Proteomic Analysis of Bovine Brain G Protein Subunit Processing Heterogeneity*□S

We characterized the variable processing of the G protein subunit isoforms associated with bovine brain G proteins, a primary mediator of cellular communication. G 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 G isoforms were detected. Significant variation in processing was found at both the amino termini and particularly the carboxyl termini of the proteins. All G isoforms contain a carboxyl-terminal CAAX 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. Molecular & Cellular Proteomics 5:671–685, 2006.