title: The 110-kD protein-calmodulin complex of the intestinal microvillus is an actin-activated MgATPase.
abstract: The microvillus 110-kD protein-calmodulin complex (designated 110K-CM) shares several properties with all myosins. In addition to its well-defined ATP-dependent binding interaction with F-actin, 110K-CM is an ATPase with diagnostically myosin-like divalent cation sensitivity. It exhibits maximum enzymatic activity in the presence of K+ and EDTA (0.24 mumol P1/mg per min) or in the presence of Ca++ (0.40 mumol P1/mg per min) and significantly less activity in physiological ionic conditions of salt and Mg++ (0.04 mumol P1/mg per min). This MgATPase is activated by F-actin in an actin concentration-dependent manner (up to 2.5-3.5-fold). The specific MgATPase activity of 110K-CM is also enhanced by the addition of 5-10 microM Ca++, but in the isolated complex, there is often also a decrease in the extent of actin activation in this range of free Ca++. Actin activation is maintained, however, in samples with exogenously added calmodulin; under these conditions, there is an approximately sevenfold stimulation of 110K-CM's enzymatic activity in the presence of 5-10 microM Ca++ and actin. 110K-CM is relatively indiscriminant in its nucleoside triphosphate specificity; in addition to ATP, GTP, CTP, UTP, and ITP are all hydrolyzed by the complex in the presence of either Mg++ or Ca++. Neither AMP nor the phosphatase substrate p-nitrophenyl phosphate are substrates for the enzymatic activity. The pH optimum for CaATPase activity is 6.0-7.5; maximum actin activation of MgATPase occurs over a broad pH range of 6.5-8.5. Finally, like myosins, purified 110K-CM crosslinks actin filaments into loosely ordered aggregates in the absence of ATP. Collectively these data support the proposal of Collins and Borysenko (1984, J. Biol. Chem., 259:14128-14135) that the 110K-CM complex is functionally analogous to the mechanoenzyme myosin.
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism / Q000648:ultrastructure; D000199:Actins / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000302:isolation & purification / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002147:Calmodulin / Q000032:analysis; D002645:Chickens; D004789:Enzyme Activation; D007413:Intestinal Mucosa / Q000378:metabolism*; D007422:Intestines / Q000648:ultrastructure; D008274:Magnesium / Q000378:metabolism; D008871:Microvilli / Q000378:metabolism; D011188:Potassium / Q000378:metabolism
chemical_list: D000199:Actins; D002147:Calmodulin; D000255:Adenosine Triphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D008274:Magnesium; D011188:Potassium; D002118:Calcium