Paramyosin and myosin content of the thick filament in the striated muscle of Limulus.

The thick myofilaments of the striated muscle from the horseshoe crab (Limulus) are composed of the proteins paramyosin and myosin. Using quantitative gel electrophoresis we find that there are 1.02 paramyosin molecules for every myosin. This protein composition is consistent with scanning transmission electron microscopy measurements of isolated thick filament mass and the four-stranded helical filament structure reported previously by others if one assumes one myosin molecule per surface subunit as has been suggested. Polyclonal antibodies were used to distinguish myosin and paramyosin bands from others of similar electrophoretic mobility as well as to detect aggregates and breakdown products by examination of gel patterns transferred to nitrocellulose membranes. Two-dimensional gel maps were utilized as a check for coelectrophoresing bands that might go undetected in one dimension. Coomassie Blue binding per mg of purified myosin or paramyosin was found to differ by less than 5%. Extinction coefficients for Limulus myosin and paramyosin as calibrated to dry weight measurements are also reported.