Role of the PsbI Protein in Photosystem II Assembly and Repair in the Cyanobacterium Synechocystis sp. PCC 6803

Abstract

The involvement of the PsbI protein in the assembly and repair of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. Analysis of PSII complexes in the wild-type strain showed that the PsbI protein was present in dimeric and monomeric core complexes, core complexes lacking CP43, and in reaction center complexes containing D1, D2, and cytochrome b-559. In addition, immunoprecipitation experiments and the use of a histidine-tagged derivative of PsbI have revealed the presence in the thylakoid membrane of assembly complexes containing PsbI and either the precursor or mature forms of D1. Analysis of PSII assembly in the psbI deletion mutant and in strains lacking PsbI together with other PSII subunits showed that PsbI was not required for formation of PSII reaction center complexes or core complexes, although levels of unassembled D1 were reduced in its absence. However, loss of PsbI led to a dramatic destabilization of CP43 binding within monomeric and dimeric PSII core complexes. Despite the close structural relationship between D1 and PsbI in the PSII complex, PsbI turned over much slower than D1, whereas high light-induced turnover of D1 was accelerated in the absence of PsbI. Overall, our results suggest that PsbI is an early assembly partner for D1 and that it plays a functional role in stabilizing the binding of CP43 in the PSII holoenzyme.The PSII complex is the multisubunit membrane protein complex catalyzing oxidation of water and reduction of plastoquinone in the thylakoid membranes of higher plants, algae, and cyanobacteria. The membrane-embedded core complex of PSII consists of the D1 and D2 reaction center (RC) subunits, the inner chlorophyll (Chl)-binding antenna proteins, CP47 and CP43, and a number of smaller polypeptides (for review, see Barber, 2006). D1 and D2 form a heterodimer that binds the cofactors involved in primary charge separation and subsequent electron transfer within PSII, while the main purpose of CP47 and CP43 is to deliver energy to the RC for driving electron transfer and, in the case of CP43, to help ligate the CaMn 4 cluster.

The Cyanobacterial Homologue of HCF136/YCF48 Is a Component of an Early Photosystem II Assembly Complex and Is Important for Both the Efficient Assembly and Repair of Photosystem II in Synechocystis sp. PCC 6803

“…In the case of D1, unassembled D1 protein can only be detected in WT when using a much shorter radioactive pulse and a lower temperature (15). Under these conditions, all three forms of D1 can be detected: the unprocessed form, pD1; the incompletely processed form, iD1; and mature D1 (15).…”

“…The remaining complex, RCb, was present in too low amounts for reliable detection of YCF48 (see also Ref. 15), but after sub-jecting thylakoids to multiple freeze/thaw cycles, its content markedly increased. Subsequent immunoblotting failed to detect YCF48 in RCb.…”

“…2, see also Ref. 15). Their precise subunit composition is unclear but each contains D2, either D1 or its processing intermediate iD1, both subunits of cytochrome b 559 and PsbI, and each lacks both CP43 and CP47.…”

“…PSII assembly appears to occur in a stepwise fashion and involves the formation of a PSII RC-like complex, possibly from D2-cytochrome b 559 and pD1-PsbI pre-complexes (14,15). Then the antennae, CP47 and CP43, plus other low-molecular mass polypeptides, become sequentially attached to form the PSII non-oxygen-evolving core complex.…”

“…1). Small amounts of the PSII proteins, D2 and D1, also accumulate in two smaller PSII complexes: a core subcomplex lacking CP43 (termed RC47) and a RC-like assembly complex (RCa) lacking both CP47 and CP43 (15,17). YCF48 could not be detected in RCC(1) and RCC(2) and was found to be mainly present in the low-molecular weight region, probably in an unassembled state.…”