Microsomal and cytosolic fractions of guinea pig hepatocytes contain 100-kilodalton GTP-binding proteins reactive with antisera against alpha subunits of stimulatory and inhibitory heterotrimeric GTP-binding proteins.

Guinea pig hepatocytes fractionated by differential centrifugation into plasma membrane-enriched, microsomal, and cytosolic fractions were examined for their content of alpha and beta subunits of heterotrimeric GTP-binding proteins (G proteins) involved in signal transduction. alpha subunits of stimulatory (Gs) and inhibitory (Gi) proteins were detected by immunoblots with antisera reactive with the carboxyl-terminal decapeptide regions of these proteins. Unexpectedly, antisera (including immunopurified) to the alpha subunit but not the beta subunit reacted with a band of 100-kDa proteins in both the microsomal and cytosolic fractions. The immunoreactive 100-kDa proteins are not substrates for ADP-ribosylation catalyzed by pertussis toxin, cholera toxin, or diptheria toxin. Protease digests of the 100-kDa proteins yielded immunoreactive peptides that are distinctly different from those obtained from protease digests of alpha subunits of heterotrimeric G proteins. The 100-kDa protein(s) reactive with antisera to Gi alpha subunit bind to GTP-agarose but not to ATP-agarose. It is concluded that the immunoreactive 100-kDa proteins in microsomal and cytosolic fractions are structurally distinct G proteins from those linked to receptors in the plasma membrane and other G proteins such as elongation factor 2. Conceivably, the 100-kDa proteins represent a new class of G proteins.