Evidence for an actin-containing cytoplasmic precursor of the focal contact and the timing of incorporation of vinculin at the focal contact

The distribution of F-actin and vinculin in chicken embryo fibroblasts has been examined by nitrobenzoxadiazol (NBD)-phallacidin and indirect immunofluorescent staining, respectively, and related to the process of focal contact formation by recording the motility of the cell with differential interference contrast (DIC) or interference reflection microscopy (IRM) before fixation for staining. Linear cytoplasmic precursors of the focal contact, present within unattached lamellipodia, stained intensely with NBD-phallacidin. Without exception new focal contacts, 8 s and older at fixation, were associated with either a longer F-actin rib in the lamellipodium or, in older contacts, an F-actin structure of similar dimensions to the contact. This change in distribution of F-actin over the new contacts was accounted for by the segregation of the structural precursor into an attached part over the focal contact and a separate motile part. These results show that F- actin accumulates in the precursor adjacent to areas of the membrane competent to form the focal contact, and are consistent with the interpretation that this F-actin contributes to the initial adhesion plaque associated with the new contact. Vinculin was essentially absent from motile lamellipodia, showed no preferential association with F- actin rich precursors or very young focal contacts, but accumulated over new contacts during a 90-s period. Therefore, the association of F- actin with the membrane that precedes and persists in the initial focal contact is independent of vinculin, and the role of vinculin in development of the focal contact remains unclear.