Lysyl residues of cardiac myosin accessible to labeling with a fluorescent reagent, N-methyl-2-anilino-6-naphthalenesulfonyl chloride

Two mol of N-methyl-2-anilino-6-naphthalenesulfonyl (Mns) groups was preferentially incorporated into pig cardiac myosin in the absence of divalent metal ions, 1 mol rapidly and 1 mol slowly. In the presence of divalent metal ions. 1 mol was rapidly incorporated but subsequent incorporation was strongly suppressed. No substantial effect of incorporation of Mns groups in the presence or absence of divalent metal ions on the Ca2+- and K+-ATPase activities of myosin was found. However, the fluorescence spectra due to attached Mns groups were different in the two cases. Extensive pronase digestion of labeled myosin indicated that the Mns groups were attached predominantly to lysyl residues, regardless of the labeling conditions. Peptide mapping of the labeled myosin digested with subtilisin, pepsin or trypsin uniformly showed the selective incorporation of an Mns group into essentially one species of peptide. However, the peptide labeled in the absence of divalent metal ions was clearly different from that labeled in their presence. The present results confirm that pig cardiac myosin heavy chains contain two distinct lysyl residues, which are both accessible to labeling with Mns groups only when divalent metal ions are absent. The results also suggest that conformational changes occur around these residues when divalent metal ions are added.