Identification and properties of a Gs protein in catfish liver membranes

The presence of G proteins and their involvement in adrenergic signaling has been investigated in catfish (Ictalurus melas) liver membranes. Adenylyl cyclase activity was potently stimulated by the nonhydrolyzable analog of GTP, [35S]guanosine 5'-O-(gamma-thiotriphosphate) (GTPgammaS) (maximal activation of about eightfold at 10(-5) M; half-maximal activation at 1.31 x 10(-7) M), and reduced by the competitive inhibitor of GTP, GDPbetaS (70% maximal inhibition at 10(-4) M; half-maximal inhibition at 1.98 x 10(-7) M). Forskolin dramatically enhanced enzyme activity (up to about 3500% at 100 microM), and its action was not affected by guanine nucleotides, confirming that the diterpene effect occurred only at targets downstream of the G proteins. Receptor-dependent G protein activity was evaluated by a [(35)S]GTPgammaS binding assay. At 100 microM GDP, 100 mM NaCl, and 5 mM MgCl2, after an incubation of 90 min at 20 degrees, a Kd of 18.6 nM and a Bmax of 105.7 pmol/mg protein for [35S]GTPgammaS binding to catfish liver membranes were determined. The binding of the tracer was enhanced by 1 microM epinephrine, up to a maximum of 158%, and inhibited by NF 449, a G(s)alpha-selective antagonist with half-maximal effect in the micromolar range. Immunoblotting analysis with a specific anti-G(s)alpha antibody revealed a single band of about 45 kDa mass. This result represents the first demonstration of the presence of G protein alpha(s) subunits in the liver of an ectothermal vertebrate.