Interaction of Arabidopsis kinesin-like calmodulin binding protein with tubulin subunits:: modulation by Ca2+-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 Ca2+-calmodulin. The results show that the carboxyterminal 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 Ca2+-calmodulin the motor with calmodulin-binding domain does not bind to tubulin. This Ca2+-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 Ca2+-calmodulin. These results indicate that Ca2+-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.