CALCIUM REGULATION OF ACTIN FILAMENT CAPPING AND MONOMER BINDING BY MACROPHAGE CAPPING PROTEIN

Macrophage capping protein (MCP) is a unique member of the gelsolin-villin family of calcium-activated barbed end capping proteins which in micromolar Ca2+ also binds actin monomers and nucleates actin assembly. Unlike gelsolin, MCP cannot sever actin filaments, and its Ca2+ dependent interaction with actin is completely reversible. The Ca2+ binding properties of MCP and its Ca2+-dependent functions were studied quantitatively. MCP undergoes a Ca2+-induced conformational change as evidenced by different chymotryptic digest patterns in 0.2 mM CaCl2 compared with 2 mM EGTA. MCP has a single low affinity Ca2+ binding site (K-D = 37 mu M). Binding of MCP to actin monomers requires a similar Ca2+ concentration ([Ca2+](0.5) = 62 mu M) suggesting that MCP.Ca2+ complex formation promotes monomer binding. In contrast, filament capping by MCP requires 1/60th of the Ca2+ concentration required for monomer binding, half-maximal capping occurring at 1 mu M Ca2+. The marked difference in the Ca2+ sensitivity of these two functions indicate the MCP's primary actin regulatory role in the macrophage is likely to be capping of the barbed ends of actin filaments.