A precursor of the focal contact in cultured fibroblasts.

Evidence for a structural precursor of the focal contact in cultured fibroblasts and continuing studies on the development of the precursor and contact are discussed. The structural precursor consists of an F-actin-rich, rib-like fiber within the motile lamellipodium. The focal contact forms beneath the fiber, part of which is retained at the contact as the initial adhesion plaque. Therefore, F-actin is present at the contact from the beginning. Vinculin accumulates at the plaque during a 90-second period after the contact forms. A novel feature of the distribution of talin has been found. The protein is present along the distal margin of the lamellipodium, where it is further concentrated as a series of nodes at the tips of each precursor and between precursors. This distribution of talin is independent of that which develops at the plaque after the contact forms. The structural development of the precursor has been followed with AVEC-DIC optics. The process begins with the development of fine oblique fibers from small structural nodes at the margin of the lamellipodium, and continues with the fusion of the nodes at the margin and inward coalescence of the fibers. It is suggested that talin may function as a cross-linking protein in the convergence of actin filaments at the membrane, while other actin-bundling proteins participate in the inward coalescence of the filaments to form fibers. The F-actin core of the precursor could provide a structural framework against which differences at the external surface of the membrane develop prior to contact formation.