Interaction of Arabidopsis kinesin-like calmodulin-binding protein with tubulin subunits: modulation by Ca(2+)-calmodulin.

Kinesin-like calmodulin-binding protein (KCBP) is a recently identified novel kinesin-like protein that appears to be unique to and ubiquitous in plants. KCBP is distinct from all other known KLPs in having a calmodulin-binding domain adjacent to its motor domain. We have used different regions of KCBP to study its interaction with tubulin subunits and the regulation of this interaction by Ca(2+)-calmodulin. The results show that the carboxy-terminal part of the KCBP, with or without calmodulin-binding domain, binds to tubulin subunits and this binding is sensitive to nucleotides. In the presence of Ca(2+)-calmodulin the motor with calmodulin-binding domain does not bind to tubulin. This Ca(2+)-calmodulin modulation is abolished in the presence of antibodies specific to the calmodulin-binding domain of KCBP. Similar binding studies with the carboxy-terminal part of KCBP lacking the calmodulin-binding domain show no effect of Ca(2+)-calmodulin. These results indicate that Ca(2+)-calmodulin modulates the interaction of KCBP with tubulin subunits and this modulation is due to the calmodulin-binding domain in the KCBP. Calcium-dependent calmodulin modulation of KCBP interaction with tubulin suggests regulation of KCBP function by calcium, the first such regulation of a kinesin heavy chain among all the known kinesin-like proteins.