Asymmetrical distribution of G-proteins among the apical and basolateral membranes of rat enterocytes

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Abstract
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 i2, alpha ji3, 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 s 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 Gi-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.