MYOSIN-II FILAMENT ASSEMBLIES IN THE ACTIVE LAMELLA OF FIBROBLASTS - THEIR MORPHOGENESIS AND ROLE IN THE FORMATION OF ACTIN FILAMENT BUNDLES

The morphogenesis of myosin II structures in active lamella undergoing net protrusion was analyzed by correlative fluorescence and electron microscopy. In rat embryo fibroblasts (REF 52) microinjected with tetr amethylrhodamine-myosin II, nascent myosin spots formed close to the a ctive edge during periods of retraction and then elongated into wavy r ibbons of uniform width. The spots and ribbons initially behaved as di stinct structural entities but subsequently aligned with each other in a sarcomeric-like pattern. Electron microscopy established that the s pots and ribbons consisted of bipolar minifilaments associated with ea ch other at their head-containing ends and arranged in a single row in an ''open'' zig-zag conformation or as a ''closed'' parallel stack. R ibbons also contacted each other in a nonsarcomeric, network-like arra ngement as described previously (Verkhovsky 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 o f other mammalian cells, At the edge of the cell, individual spots and open ribbons were associated with relatively disordered actin filamen ts. Further from the edge, myosin filament alignment increased in para llel with the development of actin bundles. In actin bundles, the acti n cross-linking protein, alpha-actinin, was excluded from sites of myo sin localization but concentrated in paired sites flanking each myosin ribbon, suggesting that myosin filament association may initiate a pa thway for the formation of actin filament bundles. We propose that zig -zag assemblies of myosin II filaments induce the formation of actin b undles by pulling on an actin filament network and that co-alignment o f actin and myosin filaments proceeds via folding of myosin II filamen t assemblies in an accordion-like fashion.