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KIC, a Novel Ca²⁺ Binding Protein with One EF-Hand Motif, Interacts with a Microtubule Motor Protein and Regulates Trichome Morphogenesis

Department of Biology and Program in Cell and Molecular Biology, Colorado State University, Fort Collins, Colorado 80523

³ To whom correspondence should be addressed. E-mail reddy{at}colostate.edu; fax 970-491-0649

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 Ca²⁺ binding protein (KIC). KIC, with its single EF-hand motif, bound Ca²⁺ 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 Ca²⁺-dependent manner. Interestingly, although both Ca²⁺-KIC and Ca²⁺-calmodulin were able to interact with KCBP and inhibit its microtubule binding activity, the concentration of Ca²⁺ required to inhibit the microtubule-stimulated ATPase activity of KCBP by KIC was threefold less than that required for calmodulin. Two KIC-related Ca²⁺ binding proteins and a centrin from Arabidopsis, which contain one and four EF-hand motifs, respectively, bound Ca²⁺ but did not affect microtubule binding and microtubule-stimulated ATPase activities of KCBP, indicating the specificity of Ca²⁺ 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 Ca²⁺ and regulates trichome morphogenesis.

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