Glucagon induces disaggregation of polymer-like structures of the α subunit of the stimulatory G protein in liver membranes

Abstract

The hydrodynamic behavior of Gαs, the α subunit of the stimulatory guanine nucleotide-binding regulatory protein (G protein), in octyl glucoside extracts of rat liver membranes was investigated. As was previously shown for G proteins similarly extracted from brain synaptoneurosomes, Gαs behaved as polydisperse structures with S values higher than that of heterotrimeric G proteins. At concentrations of guanosine 5′-[γ-thio]triphosphate (GTP[γS]) >100 μM, incubation with membranes led to smaller structures having S values in the range of 4-5 S. Incubation of liver membranes with glucagon also caused a marked increase in structures having these S values; glucagon action required the presence of low concentrations of GTP[γS] (maximal, 10 μM), was rapid (within 10 sec), and was not observed with vasopressin, angiotensin II, or glucagon-(19-29). When Gαs in its membrane-bound form was [32P]ADP-ribosylated by cholera toxin and the treated membranes were extracted with octyl glucoside, >35% of the labeled Gαs was found in material that sedimented through sucrose gradients and contained relatively low levels of immunoreactive Gαs. Glucagon selectively converted the apparently large molecular weight structures to the 4-5 S structures in the presence of GTP[γS], even at 1 mM (the maximal effect of the nucleotide alone), when incubated with the toxin-treated membranes. These findings suggest that the glucagon receptor selectively interacts with polymer-like structures of Gas and that activation by GTP[γS] results in disaggregation. The role of the β and γ subunits of G proteins in the hormone-induced process is not clear since the polymer-like structures extracted with octyl glucoside are devoid of β and γ subunits.