Proton Nuclear‐Magnetic‐Resonance Spectroscopy of Myosin Subfragment 1 Isoenzymes

High‐resolution proton NMR spectrocopy has been used to study the solution structures of the subfragment 1 (S1)isoenzymes (containing either the A1 or A2 light chains) from rabbit skeletal muscle myosin and to investigate their interaction with actin. Superimposed upon broad components, the narrow signals of the S1 spectra are unexpectedly sharp, indicating that domains of varying sidechain mobility occur in the conformation adopted solution. These observations are in agreement with previous studies of the mixed isoenzymes [Highsmith et al. (1979) Biochemistry, 18, 4238 ‐42431]. Peptide amide exchange studies show also that the S1 structure accommodates fluctuations of sufficient amplitude to allow most of the peptide groups to come into contact with the solvent on the time scale‐of the 1H‐NMR experiment. The overall impression is that S1 is a molecule possessing backbone motility well as domains of different sidechain mobility.Close comparison of the Sl(A1) and Sl(A2) spectra indicate that the N‐terminal41 residues of the A1 light chain, rich in lysine, proline and alanine, display a high degree of segmental mobility. The difference spectrum [S1(A1)‐S1(A2)] obtained closely resembles the spectral simulation of the 41‐residue segment. Upon addition of actin, many of the narrow S1 resonances decrease in intensity or progressively disappear altogether, indicative of intermediate‐ slow exchange conditions consistent with the recognised high affinity between the two proteins. These changes are interpreted as an overall modulation in the observed and hence more mobile regions of S1 as has been suggested earlier 1H‐NMR studies referred to above. In particular, the differences noted between S1(A1) and S1(A2) have now largely disappeared in their complexes with actin indicating a marked reduction in the segmental mobility of the N‐terminal region of the light chain in S1(A1). Together with other affinity chromatography results [Winstanley and Trayer (1979) Biochem. Soc. Trans. 7, 703–704], this is good evidence for a direct interaction between this area of Sl(A1) and actin.