Association of Psb28 and Psb27 Proteins with PSII-PSI Supercomplexes upon Exposure of Synechocystis sp. PCC 6803 to High Light

Abstract

Formation of the multi-subunit oxygen-evolving photosystem II (PSII) complex involves a number of auxiliary protein factors. In this study we compared the localization and possible function of two homologous PSII assembly factors, Psb28-1 and Psb28-2, from the cyanobacterium Synechocystis sp. PCC 6803. We demonstrate that FLAG-tagged Psb28-2 is present in both the monomeric PSII core complex and a PSII core complex lacking the inner antenna CP43 (RC47), whereas Psb28-1 preferentially binds to RC47. When cells are exposed to increased irradiance, both tagged Psb28 proteins additionally associate with oligomeric forms of PSII and with PSII-PSI supercomplexes composed of trimeric photosystem I (PSI) and two PSII monomers as deduced from electron microscopy. The presence of the Psb27 accessory protein in these complexes suggests the involvement of PSI in PSII biogenesis, possibly by photoprotecting PSII through energy spillover. Under standard culture conditions, the distribution of PSII complexes is similar in the wild type and in each of the single psb28 null mutants except for loss of RC47 in the absence of Psb28-1. In comparison with the wild type, growth of mutants lacking Psb28-1 and Psb27, but not Psb28-2, was retarded under high-light conditions and, especially, intermittent high-light/dark conditions, emphasizing the physiological importance of PSII assembly factors for light acclimation.

Mass spectrometry-based cross-linking study shows that the Psb28 protein binds to cytochrome b 559 in Photosystem II

“…Approximately one in three ΔO-M PSII complexes contained Psb28 compared with approximately one in seven His47-M and His47-D PSII complexes. Other studies found Psb28 mainly in the monomeric RC47 complex (19)(20)(21)(22), whereas we detected approximately equal levels in monomeric and dimeric His47-PSII. A dimeric RC47 complex containing Psb28 was detected, however, in two previous studies (20,21).…”

“…The absence of Psb28 indicates that it dissociates from PSII before, or concomitant with, binding of Psb27. This observation is consistent with the presence of Psb28 mainly in the RC47 complex (19)(20)(21)(22), to which CP43, and therefore Psb27, have not yet bound (12)(13)(14). Nowaczyk et al (32) detected both Psb27 and Psb28 in a subcomplex purified from the ΔpsbJ mutant of Thermosynechococcus elongatus.…”

“…1D) and grows photoautotrophically, but the complexes are more susceptible to photodamage and require more frequent repair than wild-type cells (30). The elevated level of Psb28 implies an increased steady-state concentration of the RC47 complex (19)(20)(21)(22), which is expected, as RC47 is a key intermediate in the PSII repair cycle (8).…”

“…Psb28 is found across a wide range of cyanobacteria and has homologs in higher plants, but relatively little is known about its binding site and function (18). Dobáková et al (19) found that Psb28 binds mainly to the monomeric CP43-less PSII assembly intermediate, referred to as "RC47," a finding confirmed subsequently (20)(21)(22). Although deletion of the psb28 gene had little physiological effect under typical growth conditions (19,20,22), its absence impaired PSII recovery after photodamage under high-light conditions, especially at high temperature (20), and under intermittent high-light/dark conditions (22), suggesting a critical role of this protein during increased PSII turnover.…”

“…Dobáková et al (19) found that Psb28 binds mainly to the monomeric CP43-less PSII assembly intermediate, referred to as "RC47," a finding confirmed subsequently (20)(21)(22). Although deletion of the psb28 gene had little physiological effect under typical growth conditions (19,20,22), its absence impaired PSII recovery after photodamage under high-light conditions, especially at high temperature (20), and under intermittent high-light/dark conditions (22), suggesting a critical role of this protein during increased PSII turnover. Psb28 is the only known extrinsic cyanobacterial PSII protein found on the cytoplasmic surface of PSII (6,8,9,19), and it may interact with CP47, based on the comigration of these two proteins during native gel electrophoresis (19).…”