GO associates with another 40 kDa brain protein.

Guanine nucleotide binding proteins (G proteins) mediate a variety of cellular responses to external stimuli. Pure G protein, receptor, and effector are sufficient to reconstitute hormonal activation of an effector in phospholipid vesicles, but other components may be important for specificity or localization in vivo. If another protein associates with GO, the molecular weight of GO solubilized from membranes would be larger than the molecular weight of GO after purification. We find that GO solubilized from bovine brain membranes by Triton X-100 behaves as a single population of molecules on sucrose density gradients and gel filtration columns. Its molecular mass is about 40 kDa larger than pure GO. Association of GO with the other protein is fragile as the proteins dissociate on further purification. There was no difference in ADP-ribosylation or tryptic cleavage of GO in larger and smaller form. These studies provide a basis for future experiments to stabilize the interaction and identify the protein.