The morphogenesis of myosin II strucutre in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetramethylrhodamine-myosin II, nascent myosin spots formed close to the active edge during periods of retraction and then elongated into wavy ribbons of uniform width. The spots and ribbons initially behaved as distinct structural entities by subsequently aligned with each other in a sarcomeric-like patten. Electron microscopy established that the spots and ribbons consisted of bipolar minifilaments associated with each other at theyir head-containing end and arranged in a single row in and "open" zig-zag conformation or as a "closed" parallel stack. Ribbons also contacted each other in a nonsarcomeric, network-like arrangement as described previously (Verhovsky and Borisy, 1993. J. Cell Biol.. 123:637-652). Myosin ribbons were particularly pronounced in REF 52 cells, but small ribbons and networks were found also in a range of other mammalian cells.
At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filament. Further from the edge, myosin filament alignments increased in parallel with the development of actin bundles. In actin bundles, the actin cross-linking protein, alpha-actinin, was excluded from sites of myosin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may intitiate a pathway for the formation of actin filament bundles. We propose that zig-zag assemblies of myosin II filaments induce the formation of actin bundles by pulling on an actin filament network and that co-alignment of actin and mysosin filaments proceeds via folding of myosin II filament assemblies in an accordion-like fashion.
