Abstract
Photosystem II (PSII) is a multisubunit membrane protein complex that is assembled in a sequence of steps. However, the molecular mechanisms responsible for the assembly of the individual subunits into functional PSII complexes are still largely unknown. Here, we report the identification of a chloroplast protein, Low PSII Accumulation3 (LPA3), which is required for the assembly of the CP43 subunit in PSII complexes in Arabidopsis (Arabidopsis thaliana). LPA3 interacts with LPA2, a previously identified PSII CP43 assembly factor, and a double mutation of LPA2 and LPA3 is more deleterious for assembly than either single mutation, resulting in a seedling-lethal phenotype. Our results indicate that LPA3 and LPA2 have overlapping functions in assisting CP43 assembly and that cooperation between LPA2 and LPA3 is essential for PSII assembly. In addition, we provide evidence that LPA2 and LPA3 interact with Albino3 (Alb3), which is essential for thylakoid protein biogenesis. Thus, the function of Alb3 in some PSII assembly processes is probably mediated through interactions with LPA2 and LPA3.
Highlights
Photosystem II (PSII) is a multisubunit membrane protein complex that is assembled in a sequence of steps
The thylakoid lumen protein HCF136 is known to be required for the formation of PSII, since the hcf136 mutant is capable of synthesizing plastid-encoded proteins, but it does not appear to accumulate any stable PSII complexes, due to blockage of the assembly of the PSII reaction center (Meurer et al, 1998; Plucken et al, 2002)
The results showed that levels of the PSII core proteins D1, D2, CP47, and CP43 were reduced to approximately 30% of wild-type levels, contents of the oxygen-evolving complex protein PsbO, LHCII, and the PSI reaction center proteins PsaA/B were approximately 80%, 90%, and 75% of wild-type levels, respectively, while contents of cytochrome f of the cytochrome b6/f complex and the b-subunit of ATP synthase were slightly increased in both 12-d-old and 5-week-old lpa3 plants (Fig. 1)
Summary
Photosystem II (PSII) is a multisubunit membrane protein complex that is assembled in a sequence of steps. Alb3.1, a homolog of Arabidopsis (Arabidopsis thaliana) Albino (Alb3), is essential for the efficient assembly of PSII in Chlamydomonas reinhardtii, probably through interactions with D1 following its insertion (Ossenbuhl et al, 2004), and another Alb homolog, Alb3.2, appears to be required for photosystem assembly in Chlamydomonas (Gohre et al, 2006). The LPA1 protein appears to be an integral membrane chaperone required for efficient assembly of the PSII core complex, probably through direct interaction with D1 (Peng et al, 2006). LPA2, which interacts with Alb, forms a protein complex that assists CP43 assembly within PSII (Ma et al, 2007) These findings suggest that each stage of the PSII assembly process is assisted by one or more specific assembly factors, most of which have not yet been identified
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