Organization of talin and vinculin in adhesion plaques of wet-cleaved chicken embryo fibroblasts

We have studied the fine structure of adhesion plaques in chicken embryo fibroblasts (CEF) and visualized the localization of vinculin and talin using immunoelectron microscopy on CEF opened by ‘wet-cleaving’. This procedure, performed with nitrocellulose on cells grown on electron microscope grids, cleaved the CEF close to the inner face of the ventral membrane or at a slightly higher level through the cytoplasm. In the resulting preparations, adhesion plaques were identified by their localization at the end of microfilament bundles and by their density of vinculin and talin. The plaques showed a substructure of moderately electron-dense parallel bands that were interconnected. Both the parallel bands as well as the interconnecting threads showed a high density of vinculin and talin labels, whereas neither the surrounding membrane cytoskeleton nor the overlaying bundled microfilaments were labeled. In stereomicrographs, we observed no difference between the distances from vinculin or talin label, respectively, to the plasma membrane. In early spreading cells, vinculin and talin were found to be deposited simultaneously in fine radiating streaks that covered rather large parts of the ventral membrane at areas of close contact with the substratum. These streaks, which were initially overlayed by an isotropic cytoskeletal network without filament bundles, were the apparent precursors of later formed adhesion plaques. These observations suggest that there are no separate layers of talin and vinculin, but rather that adhesion plaques consist of a dense network of talin and vinculin. The observations strongly support the model proposed by Bendori et al. (1989), J. Cell Biol. 108, 2383–2393, that was based on the location of vinculin- and talin-binding sites in the vinculin molecule.