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 microtubulestimulated 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.