Microtubule Motor Proteins in the Eukaryotic Green Lineage: Functions and Regulation

Abstract

Kinesins and dyneins are two superfamilies of microtubule motor proteins that regulate many diverse fundamental cellular and developmental processes including cell shape, cell division and intracellular transport as well as spatial and temporal organization of molecules and organelles within the eukaryotic cells. These motor proteins use chemical energy from ATP to move unidirectionally using microtubules as roadways or to regulate microtubule dynamics. This review focuses on a comparative analysis of kinesins in eukaryotes, especially in the green lineage and their roles in plants. Comprehensive comparative analysis of kinesins among completed genome sequences of animal and several photosynthetic eukaryotes ranging from algae to monocots revealed considerable expansion of kinesins in flowering plants. Much of this expansion is due to an increase in members of two families (Kinesin-7 and Kinesin-14). Of the fourteen recognized families of kinesins in eukaryotes, members of four families are not found in flowering plants. However, a group of plant-specific kinesins does not fall into any of the recognized families, and some plant kinesins form plant-specific clades inside of their respective families. Some known domains are found exclusively either in plant and animal lineages, suggesting their functional specialization. Arabidopsis has the highest number of kinesins of any known multicellular eukaryotes, including humans, with a total of 61 kinesins. Although the processes regulated by many plant kinesins are yet to be discovered, functions of some kinesins have been elucidated in recent years using cell biological, molecular and genetic approaches and these are discussed briefly here. In addition, insights into regulatory mechanisms of a unique plant Ca2+/CaM-interacting motor called kinesin-like calmodulin-binding protein (KCBP) obtained through biochemical assays and crystal structure studies of its motor domain alone and as a complex with a calcium-binding protein are presented.