HCF243 Encodes a Chloroplast-Localized Protein Involved in the D1 Protein Stability of the Arabidopsis Photosystem II Complex    

Abstract

Numerous auxiliary nuclear factors have been identified to be involved in the dynamics of the photosystem II (PSII) complex. In this study, we characterized the high chlorophyll fluorescence243 (hcf243) mutant of Arabidopsis (Arabidopsis thaliana), which shows higher chlorophyll fluorescence and is severely deficient in the accumulation of PSII supercomplexes compared with the wild type. The amount of core subunits was greatly decreased, while the outer antenna subunits and other subunits were hardly affected in hcf243. In vivo protein-labeling experiments indicated that the synthesis rate of both D1 and D2 proteins decreased severely in hcf243, whereas no change was found in the rate of other plastid-encoded proteins. Furthermore, the degradation rate of the PSII core subunit D1 protein is higher in hcf243 than in the wild type, and the assembly of PSII is retarded significantly in the hcf243 mutant. HCF243, a nuclear gene, encodes a chloroplast protein that interacts with the D1 protein. HCF243 homologs were identified in angiosperms with one or two copies but were not found in lower plants and prokaryotes. These results suggest that HCF243, which arose after the origin of the higher plants, may act as a cofactor to maintain the stability of D1 protein and to promote the subsequent assembly of the PSII complex.PSII is a multisubunit protein-pigment complex embedded in the thylakoid membrane that harnesses light energy to split water into oxygen, protons, and electrons. It comprises more than 20 different subunits, most of which are integral membrane proteins and bind numerous cofactors (Wollman et al., 1999;Iwata and Barber, 2004;Nelson and Yocum, 2006). The PSII reaction center complex is composed of the D1 and D2 proteins, the a-and b-subunits of cytochrome b 559 , and the PsbI protein. The D1 and D2 heterodimers bind all the essential redox components of PSII required for primary charge separation and subsequent electron transfer (Nanba and Satoh, 1987). In addition, PSII core complexes also contain the intrinsic chlorophyll a-binding proteins (CP43 and CP47), the extrinsic oxygen-evolving complex (33-, 23-, and 17-kD proteins), and the other low-molecular-mass proteins (Bricker and Ghanotakis, 1996). The functional form of PSII in the thylakoid membrane consists of the PSII core and the associated light-harvesting complex (Nelson and Yocum, 2006).Despite considerable advances in the elucidation of the structure and function of PSII, knowledge of the assembly of this multiprotein complex is only in its infancy. A number of studies have proved that this assembly is likely to involve multistep processes (Rokka et al., 2005). The first step is formation of the PSII reaction center, in which the D1 protein is incorporated into a precomplex, probably consisting of the D2, cytochrome b 559 , and PsbI proteins (Adir et al., 1990;van Wijk et al., 1997;Mü ller and Eichacker, 1999;Zhang et al., 1999). Subsequently, CP47 and CP43, core antenna subunits that bind chlorophyll a, are recruited to form the PSII core compl...

Evidence for a Role of Chloroplastic m-Type Thioredoxins in the Biogenesis of Photosystem II in Arabidopsis

“…2B). These spectroscopic properties also have been observed in a number of mutants with defects in PSII, such as low PSII accumulation1 (lpa1; Peng et al, 2006), lpa2 (Ma et al, 2007), lpa3 (Cai et al, 2010), photosynthesis affected mutant68 (pam68; Armbruster et al, 2010), and hcf243 (Zhang et al, 2011). Moreover, immunoblot analysis confirmed that essential subunits of the PSII complex Figure 7.…”

The Atypical Short-Chain Dehydrogenases HCF173 and HCF244 Are Jointly Involved in Translational Initiation of the psbA mRNA of Arabidopsis

“…An indirect effect is conceivable as a feedback regulation in consequence of the impaired D1 assembly. However, a feedback regulation is not supported by the known D1 assembly mutants of Arabidopsis, lpa1, pam68, and hcf243 (Peng et al, 2006;Armbruster et al, 2010;Zhang et al, 2011). For all these mutants, a reduced labeling of the D1 protein was observed, but none of them showed impaired ribosomal loading for the psbA mRNA.…”

Regulatory factors for the assembly of thylakoid membrane protein complexes

“…However, removal of PAM68 results in different compensatory mechanisms in plants and cyanobacteria: in Synechocystis the transition from the reaction centre to later PSII intermediates appears to be accelerated when PAM68 is absent [19]. The higherplant-specific HCF243 chloroplast protein has been implicated in maintaining the stability of the D1 protein and promoting subsequent PSII assembly [62]. LPA2 and LPA3 (low PSII accmulation 2 and 3), two other thylakoid membrane proteins, have overlapping functions in assisting CP43 assembly, and the cooperation between LPA2 and LPA3 is essential for PSII assembly [20,21].…”