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Regulatory light chains and scallop myosin full dissociation reversibility and cooperative effects



Regulatory light chains and scallop myosin full dissociation reversibility and cooperative effects



Journal of Molecular Biology 138(3): 473-492



Both regulatory light-chains of scallop myosin are removed from myofibrils by a short EDTA treatment at elevated temperatures (25.degree. C in the case of Placopecten magellanicus; 35.degree. C in the case of Aequipecten irradians). Essential light-chains are not lost by this treatment. Denuded myofibrils readily recombine with regulatory light-chains in the presence of Mg ions up to a stoichiometry of 2 mol of regulatory light-chains/mol of myosin. High-affinity, specific Ca binding sites are proportional to regulatory light-chain content and are lost upon complete removal of regulatory light-chains. Denuded myofibrils contain 2 non-specific divalent cation binding sites of lower affinity/myosin that are not found on intact myofibrils. Occupancy of these sites by Ca depresses the actin-activated Mg2+-ATPase activity by about 20-30%. Regulatory light-chains specifically modify the actin-activated Mg2+-ATPase of scallop myofibrils, inhibit the ATPase 10-20-fold in the absence of Ca, but increase the turnover rate by 2-4 fold if Ca is present. The elevated Mg2+-ATPase of denuded myofibrils in the absence of Ca is restored biphasically by regulatory light-chains to the low rates typical of intact myofibrils. This ATPase rate remains constant until 1 mol of regulatory light-chain is added back, then is depressed linearly upon further re-addition of regulatory light-chains. This ATPase dependency implies evidence that both regulatory light-chains must be attached to intact scallop myosin for regulation, and suggests that regulatory light-chains re-add in a negatively co-operative manner.

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Accession: 006292144

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PMID: 6251228



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