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 Ca²⁺-dependent interactions with specific target proteins. Using gel overlays, we identified a ∼-210-kDa protein binding specifically and in a Ca²⁺-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 (Kd ∼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.