title: Caldesmon inhibits the cooperative turning-on of the smooth muscle heavy meromyosin by tropomyosin-actin.
abstract: The 38-kDa chymotryptic fragment of caldesmon, which possesses the actin/calmodulin binding domain, was purified and utilized to study the mechanism for the inhibition of acto-myosin ATPase by caldesmon. The intact caldesmon inhibited the acto-HMM ATPase although it caused an increase in the binding of HMM to actin, presumably due to the interaction between the S-2 region of HMM and the caldesmon located on the actin filament. The 38-kDa fragment, which lacks the S-2 binding domain, inhibited both the acto-HMM ATPase and the HMM binding to actin. The ATPase and the HMM binding to actin decreased in parallel on increasing the 38-kDa fragment bound to actin. In the presence of tropomyosin, the ATPase activity fell more rapidly than did the HMM binding to actin. Binding of intact caldesmon or 38-kDa fragment to actin inhibited the cooperative turning-on of tropomyosin-actin by NEM.S-1, which forms rigor complexes in the presence of ATP. The absence of cooperative turning-on of the acto-HMM ATPase by rigor complexes in the presence of 38-kDa fragment was associated with an inhibition of the binding of HMM to tropomyosin-actin. Addition of NEM.S-1 to tropomyosin-actin-caldesmon caused a gradual decrease in the caldesmon-induced binding of HMM to actin. The calmodulin restored the caldesmon-induced binding of HMM to tropomyosin-actin, but it had only a slight effect on the acto-HMM ATPase. These data suggest that the cooperative turning-on of the smooth muscle tropomyosin-actin by rigor bonds is modulated by the interaction of caldesmon, tropomyosin, and calmodulin on the thin filament.
authors.affiliation: Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia 19104.
authors.forename: K Y
authors.initials: KY
authors.lastname: Horiuchi
authors.forename: S
authors.initials: S
authors.lastname: Chacko
mesh_terms: D000199:Actins / Q000378:metabolism; D000494:Allosteric Regulation; D000818:Animals; D002148:Calmodulin-Binding Proteins / Q000494:pharmacology*; D002645:Chickens; D002918:Chymotrypsin; D005895:Gizzard, Avian / Q000187:drug effects / Q000378:metabolism*; D009124:Muscle Proteins / Q000378:metabolism*; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D015879:Myosin Subfragments / Q000378:metabolism; D009218:Myosins / Q000037:antagonists & inhibitors*; D014335:Tropomyosin / Q000378:metabolism
chemical_list: D000199:Actins; D002148:Calmodulin-Binding Proteins; D009124:Muscle Proteins; D015879:Myosin Subfragments; D014335:Tropomyosin; D002918:Chymotrypsin; D009218:Myosins