Glucagon Induces Disaggregation of Polymer-like Structures of the Alpha Subunit of the Stimulatory G Protein in Liver Membranes

The hydrodynamic behavior of G alpha s, the alpha 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, G alpha s behaved as polydisperse structures with S values higher than that of heterotrimeric G proteins. At concentrations of guanosine 5'-[gamma-thio]triphosphate (GTP[gamma S]) greater than 100 microM, 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[gamma S] (maximal, 10 microM), was rapid (within 10 sec), and was not observed with vasopressin, angiotensin II, or glucagon-(19-29). When G alpha s in its membrane-bound form was [32P]ADP-ribosylated by cholera toxin and the treated membranes were extracted with octyl glucoside, greater than 35% of the labeled G alpha s was found in material that sedimented through sucrose gradients and contained relatively low levels of immunoreactive G alpha s. Glucagon selectively converted the apparently large molecular weight structures to the 4-5 S structures in the presence of GTP[gamma S], 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 G alpha s and that activation by GTP[gamma S] results in disaggregation. The role of the beta and gamma subunits of G proteins in the hormone-induced process is not clear since the polymer-like structures extracted with octyl glucoside are devoid of beta and gamma subunits.