The tumor-sensitive calmodulin-like protein is a specific light chain of human unconventional myosin X

Human calmodulin-like protein (CLP) is an epithelial-specific Ca(2+)-binding protein whose expression is strongly down-regulated in cancers. Like Calmodulin, CLP is thought to regulate cellular processes via Ca(2+)-dependent interactions with specific target proteins. Using gel overlays, we identified a approximately 210-kDa protein binding specifically and in a Ca(2+)-dependent manner to CLP, but not to Calmodulin. Yeast two-hybrid screening yielded a CLP-interacting clone encoding the three LIGHT chain binding IQ motifs of human "unconventional" Myosin X. Pull-down experiments showed CLP binding to the IQ domain to be direct and Ca(2+)-dependent. CLP interacted strongly with IQ motif 3 (K(d) approximately 0.5 nm) as determined by surface plasmon resonance. Epitope-tagged Myosin X was localized preferentially at the cell periphery in MCF-7 cells, and CLP colocalized with Myosin X in these cells. Myosin X was able to coprecipitate CLP and, to a lesser extent, Calmodulin from transfected COS-1 cells, indicating that CLP is a specific LIGHT chain of Myosin X in vivo. Because unconventional myosins participate in cellular processes ranging from membrane trafficking to signaling and cell motility, Myosin X is an attractive CLP target. Altered Myosin X regulation in (tumor) cells lacking CLP may have as yet unknown consequences for cell growth and differentiation.