Abstract. The subcellular distribution of the 43,000-D protein (43 kD or u~) and of some major cytoskeletal proteins was investigated in Torpedo marmorata electrocytes by immunocytochemical methods (immunofluorescence and immunogold at the electron microscope level) on frozen-fixed sections and homogenates of electric tissue. A monoclonal antibody directed against the 43-kD protein (Nghirm, H. O., J. Cartaud, C. Dubreuil, C. Kordeli, G. Buttin, and J. P. Changeux, 1983, Proc. Natl. Acad. Sci. USA, 80:6403-6407), selectively labeled the postsynaptic membrane on its cytoplasmic face. Staining by anti-actin and anti-desmin antibodies appeared evenly distributed within the cytoplasm: anti-desmin antibodies being associated with the network of intermediate-sized filaments that spans the electrocyte, and anti-actin antibodies making scattered clusters throughout the cytoplasm without preferential labeling of the postsynaptic membrane. On the other hand, a dense coating by anti-actin antibodies became apparent on the postsynaptic membrane in homogenates of electric tissue pointing to the possible artifactual redistribution of a soluble cytoplasmic actin pool.Anti-fodrin and anti-ankyrin antibodies selectively labeled the non-innervated membrane of the cell. F actin was also detected in this membrane. Filamin and vinculin, two actin-binding proteins recently localized at the rat neuromuscular junction (Bloch, R.J., and Z. W. Hall, 1983, J. CellBiol., 97:217-223), were detected in the electrocyte by the immunoblot technique but not by immunocytochemistry.The data are interpreted in terms of the functional polarity of the electrocyte and of the selective interaction of the cytoskeleton with the innervated and noninnervated domains of the plasma membrane.T HE postsynaptic membrane of the neuromuscular junction and of the electromotor synapse corresponds to a local differentiation of the plasma membrane characterized by an accumulation of the nicotinic acetylcholine receptor (Ach-R).t This membrane specialization persists for a long period after denervation (reviewed in references 10 and 18), indicating that physical constraints maintain the Ach-R molecules in place and, in particular, prevent against their lateral diffusion. Interactions of the Ach-R with extrinsic components from the extracellular matrix (13, 55) and/or with the cytoskeleton (17, 48, 67) have been postulated to contribute to this differentiation process. cell--the non-innervated face--is specialized in the regeneration of the electrochemical gradient (40). The Ach-R is present exclusively on the innervated membrane, and Na Ă· K Ă· ATPase accumulates on the non-innervated one. These membranes thus constitute fully differentiated and stable domains of the plasma membrane. From this standpoint the diskshaped electrocyte shows a striking functional and structural polarity ( 18, 21).Postsynaptic membrane fractions purified from Torpedo comprise essentially the intrinsic Ach-R polypeptides plus few extrinsic components of apparent molecular mass 43 kD (63, 64) na...