Localization of a small chloroplast-encoded polypeptide PsbK in photosystem II core complex

Abstract

Photosystem II (PSII) forms a multi-protein complex that consists of more than twenty subunits. PsbK is a small hydrophobic polypeptide with a putative single membrane-spanning helix. In eukaryotes, this polypeptide is encoded by the chloroplast psbK gene and is synthesized as a precursor with an extension at its N-terminus (1). Analyses of psbK disruption mutants of cyanobacterium (1) and green alga (2) have revealed that PsbK is required for an optimal function of PSII and/or normal accumulation of PSII complex. It is reported that the binding of PsbK to PSII complex is weak compared with other small hydrophobic polypeptides, while exact localization of this polypeptide in PSII core complex is not yet known. In the present study, we have raised antibody against Chlamydomonas PsbK overexpressed in E. coli and used it to localize PsbK in PSII core complex. We found by Western blotting analysis that almost all PsbK is present in thylakoids and also in PSII core complex purified from thylakoid extracts. The purified PSII core complex was subsequently disintegrated with a chaotrophic reagent, KSCN, and was fractionated into subcore complexes, CP47, CP43 and D1/D2 heterodimer. Localization of PsbK in these PSII subcore complexes will be presented.