Possible involvement of two-step processing in the C-terminal cleavage of precursor D1 protein in Synechocystis sp. PCC6803

Abstract

The D1 protein of photosystem II reaction center is synthesized as a precursor form with an extra C-terminal sequence which is excised by a specific endoprotease, CtpA. The proteolytic processing of the precursor D1 protein is demonstrated to be absolutely critical to the integration of manganese cluster of oxygen-evolving complex. However, exact biological function of the extension has not been elucidated at present. It is noteworthy that length of the extension is variable depending on the class of organisms; in the case of cyanobacteria, it consists of 16 amino acid residues; the extension in higher plants generally consists of 9 residues, which might be caused by a deletion of successive 7 residues from the cyanobacterial counterpart. In this study, we carried out a pulse-chase experiment to analyze the process of C-terminal cleavage of D1 protein in cyanbacterium using Synechocystis sp. PCC6803 and detected a novel form of D1 band which temporary appeared at the position between the precursor and mature forms in SDS-PAGE analyses. Immunoprecipitation and peptide mapping analyses supported a view that this band might be corresponded to an intermediate form of D1 protein in the proteolytic processing, suggesting that the C-terminal extension is excised with a two-step mechanism in Synechocystis.