ASYMMETRICAL DISTRIBUTION OF G-PROTEINS AMONG THE APICAL AND BASOLATERAL MEMBRANES OF RAT ENTEROCYTES

The distribution of the alpha and beta-subunits of guanosine-nucleotide-binding proteins (G-proteins) among the apical and basolateral membranes of polarized rat enterocytes was investigated by ADP-ribosylation assays in vitro and immunoblotting with G-protein-subunit-specific antisera. The enterocytes were found to express alpha-i-2, alpha-i-3, alpha-s and beta-subunits, whereas alpha-i1 and alpha-o subunits could not be detected. The alpha-i2 and alpha-i3 subunits were located predominantly in the basolateral membrane, in contrast with the alpha and beta-subunits, which were distributed uniformly among both membranes. Furthermore, 39 kDa and 78 kDa proteins, recognized by anti-alpha-i1/2 but not anti-alpha-i1 or anti-alpha-i3 specific antisera, and resistant to ADP-ribosylation by pertussis toxin, were localized exclusively at the apical border. These G(i)-related proteins might represent novel members of the G-protein family. Activation of apical G-proteins by GTP or its analogues failed to release the alpha-s, alpha-i, and beta-subunits or the 39 kDa and 78 kDa alpha-i-like proteins from the membrane, suggesting a functional role for these proteins in the apical membrane itself. Our recent finding of a guanosine 5'-[gamma-thio]triphosphate-sensitive Cl- conductance in the apical membrane of rat enterocytes suggests that one or more of these G-proteins may act as local regulators of specific apical transport functions.