548 Cell SnapShot: Motor Proteins in Spindle Assembly 134 Rose Loughlin, Blake Riggs, and Rebecca Heald , August 8,2008©2008 Elsevier Inc. DOI 10.1016/j.cell.2008.07.038 Molecular and Cell Biology Department, University of California, Berkeley, Berkeley, CA 94720, USA Motor-based mechanisms Molecular motors Localization Proposed role Inhibition phenotypes of spindle assembly (in red) MONOPOLAR SPINDLE + Kinesin-3 (+) Klp38B (fly) Connects spindle MTs to chromosome arms; Microtubule Kinesin-4 (+) promotes spindle stability (MT) MT depolymerization Xklp1 (frog), Klp3A (fly) Centrosome - + + - Kinesin-5 (+) Crosslinks MTs and + Motor movement Eg5 (frog, human) slides antiparallel HALF SPINDLE + Klp61F (fly) MTs outward Spindle bipolarity Kinesin-13 (i) Regulates MT dynamics Klp10A (fly) and spindle stability Kif2a (frog, LONGER SPINDLE Cell cortex + Chromosome mouse, human) Depolymerizes MTs at Kif2b (human) spindle poles - Kinesin-14 (-) Ncd (fly) Slides MTs poleward; - - XCTK2 (frog) exerts antagonizing force CHO2 (hamster) against Kinesin-5 LOSS OF CENTROSOMES SPLIT/SPLAYED POLES + HSET (human) formation Spindle pole + Slides MTs poleward; Dynein (-) generates cortical pulling (fly, frog, mouse, force human) Promotes kinetochore- KINETOCHORE MISALIGNMENT, Kinetochore MT attachment MISORIENTATION Kinesin-4 (+) Kinetochore fiber Klp3A (fly) Attaches chromosome (K-fiber) Xklp1 (frog) arms to spindle and slides Kinesin-10 (+) toward center Tubulin dimers Nod (fly), Kid (human) See online version for legend and references. + Kinesin-7 (+) Slides unattached MISALIGNED CHROMOSOME ARMS CENP-E (mouse, kinetochores along a + human) K-fiber toward spindle CENP-meta (fly) center Kinesin-8 (+, i) Dampens kinetochore + - Klp67A (fly) oscillations Kif18A (human) BENT SPINDLE Chromosome positioning Kinesin-13 (i) + Klp59C (fly) Depolymerizes MCAK (frog, kinetochore MTs human) (+) Plus end-directed (-) Minus end-directed (i) Internal motor domain SnapShot: Motor Proteins in Spindle Assembly Rose Loughlin, Blake Riggs, and Rebecca Heald Molecular and Cell Biology Department, University of California, Berkeley, Berkeley, CA 94720, USA Purpose of Spindle Assembly Proper segregation of chromosomes during cell division is orchestrated by the mitotic spindle, a dynamic assembly of polar microtubule (MT) polymers composed of tubulin subunits, as well as many associated factors. Microtubule-based molecular motors mediate crucial interactions among spindle components. They promote antiparallel organization of the MTs emanating from centrosomes, with MT minus ends (−) focused at the two spindle poles and MT plus ends (+) making dynamic attachments with chromosomes, which align in the center of the spindle at metaphase. Sister chromatids of each duplicated chromosome are held together along their length and attach at their kinetochores to bundles of spindle microtubules (K-fibers) that link them to opposite spindle poles. The metaphase bipolar spindle is thus poised to pull chromatids to opposite ends of the cell before division, ensuring that each daughter cell receives a complete genome. Motor Protein Functions Members of each motor protein family (column 2) contribute to at least one spindle assembly mechanism by generating force and/or controlling MT dynamics at specific regions in the spindle. The localization of motor proteins is shown in red (column 3). Motor protein families may function in more than one spindle assembly mechanism (and so may appear more than once in column 1), and there may be overlap in the phenotypes observed when the motors are inhibited (column 5). Through these molecular mo- tors, MTs can interact with the kinetochore, associate laterally with chromosomes, focus at the spindle pole, crosslink with antiparallel MTs from the opposite pole, or pull against the cell cortex. The proposed function of each motor family member is briefly described in column 4. Spindle Bipolarity Kinesin-3 and kinesin-4 (green) motors are plus end-directed and have an N-terminal motor domain and a putative DNA-binding C-terminal domain. They orient and slide MT minus ends away from the chromosomes, aiding interpolar MT contact and separating centrosomes. The kinesin-5 (orange) motor is a plus end-directed tetramer, with an N-terminal motor domain and a C-terminal coiled-coil tetramerization domain. Kinesin-5 is found throughout the spindle, where it bundles MTs, and crosslinks and slides antiparallel MTs outward toward the spindle poles. The kinesin-13 (blue) motor is a nonmotile dimer, with an internal motor domain. It limits MT growth by promoting MT disassembly at the kinetochore and at MT ends throughout the spindle. Spindle Pole Formation The kinesin-14 (red) motor is a minus end-directed dimer, with a C-terminal motor domain, an internal coiled-coil dimerization domain, and an MT-binding globular N-terminal domain. It transports one MT toward the minus end of an adjacent MT, antagonizing the outward sliding of kinesin-5 and clustering MT minus ends at the spindle pole. Cytoplasmic dynein (orange) is a minus end-directed dimer, with an N-terminal tail dimerization domain, an internal MT-binding stalk, and a C-terminal motor domain with six tandem ATPase domains. It transports both short MTs and molecules contributing to pole organization toward MT minus ends and the pole. Dynein also maintains attach- ment of the centrosome to the spindle pole. Kinetochore-associated dynein aids in chromosome capture and promotes proper kinetochore attachment and orientation. The kinesin-13 (blue) motor depolymerizes MTs sliding into the spindle pole to maintain pole coherence and spindle length. Chromosome Positioning The kinesin-7 (purple) motor is a plus end-directed dimer, with an N-terminal motor domain and an internal coiled-coil dimerization domain. It promotes the end-on con- nection between k-MTs and the kinetochore and promotes chromosome congression to the spindle center by moving unattached kinetochores along K-fibers of bioriented chromosomes. The kinesin-8 (yellow) motor is a plus end-directed dimer, with an N-terminal motor domain. It regulates K-fiber length and thus chromosome oscillations by accumulating at K-fiber plus ends and stimulating MT disassembly. The kinesin-13 (blue) motor destabilizes MT plus ends at the kinetochore in a highly regulated manner to control kinetochore positioning and microtubule attachment. The kinesin-4 and kinesin-10 (green) motors are plus end-directed, with an N-terminal motor domain and a C-terminal DNA-binding domain. They push chromosome arms to the center of the spindle, contributing to chromosome congression and chromosome arm alignment perpendicular to the pole-to-pole axis. Abbreviations Microtubule (MT), kinetochore-fiber (K-fiber), kinetochore-MT (k-MT), fruit fly Drosophila melanogaster, frog Xenopus laevis, human Homo sapiens, Chinese hamster Crice- tulus griseus, mouse Mus musculus. Kinesin-like protein (Klp), kinesin family (Kif), non-claret disjunction (Ncd), Xenopus C-terminal kinesin (XCTK), Chinese hamster ovary (CHO), human spleen embryo testes (HSET), centromere protein (CENP), mitotic centromere-associated kinesin (MCAK), no distributive disjunction (Nod), kinesin-like DNA- binding protein (Kid). ACKNOWLEDGMENTS We thank Claire Walczak for helpful comments. R.L. is supported by the National Science Foundation. B.R. and R.H. are supported by the National Institutes of Health. REFERENCES Brust-Mascher, I., and Scholey, J.M. (2007). Mitotic spindle dynamics in Drosophila. Int. Rev. Cytol. 259, 139–172. Gardner, M.K., Odde, D.J., and Bloom, K. (2008). Kinesin-8 molecular motors: putting the brakes on chromosome oscillations. Trends Cell Biol. 18, 307–310. Mazumdar, M., and Misteli, T. (2005). Chromokinesins: multitalented players in mitosis. Trends Cell Biol. 15, 349–355. 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