The extrinsic 18-kDa protein in photosystem II restores the ion-retention activity of a mutant extrinsic 23-kDa protein lacking 19 amino-acid residues on the amino terminus.

Abstract

The extrinsic 23-kDa protein in oxygen-evolving complex (OEC23) of photosystem II (PS II) has a function of retaining Ca2+ and Cl-, essential cofactors for photosynthetic oxygen evolution. We previously showed that the activities of ion retention differed among OEC23s derived from different plant species. Further reconstitution experiments of PS II activity with chimeric OEC23s showed that N-terminal amino-acid sequence of OEC23 was important for ion retention (1). We here report the characterization of a mutant OEC23, which lacked 19 residues on its N-terminus (OEC23 ?18). OEC23 ?18 had no ability to restore the oxygen-evolving activity of salt-washed PSII particles, although it bound to PSII normally, and circular dichroism spectrum indicated the similar overall structure to OEC23. These facts showed that N-terminal 18 residues of OEC23 were indispensable for ion retention but not for its general folding. On the other hands, OEC23 ?18 could partially restore PS II activity when OEC18, the extrinsic 18-kDa protein in oxygen-evolving complex, was present. This suggests the possible cooperation of OEC23 and OEC18 in both Ca2+ and Cl- retention. To understand the molecular mechanism of OEC23 function, X-ray analysis of OEC23 crystal is undergoing.