Abstract
The biogenesis and assembly of photosynthetic multisubunit protein complexes is assisted by a series of nucleus-encoded auxiliary protein factors. In this study, we characterize the dac mutant of Arabidopsis (Arabidopsis thaliana), which shows a severe defect in the accumulation of the cytochrome b(6)/f complex, and provide evidence suggesting that the efficiency of cytochrome b(6)/f complex assembly is affected in the mutant. DAC is a thylakoid membrane protein with two predicted transmembrane domains that is conserved from cyanobacteria to vascular plants. Yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitation analyses revealed a specific interaction between DAC and PetD, a subunit of the cytochrome b(6)/f complex. However, DAC was found not to be an intrinsic component of the cytochrome b(6)/f complex. In vivo chloroplast protein labeling experiments showed that the labeling rates of the PetD and cytochrome f proteins were greatly reduced, whereas that of the cytochrome b(6) protein remained normal in the dac mutant. DAC appears to be a novel factor involved in the assembly/stabilization of the cytochrome b(6)/f complex, possibly through interaction with the PetD protein.
Highlights
The biogenesis and assembly of photosynthetic multisubunit protein complexes is assisted by a series of nucleus-encoded auxiliary protein factors
The results showed that the levels of the Cyt b6/f subunits, Cyt f, Cyt b6, PetC, and PetD, were decreased to 10% to 15% of wild-type levels and that the protein contents of D1 and PsbO of PSII were reduced to 50% to 70% of wild-type levels
The biogenesis and assembly of the Cyt b6/f complex is a complicated process that is regulated at the levels of transcription, translation, and complex assembly (Bruce and Malkin, 1991; Saint-Marcoux et al, 2009; Boulouis et al, 2011)
Summary
The biogenesis and assembly of photosynthetic multisubunit protein complexes is assisted by a series of nucleus-encoded auxiliary protein factors. DAC appears to be a novel factor involved in the assembly/stabilization of the cytochrome b6/f complex, possibly through interaction with the PetD protein. The CCS pathway was originally discovered in the green alga C. reinhardtii through genetic studies of ccs mutants (for cytochrome c synthesis) that display a specific defect in membrane-bound Cyt f and soluble Cyt c6, two thylakoid lumen-resident c-type cytochromes functioning in photosynthesis (Xie and Merchant, 1998). In the CCS pathway, six loci that include plastid ccsA and nuclear CCS1 to CCS5 have been found in C. reinhardtii (Xie and Merchant, 1998) In these mutants, the apocytochrome is normally synthesized, targeted, and processed, but heme attachment is perturbed. Heme binding is not a prerequisite for the assembly of Cyt b6 into the Cyt b6/f complex, the fully formed Cyt b6/f showed an increased sensitivity to protease (Saint-Marcoux et al, 2009)
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