Determination of the complete covalent structure of the gamma 2 subunit of bovine brain G proteins by mass spectrometry.

The complete covalent structure of the gamma 2 subunit of bovine brain G proteins was determined by peptide mapping using matrix-assisted laser desorption ionization and electrospray ionization mass spectrometry and by partial sequencing using tandem mass spectrometry. Fragments were identified in proteolytic digests corresponding to all predicted internal sequences of gamma 2, but not for N- or C-terminal sequences. Fragments consistent with modifications of the terminal peptides were observed, however. The presence of these postulated modifications was verified by tandem mass spectrometry. These studies define the structure of the native protein and provide a basis for comparison to expressed proteins.