IN-VITRO MOTILITY OF ATKCBP, A CALMODULIN-BINDING KINESIN PROTEIN OF ARABIDOPSIS

AtKCBP is a calcium-dependent calmodulin-binding protein from Arabidop sis that contains a conserved kinesin microtubule motor domain, Calmod ulin has been shown previously to bind to heavy chains of the unconven tional myosins, where it is required for in vitro motility of brush bo rder myosin 1, but AtKCBP is the first kinesin-related heavy chain rep orted to be capable of binding specifically to calmodulin. Other kines in proteins have been identified in Arabidopsis, but none of these bin ds to calmodulin, and none has been demonstrated to be a microtubule m otor. We have tested bacterially expressed AtKCBP for the ability to b ind microtubules to a glass surface and induce gliding of microtubules across the glass surface. We find that AtKCBP is a microtubule motor protein that moves on microtubules toward the minus ends, with the opp osite polarity as kinesin, In the presence of calcium and calmodulin, AtKCBP no longer binds microtubules to the coverslip surface, This con trasts strikingly with the requirement of calmodulin for in vitro moti lity of brush border myosin I. Calmodulin could regulate AtKCBP bindin g to microtubules in the cell by inhibiting the binding of the motor t o microtubules. The ability to bind to calmodulin provides an evolutio nary link between the kinesin and myosin motor proteins, but our resul ts indicate that the mechanisms of interaction and regulation of kines in and myosin heavy chains by calmodulin are likely to differ signific antly.