MICROFILAMENT-MEMBRANE INTERACTIONS IN XENOPUS MYOCYTES

We used quick-freeze, deep-etch, rotary-replication transmission elect ron microscopy to determine at molecular resolution the organization o f microfilaments at the cytoplasmic surface of the sarcolemma of Xenop us myocytes. We demonstrate that actin microfilaments interact with th e sarcolemma in two distinct ways. In one, which resembled focal conta cts in Xenopus fibroblasts [Samuelsson et al., 1993: J. Cell Biol. 122 :485-496], bundles of microfilaments approached the sarcolemma at site s containing aggregates of membrane-associated particles. Immunogold c ytochemistry showed that these particle aggregates contained vinculin, talin and beta(1)-integrin, In the second, which covered most of the cytoplasmic surface of the sarcolemma, individual actin microfilaments formed an extensive, lattice-Like array. Particle aggregates associat ed with this array of actin microfilaments also labeled with antibodie s to vinculin, talin and beta(1)-integrin. The unique, lattice-like as sociation of actin microfilaments with the membrane in Xenopus myocyte s suggests that the organization of actin filaments over most of the s arcolemma is distinct from focal contacts, mediating widespread associ ations of the actin cytoskeleton with the cytoplasmic membrane face. ( C) 1996 Wiley-Liss, Inc.