Evidence for a Role of VIPP1 in the Structural Organization of the Photosynthetic Apparatus in Chlamydomonas

Abstract

The vesicle-inducing protein in plastids (VIPP1) was suggested to play a role in thylakoid membrane formation via membrane vesicles. As this functional assignment is under debate, we investigated the function of VIPP1 in Chlamydomonas reinhardtii. Using immunofluorescence, we localized VIPP1 to distinct spots within the chloroplast. In VIPP1-RNA interference/artificial microRNA cells, we consistently observed aberrant, prolamellar body-like structures at the origin of multiple thylakoid membrane layers, which appear to coincide with the immunofluorescent VIPP1 spots and suggest a defect in thylakoid membrane biogenesis. Accordingly, using quantitative shotgun proteomics, we found that unstressed vipp1 mutant cells accumulate 14 to 20% less photosystems, cytochrome b 6 f complex, and ATP synthase but 30% more light-harvesting complex II than control cells, while complex assembly, thylakoid membrane ultrastructure, and bulk lipid composition appeared unaltered. Photosystems in vipp1 mutants are sensitive to high light, which coincides with a lowered midpoint potential of the Q A /Q A 2 redox couple and increased thermosensitivity of photosystem II (PSII), suggesting structural defects in PSII. Moreover, swollen thylakoids, despite reduced membrane energization, in vipp1 mutants grown on ammonium suggest defects in the supermolecular organization of thylakoid membrane complexes. Overall, our data suggest a role of VIPP1 in the biogenesis/assembly of thylakoid membrane core complexes, most likely by supplying structural lipids.

Conditional Depletion of the Chlamydomonas Chloroplast ClpP Protease Activates Nuclear Genes Involved in Autophagy and Plastid Protein Quality Control

“…The luciferase protein was weakly expressed under other conditions except in cells subjected to high light and followed essentially the same pattern as the endogenous Vipp2 protein. Induction of Vipp2 protein and mRNA during high-light exposure was previously reported (Nordhues et al, 2012). Importantly, the induction of Vipp2-gLUX also occurred during ClpP1 inactivation in the dark.…”

“…Instead, it may be triggered by accumulation of unfolded proteins and/or damaged thylakoid membranes, which is also likely to occur upon photooxidative damage induced under high light (Figure 8). However, VIPP2 is not induced by heat shock, a treatment that is also expected to unfold proteins (Nordhues et al, 2012). Possibly, depletion of ClpP1 may induce a stronger and more chloroplast-specific response to which Vipp2 is reactive.…”

“…The expression and localization of these two proteins is regulated in an interdependent way; Alb3.2 is associated with Vipp1 and Vipp1 expression is considerably enhanced when the expression of Alb3.2 is downregulated (Göhre et al, 2006). Moreover, Vipp1 expression is influenced by the light conditions (Liu et al, 2005;Nordhues et al, 2012).…”

“…Whether the released polar lipids serve as a cellular membrane source during autophagy remains an open question. Vipp1 and Vipp2 are partially redundant and have been proposed to be involved in the assembly and biogenesis of thylakoid core complexes, perhaps by providing structural lipids (Nordhues et al, 2012). The absence of induction of Lhcsr3, Hsp70B, and Hsp90C in Vipp1-RNAi strains upon high-light exposure suggests that Vipp1 might play a role in the high-light retrograde signaling pathway.…”

IM30 triggers membrane fusion in cyanobacteria and chloroplasts

“…However, further studies have indicated that IM30 is involved in maintaining the structure and stability of internal membranes in chloroplasts and cyanobacteria, similar to its bacterial homologue PspA, and thus has a more protective function 27,28 . In addition, IM30 has been found to enhance protein translocation via the chloroplast Tat pathway 29 and involvement in photosystem biogenesis has been suggested 21,[30][31][32] . Currently, no model combining these diverse IM30 functions has been proposed, and the exact role of IM30 remains enigmatic.…”