Kinesin and ncd bind through a single head to microtubules and compete for a shared MT binding site.

Kinesin and non claret disjunctional are closely related molecular motors that move in opposite directions along microtubules. We have used recombinant single-headed and double-headed constructs of both rat kinesin heavy chain and non claret disjunctional to investigate the interactions of these motor proteins with microtubules. At saturation the stoichiometry of binding for non claret disjunctional and kinesin to microtubules is one molecule (single or double-headed) per tubulin heterodimer. In the absence of added nucleotide, addition of increasing amounts of one motor results in the competitive displacement of the other motor from the microtubules. This effect is apparent also in the presence of the nucleotide analogue 5'-adenylimidodiphosphate, which tightens the binding of both kinesin and non claret disjunctional. Competition for binding sites occurs also under conditions of steady-state ATP turnover. We conclude that despite their opposite directionality, kinesin and non claret disjunctional compete for overlapping binding sites on the MT surface. Since the binding of the second head of a double-headed motor is sterically blocked, the data imply also that both kinesin and non claret disjunctional may translocate via a processive (alternating heads) mechanism with a minimum step size of approximately 8 nm.