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.