title: The myosin cardiac loop participates functionally in the actomyosin interaction.
abstract: The motor protein myosin in association with actin transduces chemical free energy in ATP into work in the form of actin translation against an opposing force. Mediating the actomyosin interaction in myosin is an actin binding site distributed among several peptides on the myosin surface including surface loops contributing to affinity and actin regulation of myosin ATPase. A structured surface loop on beta-cardiac myosin, the cardiac or C-loop, was recently demonstrated to affect myosin ATPase and was indirectly implicated in the actomyosin interaction. The C-loop is a conserved feature of all myosin isoforms with crystal structures, suggesting that it is an essential part of the core energy transduction machinery. It is shown here that proteolytic digestion of the C-loop in beta-cardiac myosin eliminates actin-activated myosin ATPase and reduces actomyosin affinity in rigor more than 100-fold. Studies of C-loop function in smooth muscle myosin were also undertaken using site-directed mutagenesis. Mutagenesis of a single charged residue in the C-loop of smooth muscle myosin alters actomyosin affinity and doubles myosin in vitro motility and actin-activated ATPase velocities, thereby involving a charged region of the loop in the actomyosin interaction. It appears likely that the C-loop is an essential electrostatic binding site for actin involved in modulation of actomyosin affinity and regulation of actomyosin ATPase velocity.
authors.affiliation: Department of Biochemistry and Molecular Biology, Mayo Clinic Rochester, Rochester, Minnesota 55905, USA.
authors.forename: Katalin
authors.initials: K
authors.lastname: Ajtai
authors.forename: Susanna P
authors.initials: SP
authors.lastname: Garamszegi
authors.forename: Shinya
authors.initials: S
authors.lastname: Watanabe
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Ikebe
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Burghardt
mesh_terms: D000205:Actomyosin / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D002417:Cattle; D006868:Hydrolysis; D008958:Models, Molecular; D020409:Molecular Motor Proteins; D018482:Muscle, Skeletal / Q000378:metabolism; D009154:Mutation; D009206:Myocardium / Q000378:metabolism; D009218:Myosins / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011817:Rabbits
chemical_list: D020409:Molecular Motor Proteins; D000205:Actomyosin; D009218:Myosins