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.
Nakamura
S
S
Signal Transduction Section, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.
Rodbell
M
M
D000804:Angiotensin II / Q000494:pharmacology; D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D002499:Centrifugation, Density Gradient; D002772:Cholera Toxin / Q000378:metabolism / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D005934:Glucagon / Q000494:pharmacology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D046911:Macromolecular Substances; D009243:NAD / Q000378:metabolism; D051381:Rats; D014667:Vasopressins / Q000494:pharmacology
D046911:Macromolecular Substances; D009243:NAD; D014667:Vasopressins; D000804:Angiotensin II; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D005934:Glucagon; D002772:Cholera Toxin; D019204:GTP-Binding Proteins