Structural characterisation of photosystem II.
Protocols were developed to fractionate each of the five protein components present in the PSII reaction centre complex, isolated from the thylakoid membrane of pea plants. The precise molecular weights of all the purified components were then successfully determined, for the first time by electrospray and fast atom bombardment mass spectrometry. Discrepancies between the molecular weights assigned and those calculated from the respective cDNA sequences were observed for all of the reaction centre component proteins. Application of novel mapping and sequencing strategies on these proteins has assured the elucidation of full primary structures of and subunits of cytochrome b559 and the majority of the structures of the D1 and D2 proteins. In the case of the subunit to cytochrome b559 an N-terminal processing event, involving the removal of the initiating formyl-methionine residue was found. The transformations on the subunit of cytochrome b559 were characterised as a N-terminal modification, entailing the processing of the formyl-methionine residue followed by an acetylation of the amino terminus of the mature protein, and a sequence change at residue twenty six, were a serine has been replaced by a phenylalanine residue. This was presumed to be due to an error in the gene sequence, but is more probably a result of the recently described phenomenon of mRNA editing. Currently, the nucleotide sequence of the pshI gene from pea plants is not available. Using a combination of mass spectrometric techniques and automated gas phase sequencing the primary structure of this component has been obtained in these studies. In contrast to the smaller subunits, data obtained on the D1 and D2 proteins indicated that both these components exist in heterogeneous forms. Mapping studies on these subunits have identified phosphorylation and oxidation, as at least two sources of this microheterogeneity.