title: Calponin inhibits actin-activated MgATPase of myosin subfragment 1 (S1) without displacing S1 from its binding site on actin.
abstract: Calponin is a smooth-muscle thin-filament protein implicated in the regulation of contraction. Its binding to actin is a prerequisite for inhibition of actin-activated myosin MgATPase. Investigating the molecular mechanism of this inhibition, it was found that titration of acto-myosin subfragment 1 with calponin in the presence of either ADP or ATP does not displace weakly or strongly bound myosin subfragment 1 (S1) from actin. S1.ADP, however, is able to release about two-thirds of the calponin from saturated (equimolar) complexes of actin-calponin. The remaining calponin is sufficient for almost full inhibition of acto-S1 MgATPase activity. Bunding of actin filaments by calponin takes place at a higher ratio calponin/actin (above 1:3) and, therefore, is not responsible for inhibition of the ATPase. Bundle formation is inhibited by S1.ADP. These results suggest the existence of two calponin-binding sites on actin; one, that is insensitive to S1, which is responsible for inhibition of the ATPase, the other, from which calponin is readily displaced by S1.
authors.affiliation: Department of Muscle Biochemistry, Nencki Institute of Experimental Biology, Warsaw, Poland.
authors.forename: J
authors.initials: J
authors.lastname: Kołakowski
authors.forename: A
authors.initials: A
authors.lastname: Karkucińska
authors.forename: R
authors.initials: R
authors.lastname: Dabrowska
mesh_terms: D000199:Actins / Q000378:metabolism / Q000494:pharmacology*; D000244:Adenosine Diphosphate / Q000502:physiology; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase / Q000037:antagonists & inhibitors*; D002135:Calcium-Binding Proteins / Q000494:pharmacology* / Q000502:physiology; D002645:Chickens; D004789:Enzyme Activation / Q000187:drug effects; D008840:Microfilament Proteins; D015879:Myosin Subfragments / Q000037:antagonists & inhibitors* / Q000378:metabolism; D009218:Myosins / Q000037:antagonists & inhibitors*; D011817:Rabbits; D000096985:Calponins
chemical_list: D000199:Actins; D000244:Adenosine Diphosphate; D017301:Ca(2+) Mg(2+)-ATPase; D002135:Calcium-Binding Proteins; D008840:Microfilament Proteins; D015879:Myosin Subfragments; D009218:Myosins; D000096985:Calponins