Cytoskeletal Motors in Arabidopsis. Sixty-One Kinesins and Seventeen Myosins

Abstract

Cytoskeletal motor proteins are ATPases that use the energy released from ATP hydrolysis to move along the cytoskeletal elements of microtubules and actin microfilaments. Found among all eukaryotic organisms, kinesins are microtubule-based motor proteins with a conserved kinesin motor domain, and myosins are actin microfilament-based motor proteins with a conserved myosin motor domain. Cytoskeletal motor proteins directly contribute to the organization of various cytoskeletal arrays during cell division and cell growth in plant tissues. They are also responsible for the motility of molecules and organelles, and the segregation of genetic materials during mitosis and meiosis. In the genome of the model plant Arabidopsis (Arabidopsis thaliana), there are at least 61 genes encoding kinesins and 17 genes encoding myosins. Most Arabidopsis kinesins and all myosins are evolutionarily divergent from their counterparts in animals and fungi. Little is known about the functions of most plant kinesins and myosins. Arabidopsis kinesins form a number of subfamilies. The mitotic kinesins in the BIMC/Kinesin-5 and the NCD/Kinesin-14 subfamilies appear to be similar to those in fungi and animals. Others, however, are very divergent, as their nonmotor sequences are unique to plants. Some of Arabidopsis kinesins are associated with microtubules, mitochondria, Golgi stacks, and vesicles. They affect microtubule organization, organelle distribution, and vesicle transport, respectively. Ultimately, Arabidopsis kinesins contribute directly or indirectly to cell division and cell growth in various tissues. Arabidopsis myosins are classified into two subfamilies: class VIII and class XI. The class XI myosins are associated with various organelles/vesicles. Functions of Arabidopsis myosins are still elusive. Future efforts will be devoted to deciphering not only the functions of these motors by molecular genetics but also the molecular mechanisms underlying how these roles are played.