The vinculin/sarcomeric-alpha-actinin/alpha-actin nexus in cultured cardiac myocytes

Experiments are described supporting the proposition that the assembly of stress fibers in non-muscle cells and the assembly of myofibrils in cardiac cells share common mechanisms. By extensive use of the double-staining technique with a battery of labelled-antibodies against membrane-associated proteins and against myofibrillar proteins the following is demonstrated: (1) dissociated, cultured cardiac myocytes reconstitute intercalated discs consisting of adherens junctions interspersed with spot desmosomes at sites of cell-cell contact and sub-sarcolemmal adhesion plaques (SAPs) at sites of cell-substrate contact; (2) the insertion of a myofibril into an adherens junction of the intercalated disc or into a SAP at the cell-substrate junction always involves vinculin and the sarcomeric isoforms $\alpha$-actin and sarcomeric $\alpha$-actinin (s-$\alpha$-actinin); (3) adherens junctions are positive for A-CAM but negative for talin and integrin; SAPs lack A-CAM but are positive for talin and integrin; (4) adherens junctions and SAPs associate proximally with a striated myofibril, and conversely every striated myofibril is capped at either end by an adherens junction or a SAP; (5) the invariant association between a given myofibril and its SAPs is especially prominent at the earliest stages of myofibrillogenesis; nascent myofibrils are capped by a pair of oppositely oriented SAPs; (6) each pair of SAPs inserts into the ventral surface, never the dorsal surface, thus conferring a dorsal-ventral polarity on the spread myocytes; and (7) spread non-translocating, cultured cardiac cells differ from virtually all other types of spread cultured cells in that they do not assemble stress fibers consisting exclusively of such contractile proteins as $\gamma$-actin, non-sarcomeric $\alpha$-actinin (non-s-$\alpha$-actinin) and caldesmon; instead they assemble stress fiber-like bundles which without exception include linearly-aligned I-Z-I-like complexes of s-$\alpha$-actinin, $\alpha$-actin, titin and troponin-I and often sarcomeric myosin heavy chains (s-MHC)