(A) Spectrum showing the dissociation of the 34+ charge state.

(B) Expansion of the spectrum over the m/z range of 1,600–2,800. Three individual subunits are assigned Rpn5, Rpn6, and Rpn9 (orange, green, and blue circles, respectively).

(C) Expansion of the 8,000–16,000 m/z region. Three different hetrotrimers are assigned: Rpn5:Rpn6:Rpn8, Rpn5:Rpn8:Rpn9, and Rpn6:Rpn8:Rpn9 (blue, green, and orange stars, respectively). By extrapolation we can conclude that the subcomplex is composed of four subunits, Rpn5, Rpn6, Rpn8, and Rpn9. Rpn8 is not observed in its monomeric form; however, because it exists in all three different hetrotrimers, we can conclude it adopts a central position within the subcomplex.

(D) A schematic summarizes the structural data obtained from MS and MS/MS results. Red indicates core subunits, while purple and to a greater extent blue specifies peripheral subunits.