Ca2+/calmodulin regulation of the Arabidopsis kinesin-like calmodulin-binding protein.

The kinesin family motor protein KCBP (kinesin-like calmodulin binding protein) was identified during a screen for Arabidopsis calmodulin-binding proteins [Reddy, et al., 1996b: J. Biol Chem. 271:7052-7060]. KCBP contains a C-terminal motor domain and is unique among kinesin motors in that it has a calmodulin-binding site. We expressed the KCBP motor domain in Escherichia coli and examined its microtubule (MT) binding and ATPase activity. KCBP bound MTs in an ATP-dependent manner and exhibited MT-stimulated ATPase activity. Ca2+/ calmodulin inhibited binding of KCBP to MTs under conditions that normally favor tight motor-MT interactions, and the extent of inhibition was dependent on the concentration of calcium and calmodulin. Ca2+/calmodulin did not affect KCBP's basal ATPase activity, but reduced the motor's MT-stimulated ATPase activity. The substantial reduction in affinity of KCBP for MTs in the presence of Ca2+/calmodulin suggests that Ca2+/calmodulin may modulate the activity of KCBP in vivo by regulating the motor's association with MTs. KCBP is the first MT-dependent motor protein found to be regulated by direct binding of Ca2+/calmodulin to its motor subunit.