Ultrastructural evidence for myosin of the smooth muscle type at the surface of trypsin-dissociated embryonic chick cells.

Cells dissociated from embryonic chick muscle tissue using trypsin were rotated in the presence of globulin-enriched rabbit antisera against both smooth and striated muscle actomyosins originating from chicken gizzard (GAM) and pectoralis (PAM) muscles respectively. The presence of the rabbit antibodies was demonstrated using peroxidase-labelled sheep antirabbit y-globulins, the enzyme-antibody conjugate being located by electron-microscope histochemistry. Anti-GAM y-globulins reacted strongly with the plasma membrane. Judging from the complete absence of staining, y-globulins from non-immunized rabbit serum did not interact with the membrane. When y-globulins of sheep anti-rabbit IgG serum were applied alone, that is in the absence of pretreatment with rabbit y-globulin, there was an observable reaction with the cell surface. Preincubation of anti-GAM with the heavy meromyosin fraction from smooth-muscle myosin inhibited the interaction of the antibodies with the membrane, as evidenced by the absence of staining. A weak positive reaction obtained with anti-PAM was due to components of the antibody preparation which were reactive with actin and not with PAM. It was concluded that a smooth-muscle myosin-like protein is an integral part of the plasma membrane of embryonic chick muscle cells.