title: Self-association of a high molecular weight subfragment-2 of myosin induced by divalent metal ions.
abstract: The effect of divalent cations on the self-association of high molecular weight subfragment-2 (long S-2) and low molecular weight subfragment-2 (short S-2) of rabbit skeletal muscle myosin has been investigated. In the presence of millimolar concentrations of Ca2+ or Mg2+ long S-2 associates at neutral pH to form ordered, high molecular weight aggregates whereas short S-2 does not associate. The association process is co-operative and results from binding two to four divalent cations within the light meromyosin-heavy meromyosin (LMM-HMM) hinge region of long S-2. Optical diffraction of electron micrographs of the long S-2 aggregates revealed several periodicities including reflections near 143 A. High molecular weight HMM showed a similar divalent metal induced self-association. Chymotryptic digestion studies of rod filaments reveal that cleavage within the LMM-HMM hinge is also strongly dependent on the presence of divalent cations. At pH 8, in the absence of divalent cations, the S-2 region appears to be displaced away from the filament backbone resulting in rapid proteolysis in the hinge domain. At high cation concentrations (greater than 10 mM) proteolytic cleavage is suppressed. A similar depression of the (substantially lower) hinge cleavage rate was also observed at neutral pH following addition of these divalent metal ions. Results suggest that binding of Mg2+ within the hinge domain under physiological conditions may act to lock the cross-bridge onto the thick filament surface in its resting-state orientation.
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology; D002918:Chymotrypsin; D004591:Electrophoresis, Polyacrylamide Gel; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology; D008970:Molecular Weight; D009132:Muscles / Q000032:analysis; D015879:Myosin Subfragments / Q000378:metabolism; D009218:Myosins / Q000378:metabolism*; D009994:Osmolar Concentration; D010446:Peptide Fragments / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits
chemical_list: D002413:Cations, Divalent; D046911:Macromolecular Substances; D015879:Myosin Subfragments; D010446:Peptide Fragments; D002918:Chymotrypsin; D009218:Myosins; D008274:Magnesium; D002118:Calcium