title: Removal of tropomyosin overlap modifies cooperative binding of myosin S-1 to reconstituted thin filaments of rabbit striated muscle.
abstract: Cooperative binding of myosin S-1.ADP to regulated F-actin was previously reported and has been interpreted by a two-state model in which an important source of cooperativity is nearest neighbor interactions between the 7-actin.tropomyosin (TM).troponin units (functional units) (Hill, T.L., Eisenberg, E., and Greene, L. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 3186-3190). It has been postulated that the head-to-tail overlap between adjacent TM molecules is the structural basis of the nearest neighbor interactions. We tested the hypothesis by examining S-1.ADP binding to reconstituted regulated F-actin containing either intact TM or nonpolymerizable TM from which the COOH-terminal 11 residues were removed. In the absence of Ca2+, substitution of nonpolymerizable TM for TM reduced significantly the slope of the steeply rising phase of the sigmoidal S-1.ADP binding curve. Nevertheless, considerable residual cooperativity remained. Analysis of the data using the two-state model of Hill et al. suggests that removal of TM overlap abolishes nearest neighbor interactions, while the concerted change of the state of 7 actins in a functional unit can account for the residual cooperativity.
authors.affiliation: Department of Physiology and Biophysics, University of Washington, Seattle 98195.
authors.forename: B S
authors.initials: BS
authors.lastname: Pan
authors.forename: A M
authors.initials: AM
authors.lastname: Gordon
authors.forename: Z X
authors.initials: ZX
authors.lastname: Luo
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism*; D000199:Actins / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology; D003599:Cytoskeleton / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D009132:Muscles / Q000378:metabolism*; D015879:Myosin Subfragments; D009218:Myosins / Q000378:metabolism*; D010446:Peptide Fragments / Q000378:metabolism*; D011485:Protein Binding; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism*
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D010446:Peptide Fragments; D014335:Tropomyosin; D009218:Myosins; D002118:Calcium