Content area
Full Text
Unc104/KIF1A belongs to a class of monomeric kinesin motors that have been
thought to possess an unusual motility mechanism. Unlike the unidirectional motion driven by the coordinated actions of the two heads in conventional kinesins, single-headed KIF1A was reported to undergo biased diffusional motion along microtubules. Here, we show that Unc104/KIF1A can dimerize and move unidirectionally and processively with rapid velocities characteristic of transport in living cells. These results suggest that Unc104/KIF1A operates in vivo by a mechanism similar to conventional kinesin and that regulation of motor dimerization may be used to control transport by this class of kinesins.
Caenorhabditis elegans Unc104 and the mouse ortholog KIF1A are kinesin motors that transport synaptic vesicle precursors along microtubules from the neuronal cell body to the nerve terminal (1-3). For such long-range transport to be efficient, organelles that encounter a microtubule must move processively. Conventional kinesin, which belongs to a different subfamily of vesicle-transporting kinesins, is dimeric and uses its two motor domains in a coordinated manner to take successive, unidirectional 8-nm steps along the microtubule without dissociating (4). However, KIF1A (2) and Unc 104 (5) are monomeric in solution and are thought to operate using a different motility mechanism, because a single KIF1A motor domain has been shown to undergo biased diffusional movement along the microtubule (6). A novel processivity mechanism was proposed that involves an electrostatic interaction between a highly basic loop (the K loop) on the motor domain and the disordered, negatively charged COOH-terminus of tubulin (7, 8); a nucleotide-induced rotation of the motor domain is believed to create directional bias (9). However, the observed net motion (0.14 (mu)m/s) (6) is ~eight times slower than that observed for fulllength Unc 104/KIF1A-induced movement in vivo (3) and in vitro (2). In addition, biased diffusion was not observed for the C. elegans Unc104 motor domain (5) and may be an artifact of particular motor constructs and buffer condition (10). Hence, the normal mode of motility for this kinesin class [often referred to as the "monomeric" kinesins (11)] has not been clear (12).
Sequence analysis of Unc 104/KIF1A (and other members of this kinesin class) revealed potential coiled-coil regions adjacent to the motor domain, the first of which aligns with the neck coiled-coil of conventional...