The myosin filament. I. Structural organization from antibody staining observed in electron microscopy

A model for the myosin filament is presented. The model is based on: (1) the staining pattern observed with anti-myosin in electron microscopy and its relation to the M-line and pseudo-H-zone; (2) the presence of six radially distributed bridges between the filaments in the M-line; (3) the triangular profiles seen in cross-sections of the thick filament through the pseudo-H-zone. The most significant features of the model are that (1) the packing of the myosin molecules along the filament is altered by the tapered ends and the overlap of molecules in the pseudo-H-zone, (2) the myosin filaments are ordered in the A-band in positions restricted by the ability to form M-bridges between the filaments and (3) the approximately 430-Å repeat period generally seen in the A-band can be accounted for solely on the basis of superposition of the cross-bridges.The model of the myosin filament has the following important structural characteristics: (1) myosin molecules are aggregated in parallel rows; (2) in the pseudo-H-zone the myosin molecules are aggregated tail (L-meromyosin) to tail and everywhere else head to tail; (3) in the M-line region there is tail to tail abutment of myosin molecules in the same row; (4) between the pseudo-H-zone and the tapered ends, there is an overlap of one period (approx. 430 Å) in the myosin molecules along each row; (5) there are 12 rows in a cross-section through the M-line region and 18 rows in a cross-section between the pseudo-H-zone and the tapered ends; (6) the myosin molecules are packed so that a triangular profile is obtained in a cross-section through the pseudo-H-zone. Nine rows are on the surface and three centrally located; (7) these 12 rows make up three sets of four rows. In each set there are two pairs. There is a stagger of one-third of a period between the rows of a pair and a stagger of one period between pairs.