Identification of a surface actin-binding site on myosin.

The surface accessibility of mobile domains of rabbit fast muscle myosin subfragment-1 isoenzymes (subfragment-1(A1), (A2)) influenced by interaction with actin has been investigated by proton magnetic resonance spectroscopy using the soluble paramagnetic reagents Cr(CN)6(3-), Fe(CN)6(3-), Mn2+ and the Gd3+ salt of 1,4,7,10-tetraazacyclododecane-N,N',N",N"'-tetraacetic acid as probes. Anionic probes interact principally with lysine residues disposed close to other non-charged sidechains in both isoenzymes. Additional resonances in subfragment-1(A1) not present in subfragment-1(A2) are also observed to be affected, notably the sharp signal at 3.23 ppm which derives from a -N+ (CH3)3 group found in the N-terminal segment of the A1 light chain, showing that this domain of interaction with actin (Prince et al. (1981) Eur. J. Biochem. 121, 213-219) is situated at a surface location. Different probes identify a heterogeneity in the location and function of mobile sidechains. These results suggest a configurational lability in the various parts of the myosin head, differentially constrained upon interaction with actin and consistent with a structure composed of relatively rigid domains linked by more flexible regions.