Function of VIPP1 (vesicle-inducing protein in plastids) in Chlamydomonas reinhardtii
André Nordhues, Mark Aurel Schöttler, Giovanni Finazzi, Heiko Lokstein, Stephanie Schlede, Stefan Geimer, Salim Al-Babili, Jochen Golecki, and Michael Schroda
Max Planck Institute of Molecular Plant Physiology, Potsdam-Golm, Germany
The vesicle-inducing protein in plastids 1 (VIPP1) is conserved from cyanobacteria to higher plants and is believed to be essential for the biogenesis of thylakoid membranes via vesicle traffic. In Arabidopsis, reduction of VIPP1 to ~20 % of wild type levels caused severe defects in thylakoid structure. Surprisingly, Chlamydomonas VIPP1-RNAi strains that contain less than 3 % of VIPP1 compared to wild type did not display any obvious defects in thylakoid structure or in the composition of thylakoid membrane protein complexes at low light intensities. However, PSII activity and Chla/b ratio were slightly reduced in VIPP1-RNAi strains when compared to wild type. Moreover, VIPP1-RNAi strains were extremely sensitive to higher light intensities: prolonged exposure at 1000 µE m-2 s-1 resulted in bleaching and photodestruction of the photosystems and the b6f complex while the ATP synthase remained unaffected. In addition, levels of zeaxanthin and LHCSR3 barely increased in high-light treated VIPP1-RNAi strains. Reduced induction of photoprotective mechanisms in VIPP1-RNAi strains correlated with reduced linear electron transport rates and reduced thylakoid membrane energetization. Interestingly, PSII activity and/or antenna coupling rapidly decreased in VIPP1-RNAi strains exposed to heat shock in low light, suggesting that defects were present but not evident in VIPP1-RNAi strains already under low-light conditions. Taken together, our data suggest that VIPP1 most likely is not involved in vesicle-mediated thylakoid biogenesis. Rather, VIPP1 might be necessary for the supramolecular organization of protein complexes within the thylakoid membranes. Alternatively, VIPP1 might support Alb3.2-mediated import of distinct subunits of the photosystems and incompletely assembled photosystems may in turn impair the integrity of thylakoid membranes.
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