The interaction of skeletal myosin subfragment 1 with the polyanion, heparin

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Abstract
The association between chymotryptic skeletal muscle myosin subfragment 1 (S1) and the polyanion, heparin, was investigated as an experimental approach in probing the functional importance of the cationic sites on S1 and their involvement in ionic interactions within the myosin head during energy transduction. The direct binding of heparin, used at micromolar concentrations, and its influence on the structural and functional properties of S1 were followed by gel chromatography, electron microscopy, chemical cross‐linking techniques and limited digestion studies. 1 The limited tryptic digestion of S1 showed that the presence of heparin, as well as of the homopolymer, poly‐(L‐glutamic acid) causes a specific structural change in the 50‐kDa heavy chain region of S1 and accelerates the breakdown of this segment into a 45‐kDa species by a proteolytic cleavage restricted to its COOH‐terminal portion. Under similar experimental conditions, the binding of MgATP and MgADP to S1 led also to the 50‐kDa → 45‐kDa conversion, suggesting that the S1‐nucleotide interactions exhibit some resemblances to the polyanion‐S1 interactions; a distortion of the COOH‐terminal peptide stretch of the 50‐kDa fragment seems to result from the binding of polyanionic ligands to S1. This particular area is adjacent to the actin site containing the 45‐kDa and 20‐kDa segments of the S1 heavy chain. On the other hand, the polyanions as well as nucleotides induced changes in the interface between the heavy chain and the alkali light chains. 2 Moreover, the binding of heparin to S1 resulted in the self‐association of the enzyme and the production of stable small S1 oligomers, most likely dimers, which were demonstrated by the alteration of the size of the S1 particles examined by electron microscopy and their freezing by chemical cross‐linking agents. These findings are relevant to the recently reported property of skeletal chymotryptic S1 to form dimers under convenient ionic conditions, in particular in the presence of Mg‐nucleotides. The interaction of cationic sites on S1 and possibly on the 50‐kDa region of the heavy chain with polyanions promotes the dimerization of the S1 molecules. The binding of S1 to F‐actin abolished S1 aggregation.