Primary structure characterization of the photosystem II D1 and D2 subunits.
Mass spectrometry techniques have been applied in a protein mapping strategy to elucidate the majority of the primary structures of the D1 and D2 proteins present in the photosystem II reaction center. Evidence verifying the post-translational processing of the initiating methionine residue and acetylation of the free amino group, similar to those reported for other higher plant species, are presented for the two subunits from pea plants (Pisum sativum L.). Further covalent modifications observed on the D1 protein include the COOH-terminal processing with a loss of nine amino acids and phosphorylation of Thr2. In addition, the studies reported in this paper provide the first definitive characterization of oxidations on specific amino acids of the D1 and D2 proteins. We believe that these oxidations, and to a much lesser extent the phosphorylations, are major contributors to the heterogeneity observed during the electrospray analysis of the intact subunits reported in the accompanying paper (Sharma, J., Panico, M., Barber, J., and Morris, H. R. (1997) J. Biol. Chem. 272, 33153-33157). Significantly, all of the regions that have been identified as those particularly susceptible to oxidation are anticipated (from current models) to be in close proximity to the redox active components of the photosystem II complex.