Kinesin-related proteins in plant cytokinesis

Cytokinesis in higher plant cells is mediated by the phragmoplast. The framework of the phragmoplast consists of two anti-parallel sets of microtubules with their plus ends facing each other at or near the division site. During cytokinesis, the phragmoplast microtubule array undergoes dynamic reorganization froze a solid cylinder to a hollow array. Concomitant with the microtubule reorganization, Golgi-originated vesicles are rapidly transported along microtubules toward their plus ends to give rise to the centrifugally growing cell plate. Kinesin-related motor proteins play crucial roles in microtubule reorganization and vesicle transport. To date, a number of plant proteins in the kinesin superfamily have been localized to the phragmoplast, and possibly exert roles in cytokinesis. Plus end-directed motors in the BIMC subfamily play a role in sliding anti-parallel microtubules apart to establish the phragmoplast microtubule array, while microtubule minus end-directed Ncd/Kar3-like motors in the C-terminal motor subfamily likely act antagonistically to balance the force generated by the BIMC-like kinesins. The novel C-terminal motor kinesin KCBP probably regulates microtubule organization and cross-linkage of the microtubule minus ends in a Ca++/calmodulin-dependent manner. By acting at or near the plus ends of the microtubules, the Arabidopsis phragmoplast-associated kinesin-related protein AtPAKRP1 likely plays a role in establishing and/or maintaining the organization of phragmoplast microtubules. We anticipate more phragmoplast-associated motors to be revealed in the next few years as the completed Arabidopsis genome contains more than 45 genes encoding kinesin-related proteins. Orchestrated forces generated by different motors are required for microtubule-dependent activities to take place in the phragmoplast.