S3A–S3C). Adult plants exhibit a dwarfed stature, but all organs are present, although miniaturized (Figures 3E–3G, Figures S3D–S3F). Plants are fertile but produce reduced numbers of seeds. To investigate the double mutant phenotype on a cellular level, living root meristems were stained with propidium iodide (10 μg/ml in H2O), which outlines cell walls in living tissues. In wild-type root meristems, cells divide transversely and longitudinally to generate a very regular cell pattern (Figure 4A). In contrast, double mutant root meristems exhibited highly disordered cell patterns with most walls lacking parallel or perpendicular alignment to the longitudinal axis of the root and most cells exhibiting irregular shapes (Figures 4B and 4C). Examination of cell wall patterns suggested that misoriented cell divisions were largely responsible for the irregular cell pattern (e.g., arrowheads, Figures 4B and 4C). We assessed how early the mutation affects the double mutant's development. In wild-type embryos, cell divisions follow an invariant pattern [16Westhoff P. Jesker H. Jurgens G. Kloppstech K. Link G. Embryogenese.in: Molekulare Entwicklungsbiologie: Vom Gen zur Pflanze. Georg Thieme Verlag Stuttgart, New York1996: 117-126Google Scholar] (e.g., Figure 4D for octant stage embryos). In pok1;pok2 double mutants, misplaced cell walls were observed as early as the octant stage of embryogenesis (Figures 4E and 4F). Throughout embryogenesis, the mispositioning of cell walls continues (see double mutants, Figures 4H and 4I, versus wild-type, Figure 4G), eventually giving rise to slightly misshapen mature embryos with a stubby appearance (Figure 4K) compared to wild-type (Figure 4J). Incomplete cell walls and wall stubs, a common feature of cytokinesis-defective mutants [17Jurgens G. Cytokinesis in higher plants.Annu. Rev. Plant Biol. 2005; 56: 281-299Crossref PubMed Scopus (149) Google Scholar, 18Waizenegger I. Lukowitz W. Assaad F. Schwarz H. Jurgens G. Mayer U. The Arabidopsis KNOLLE and KEULE genes interact to promote vesicle fusion during cytokinesis.Curr. Biol. 2000; 10: 1371-1374Abstract Full Text Full Text PDF PubMed Scopus (119) Google Scholar, 19Muller S. Fuchs E. Ovecka M. Wysocka-Diller J. Benfey P.N. Hauser M.-T. Two new loci, PLEIADE and HYADE, implicate organ-specific regulation of cytokinesis in Arabidopsis.Plant Physiol. 2002; 130: 312-324Crossref PubMed Scopus (45) Google Scholar], were not observed in Toluidine blue-stained sections of double mutant embryos (Figures 4L and 4M), nor in propidium iodide-stained root meristems (Figures 4B and 4C). These results suggest that POK1 and POK2 are not involved in the formation or maturation of the cell wall but are required for its correct positioning.Figure 4Cell Wall Misorientation in pok Double Mutant Seedlings and EmbryosShow full caption(A–C) Propidium iodide-stained root meristems: (A) wild-type (Columbia), (B) pok1-2;pok2-2, (C) pok1-1;pok2-1. Arrowheads point to examples of misoriented cell walls. Fluorescent samples were illuminated with a 488 nM line form an argon laser and visualized with a Nikon TE-200U microscope equipped with a 100× 1.4 NA water immersion objective. Images were acquired with a Yokogawa Nipkow spinning disc confocal head, a Chroma HQ610/75 band pass emission filter, and a Coolsnap HQ cooled CCD camera controlled by MetaMorph software (Universal Imaging Corporation, Downing, PA).(D–K) Cleared embryos viewed with DIC optics: (D–F) octant stage embryos, (G–I) globular stage embryos, (J and K) mature embryos; (D, G, J) wild-type (Columbia), (E, H, K) pok1-2;pok2-2, (F, I) pok1-1;pok2-1. Arrowheads mark boundaries of cell walls. Siliques at various stages after fertilization were fixed in methanol/acetic acid (3:1). Embryos were dissected out and cleared in chloralhydrate/water (5 g/2 ml) visualized with DIC optics with a Nikon E600 microscope. Images were captured with NIH ImageJ version 1.32j software with a DAGE MTI CCD72 camera coupled to a Scion LG-3 frame grabber.(L and M) Random cell wall positions observed in pok1-1;pok2-1 3 μm sections of globular embryos. Embryos were fixed, embedded in methacrylate resin, and sectioned as described previously [24Frank M.J. Cartwright H.N. Smith L.G. Three Brick genes have distinct function in a common pathway promoting polarized cell division and cell morphogenesis in maize leaf epidermis.Development. 2003; 130: 753-762Crossref PubMed Scopus (73) Google Scholar]. Images were acquired as described above. Scale bars equal 10 μm (A–D); 25 μm (G–I); 50 μm (J and K); 10 μm (L and M).View Large Image Figure ViewerDownload Hi-res image Download (PPT) (A–C) Propidium iodide-stained root meristems: (A) wild-type (Columbia), (B) pok1-2;pok2-2, (C) pok1-1;pok2-1. Arrowheads point to examples of misoriented cell walls. Fluorescent samples were illuminated with a 488 nM line form an argon laser and visualized with a Nikon TE-200U microscope equipped with a 100× 1.4 NA water immersion objective. Images were acquired with a Yokogawa Nipkow spinning disc confocal head, a Chroma HQ610/75 band pass emission filter, and a Coolsnap HQ cooled CCD camera controlled by MetaMorph software (Universal Imaging Corporation, Downing, PA). (D–K) Cleared embryos viewed with DIC optics: (D–F) octant stage embryos, (G–I) globular stage embryos, (J and K) mature embryos; (D, G, J) wild-type (Columbia), (E, H, K) pok1-2;pok2-2, (F, I) pok1-1;pok2-1. Arrowheads mark boundaries of cell walls. Siliques at various stages after fertilization were fixed in methanol/acetic acid (3:1). Embryos were dissected out and cleared in chloralhydrate/water (5 g/2 ml) visualized with DIC optics with a Nikon E600 microscope. Images were captured with NIH ImageJ version 1.32j software with a DAGE MTI CCD72 camera coupled to a Scion LG-3 frame grabber. (L and M) Random cell wall positions observed in pok1-1;pok2-1 3 μm sections of globular embryos. Embryos were fixed, embedded in methacrylate resin, and sectioned as described previously [24Frank M.J. Cartwright H.N. Smith L.G. Three Brick genes have distinct function in a common pathway promoting polarized cell division and cell morphogenesis in maize leaf epidermis.Development. 2003; 130: 753-762Crossref PubMed Scopus (73) Google Scholar]. Images were acquired as described above. Scale bars equal 10 μm (A–D); 25 μm (G–I); 50 μm (J and K); 10 μm (L and M). To investigate the basis of cell wall placement defects in pok1;pok2 double mutants, we immunolabeled microtubules to visualize mitotic microtubule arrays. Whole-mount staining of root meristems revealed that all mitotic arrays were present in double mutants (Figures 5D–5F) as in wild-type (Figures 5A–5C). Although both straight and curved phragmoplasts could be observed in these whole mounts, the orientations of most mitotic arrays was obscure due to irregular cell shapes and orientations. To overcome this limitation, root squashes were prepared for immunolocalization of microtubules (Figures 5G–5L). In pok1-1;pok2-1 roots, 79% of PPBs were perpendicular to the longitudinal cell axis (Figure 5H), while 21% were tilted (neither parallel nor perpendicular to the long axis, Figure 5I, n = 34); only 4% of wild-type PPBs were tilted (n = 44, Figure 5G). All wild-type phragmoplasts analyzed (n = 26) were positioned parallel or perpendicular to the longitudinal cell axis (Figure 5J). In pok1-1;pok2-1 double mutants, 60% of the phragmoplasts analyzed (n = 32) were tilted from the expected position (e.g., Figure 5L), while 40% were transverse or parallel to the longitudinal cell axis (e.g., Figure 5K). Since the frequency of transverse phragmoplasts in pok1-1;pok2-1 double mutant cells (40%) is greatly reduced compared to that of transverse PPBs (79%), these results indicate that, as in maize tan1 mutants, misplacement of cell walls in pok1;pok2 double mutants can be attributed largely to a failure of phragmoplast guidance to the site previously occupied by the PPB. In this study, we identified two previously unstudied kinesins in A. thaliana, POK1 and POK2, and showed that they are functionally redundant proteins involved in the spatial control of cytokinesis. The mechanisms leading to properly oriented cell division in plants are poorly understood, and there are many possible roles for kinesins in this process. Proper orientation of cell division planes involves an interaction between the expanding phragmoplast and the cortical division site, an interaction that appears to be defective in pok1;pok2 double mutants. A dynamic population of endoplasmic microtubules links the dividing nucleus to the cell cortex throughout mitosis and cytokinesis [20Dhonukshe P. Mathur J. Hulskamp M. Gadella T.W.J. Microtubule plus-ends reveal essential links between intracellular polarization and localized modulation of endocytosis during division-plane establishment in plant cells.BMC Biol. 2005; 3: 11Crossref PubMed Scopus (97) Google Scholar, 21Wick S.M. Immunofluorescence microscopy of tubulin and microtubule arrays in plant cells. III. Transition between mitotic/cytokinetic and interphase microtubule arrays.Cell Biol. Int. Rep. 1985; 9: 357-371Crossref PubMed Scopus (79) Google Scholar], suggesting the possibility that POK1/2 could mediate microtubule-dependent interactions between the dividing nucleus and the cortex that are important for phragmoplast guidance. Maize homologs of POK1 and POK2 were initially identified as potential binding partners of TAN1 in a yeast two-hybrid screen, and the COOH terminus of POK1 (containing the putative cargo binding domain) interacts with the A. thaliana homolog of TAN1, ATN. Combined with our finding that the pok1;pok2 double mutant phenotype in A. thaliana closely resembles the tan1 mutant phenotype in maize, these findings suggest that POK1 and POK2 directly interact with ATN to achieve their functions in cytokinesis. One hypothesis is that POK1/2 transports ATN to its site(s) of action, moving ATN along microtubules within mitotic arrays or along endoplasmic microtubules linking the dividing nucleus to the cell cortex. Further work will be required to elucidate the roles of POK1 and POK2 in the spatial control of cytokinesis and their relationship to ATN. We thank Amanda Overton and Andreas Chandra for help with genotyping. We are also grateful to former and present members of the Smith lab for stimulating discussions. T-DNA insertion lines were obtained from the ABRC. This work was supported by NIH grant GM53137 to L.G.S. and a fellowship from the Austrian Science Fund to S.M. (project J2241-B04). Download .pdf (.14 MB) Help with pdf files Document S1. Three Figures and Three Tables The GenBank accession numbers for full-length cDNA sequences are DQ399529 (POK1) and DQ399530 (POK2)." @default.
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• W1987173208 title "Two Kinesins Are Involved in the Spatial Control of Cytokinesis in Arabidopsis thaliana" @default.
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