Analysis of G protein gamma subunit heterogeneity using mass spectrometry

The diversity of the gamma subunits in bovine brain G protein preparations was investigated using matrix-assisted laser desorption ionization (MALDI) mass spectrometry. Analysis of these G protein mixtures revealed at least four gamma subunit masses by the following four criteria. 1) The measured masses were in the same mass range as the predicted molecular weights of gamma isoforms. 2) The masses were reproducible between the same or different preparations of G proteins. 3) The masses were independent of the matrix used for MALDI analysis. 4) The masses comigrated with the gamma subunit, as part of the heterotrimer, the beta gamma dimer, or the separated gamma subunit. These measured masses were compared with those calculated from cDNA sequences of known bovine brain gamma isoforms with the addition of plausible post-translational modifications. The mass of each spectral peak was consistent with the calculated mass for only one of four known bovine brain gamma subunit isoforms, but the data suggest modifications of the gamma subunits in addition to those already known or suspected at their carboxyl termini. Besides these four major masses, several additional, less resolved spectral peaks were observed whose measured masses did not correlate with any known gamma subunit or plausible modification. MALDI mass spectrometry promises to be a powerful technique for the analysis of the diversity of the gamma subunit in G proteins and for the characterization of their post-translational modifications.