Proteomic Analysis of Bovine Brain G Protein γ Subunit Processing Heterogeneity

We characterized the variable processing of the G protein γ subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. Gγ subunits were isolated from purified brain G proteins and characterized by Edman sequencing, by MALDI MS, by chemical and/or enzymatic fragmentation assayed by MALDI MS, and by MS/MS fragmentation and sequencing. Multiple forms of six different Gγ isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All Gγ isoforms contain a carboxyl-terminal C AAX motif for prenylation, carboxyl-terminal proteolysis, and carboxymethylation. Characterization of these proteins indicates significant variability in the normal processing of all of these steps in the prenylation reaction, including a new variation of prenyl processing resulting from cysteinylation of the carboxyl terminus. These results have multiple implications for intracellular signaling mechanisms by G proteins, for the role of prenyl processing variation in cell signaling, and for the site of action and consequences of drugs that target the prenylation modification.