KIC, a Novel Ca^2+ Binding Protein with One EF-Hand Motif, Interacts with a Microtubule Motor Protein and Regulates Trichome Morphogenesis

Kinesin-like calmodulin binding protein (KCBP) is a microtubule motor protein involved in the regulation of cell division and trichome morphogenesis. Genetic studies have shown that KCBP is likely to interact with several other proteins. To identify KCBP-interacting proteins, we used the C-terminal region of KCBP in a yeast two-hybrid screen. This screening resulted in the isolation of a novel KCBP-interacting Ca^2+ binding protein (KIC). KIC, with its single EF-hand motif, bound Ca^2+ at a physiological concentration. Coprecipitation with bacterially expressed protein and native KCBP, gel-mobility shift studies, and ATPase assays with the KCBP motor confirmed that KIC interacts with KCBP in a Ca^2+-dependent manner. Interestingly, although both Ca^2+-KIC and Ca^2+-calmodulin were able to interact with KCBP and inhibit its microtubule binding activity, the concentration of Ca^2+ required to inhibit the microtubule-stimulated ATPase activity of KCBP by KIC was threefold less than that required for calmodulin. Two KIC-related Ca^2+ binding proteins and a centrin from Arabidopsis, which contain one and four EF-hand motifs, respectively, bound Ca^2+ but did not affect microtubule binding and microtubule-stimulated ATPase activities of KCBP, indicating the specificity of Ca^2+ sensors in regulating their targets. Overexpression of KIC in Arabidopsis resulted in trichomes with reduced branch number resembling the zwichel/kcbp phenotype. These results suggest that KIC modulates the activity of KCBP in response to changes in cytosolic Ca^2+ and regulates trichome morphogenesis.