PRIMARY STRUCTURE CHARACTERIZATION OF THE PHOTOSYSTEM-II D1 AND D2 SUBUNITS

Mass spectrometry techniques have been applied in a protein mapping st rategy to elucidate the majority of the primary structures of the D1 a nd D2 proteins present in the photosystem II reaction center. Evidence verifying the post-translational processing of the initiating methion ine residue and acetylation of the free amino group, similar to those reported for other higher plant species, are presented for the two sub units from pea plants (Pisum sativum L.). Further covalent modificatio ns observed on the D1 protein include the COOH-terminal processing wit h a loss of nine amino acids and phosphorylation of Thr(2). In additio n, the studies reported in this paper provide the first definitive cha racterization of oxidations on specific amino acids of the D1 and D2 p roteins. We believe that these oxidations, and to a much lesser extent the phosphorylations, are major contributors to the heterogeneity obs erved during the electrospray analysis of the intact subunits reported in the accompanying paper (Sharma, J., Panico, M., Barber, J., and Mo rris, H. R. (1997) J. Biol. Chem. 272, 33153-33157). Significantly, al l of the regions that have been identified as those particularly susce ptible to oxidation are anticipated (from current models) to be in clo se proximity to the redox active components of the photosystem II comp lex.