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

Microtubules (MTs), one of the three cytoskeletal structures in all eukaryotes, play key roles in many fundamental cellular and developmental processes such as cell growth, cell shape, cell division, cell differentiation and intracellular transport. MTs are cylindrical polar polymers made up of repeating aand b-tubulin. MTs undergo alternating phases of polymerization and depolymerization. Like individual MTs, arrays of MTs are highly dynamic in that they can form, breakdown and reform. In plants interphase MTs are a mainly parallel array found beneath the plasma membrane and function in cell shape and growth processes. MTs also function in intracellular transport and separation of chromosomes in cell division. During cytokinesis, MTs are part of the phragmoplast that is responsible for the formation of a new cell plate between daughter cells. MTs together with actin filaments/ microfilaments (MFs) form two distinct cytoskeletal networks that are independent but can coordinate activity in such processes as mitotic/meiotic spindle formation, cytokinesis and movement of cargo. Molecular motors use the MTs and MFs in regulating numerous cellular and developmental processes. There are three families of motors in eukaryotes: the myosins, the dyneins, and the kinesins. All three motors use energy released from ATP hydrolysis to move along either the MTs or MFs. The MT-associated motors are dyneins and kinesins. Dynein is a microtubuledependent, minus end-directed motor that is required for intraflagellar transport, is involved in organizing the Golgi apparatus and spindle poles, and has roles in moving nuclei, vesicles, pigment granules, and chromosomes [21, 27]. A comparative analysis of 24 diverse eukaryotic genomes confirmed that dyneins are not represented in higher plant genomes [77]. This chapter focuses on various aspects of kinesins in photosynthetic eukaryotes. Myosins, the actin-based motor proteins, are discussed in detail in Sect. 1, Chap. 3.