Purification and determination of intact molecular mass by electrospray ionization mass spectrometry of the photosystem II reaction center subunits.

A reverse phase high pressure liquid chromatography purification system for the rapid separation of photosystem II reaction center proteins free of salts and detergents is described. This procedure results in the isolation of the three small subunits: alpha- and beta-subunits of cytochrome b559 and PsbI protein, with near base-line resolution between each peak, although the D1 and D2 proteins were partially deconvoluted. The molecular masses obtained by electrospray ionization mass spectrometry for the purified beta-subunit of cytochrome b559, alpha-subunit of cytochrome b559, and the PsbI protein, 4,394.8 +/- 0.4, 9,283.7 +/- 0.8, and 4,209.5 +/- 0.4 Da, respectively, are in excellent agreement with values obtained from previous characterization studies (Sharma, J., Panico, M., Barber, J., and Morris, H. R. (1997) J. Biol. Chem. 272, 3935-3943). Direct electrospray analysis of the D1 and D2 proteins suggests that these components exist in heterogeneous forms. The molecular mass ascribed to a predominant form of the D1 protein, 38, 040.9 +/- 6.5 Da, and the D2 protein, 39,456.1 +/- 7.7, are also in agreement with those expected for the mature nonphosphorylated states of these subunits.