Fig. 6. The rod myosin domain is folded and supports filament formation. (A) The GFP-myosin in infected C2C12 cells fixed with paraformaldehyde. (B) mAb 10F12.3 selectively binds the native coiled-coil conformation of S2 region of the rod domain and labels the GFP-myosin in the folding intermediates as well as in the striated myofibrils. (C) Areas of infected C2C12 cells containing the myosin maturation intermediates were marked during processing for electron microscopy and sections of these regions examined. The electron micrograph of an area of one cell reveals unusual cytoplasmic structures that contain loosely packed fibrils enclosed in a protein dense matrix that is devoid of polysomes (white arrows). Individual filaments in these structures are just discernable (black arrows). These intermediates range from 1-9 µm in length, and 0.5-3.5 µm in diameter and contain filamentous material organized in loose parallel or cross-hatched patterns. The boxed region is enlarged in the upper right corner. The filaments are best seen when viewed at a glancing angle.