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 Ca(2+)-dependent interaction with actin is completely reversible. The Ca2+ binding properties of MCP and its Ca(2+)-dependent functions were studied quantitatively. MCP undergoes a Ca(2+)-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 (KD = 37 microM). Binding of MCP to actin monomers requires a similar Ca2+ concentration ([Ca2+]0.5 = 62 microM) 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 microM Ca2+. The marked difference in the Ca2+ sensitivity of these two functions indicate that MCP's primary actin regulatory role in the macrophage is likely to be capping of the barbed ends of actin filaments.
Young
C L
CL
Department of Medicine, College of Medicine, University of Florida, Gainesville 32610-0277.
Feierstein
A
A
Southwick
F S
FS
D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism*; D018260:Gelsolin / Q000737:chemistry / Q000378:metabolism*; D016676:Macrophages, Alveolar / Q000378:metabolism; D008840:Microfilament Proteins; D008969:Molecular Sequence Data; D009687:Nuclear Proteins; D011487:Protein Conformation; D011817:Rabbits; D017386:Sequence Homology, Amino Acid
D000199:Actins; D018260:Gelsolin; D008840:Microfilament Proteins; D009687:Nuclear Proteins; D002118:Calcium