The extreme Albino3 (Alb3) C terminus is required for Alb3 stability and function in Arabidopsis thaliana
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The extreme Alb3 C terminus is important for Alb3 stability in a light dependent manner, but is dispensable for LHCP insertion or D1 synthesis.
YidC/Oxa1/Alb3 dependent insertion of membrane proteins is evolutionary conserved among bacteria, mitochondria and chloroplasts. Chloroplasts are challenged by the need to coordinate membrane integration of nuclear encoded, post-translationally targeted proteins into the thylakoids as well as of proteins translated on plastid ribosomes. The pathway facilitating post-translational targeting of the light-harvesting chlorophyll a/b binding proteins involves the chloroplast signal recognition particle, cpSRP54 and cpSRP43, as well as its membrane receptor FtsY and the translocase Alb3. Interaction of cpSRP43 with Alb3 is mediated by the positively charged, stromal exposed C terminus of Alb3. In this study, we utilized an Alb3 T-DNA insertion mutant in Arabidopsis thaliana lacking the last 75 amino acids to elucidate the function of this domain (alb3∆C). However, the truncated Alb3 protein (Alb3∆C) proved to be unstable under standard growth conditions, resulting in a reduction of Alb3∆C to 20 % of wild-type levels. In contrast, accumulation of Alb3∆C was comparable to wild type under low light growth conditions. Alb3∆C mutants grown under low light conditions were only slightly paler than wild type, accumulated almost wild-type levels of light harvesting proteins and were not affected in D1 synthesis, therefore showing that the extreme Alb3 C terminus is dispensable for both, co- and post-translational, protein insertion into the thylakoid membrane. However, reduction of Alb3∆C levels as observed under standard growth conditions resulted not only in a severely diminished accumulation of all thylakoid complexes but also in a strong defect in D1 synthesis and membrane insertion.
KeywordsChloroplast Thylakoid membrane Light harvesting complex Photosystem II Chlorophyll
Michaela Häusler is greatly acknowledged for excellent technical assistance. We would further like to thank Ralph Krafczyk for help with genotyping experiments. Alb3 antisera were a kind gift from Danja Schünemann as well as HCF136 antisera from Peter Westhoff. ATP synthase antisera were kindly provided by Stephan Greiner. Financial support from the German Research Council (DFG, SFB1035, project A4, to JS., SS), the Munich Center for Integrated Protein Science (CiPSM, Exc114/2) to JS, BB and SS, as well as the Academy of Finland Centre of Excellence in Molecular Biology of Primary Producers 271832 to PM and MK and the DFG Research Unit FOR2092 (Gr936 18-1) to BG is acknowledged.
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