Tandem mass spectrometry of yeast V1-ATPase. A, electrospray ionization mass spectrometry spectrum of intact V1-ATPase (taken from Ref. 47) showing three species. The 52+ charge state of the most abundant complex (593 kDa; I), which has the stoichiometry A3B3DE3FG3H was selected for collisional activation, highlighted in gray. B-G, tandem mass spectra of V1-ATPase sprayed from aqueous 100 mm ammonium acetate, pH 6.8. Dissociation of the 52+ ion of the 593-kDa V1-ATPase complex at a collision voltage of 100 V (B) with zoom in the m/z regions 900-2,900 (C) and 9,000-34,000 (D). Dissociation of the 52+ ion of the complex at a collision voltage of 200 V (E) with zoom in the m/z regions 900-2,900 (F) and 9,000-34,000 (G). The dissociated subunits E, G, and H are represented by green circle, blue square, and red triangle, respectively. The product ions of five different complexes are indicated by the empty green circle (loss of one subunit E), empty pink rhombus (loss of two subunits E), empty blue square (loss of one subunit G), empty black triangle (loss of one subunit G and one subunit E), and empty red triangle (loss of one subunit H). The charge series marked with a asterisk corresponds to a degraded form of subunit E.