STRUCTURES LINKING MICROFILAMENT BUNDLES TO THE MEMBRANE AT FOCAL CONTACTS

We used quick-freeze, deep-etch, rotary replication and immunogold cyt ochemistry to identify a new structure at focal contacts. In Xenopus f ibroblasts, elongated aggregates of particles project from the membran e to contact bundles of actin microfilaments. Before terminating, a si ngle bundle of microfilaments interacts with several aggregates that a ppear intermittently over a distance of several microns. Aggregates ar e enriched in proteins believed to mediate actin-membrane interactions at focal contacts, including beta1-integrin, vinculin, and talin, but they appear to contain less alpha-actinin and filamin. We also identi fied a second, smaller class of aggregates of membrane particles that contained beta1-integrin but not vinculin or talin and that were not a ssociated with actin microfilaments. Our results indicate that vinculi n, talin, and beta1-integrin are assembled into distinctive structures that mediate multiple lateral interactions between microfilaments and the membrane at focal contacts.