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VIPP proteins aid thylakoid biogenesis and membrane maintenance in cyanobacteria, algae, and plants. Some members of the Chlorophyceae contain two VIPP paralogs termed VIPP1 and VIPP2, which originate from an early gene duplication event during the evolution of green algae. VIPP2 is barely expressed under nonstress conditions but accumulates in cells exposed to high light intensities or H2O2, during recovery from heat stress, and in mutants with defective integration (alb3.1) or translocation (secA) of thylakoid membrane proteins. Recombinant VIPP2 forms rod‐like structures in vitro and shows a strong affinity for phosphatidylinositol phosphate. Under stress conditions, >70% of VIPP2 is present in membrane fractions and localizes to chloroplast membranes. A vipp2 knock‐out mutant displays no growth phenotypes and no defects in the biogenesis or repair of photosystem II. However, after exposure to high light intensities, the vipp2 mutant accumulates less HSP22E/F and more LHCSR3 protein and transcript. This suggests that VIPP2 modulates a retrograde signal for the expression of nuclear genes HSP22E/F and LHCSR3. Immunoprecipitation of VIPP2 from solubilized cells and membrane‐enriched fractions revealed major interactions with VIPP1 and minor interactions with HSP22E/F. Our data support a distinct role of VIPP2 in sensing and coping with chloroplast membrane stress. The VIPP2 protein from Chlamydomonas reinhardtii localizes predominantly to chloroplast membranes under stresses inducing protein mistargeting and misfolding. VIPP2 interacts with VIPP1 as major partner and with HSP22E/F as minor partner and modulates a retrograde signal for HSP22E/F gene expression in high light. [ABSTRACT FROM AUTHOR]

A concise overview on the current knowledge of the proteolytic activities in chloroplasts is presented, with an emphasis on the proteolytic events associated with thylavoid membranes. The D1 reaction centre protein of photosystem II undergoes rapid light-dependent turnover and chlorophyll a/b-binding proteins are effectively degraded upon acclimation of plants to higher irradiances. Insights into the partially characterized proteolytic systems in each case will be presented, but the proteases involved still remain unknown. It can be envisaged, however, that the proteolysis is probably an as highly regulated phenomenon as the various steps during biosynthesis of the photosynthetic multiprotein complexes. From the protease point of view, more progress has recently been made in characterization of processing proteases involved m protein import into chloroplasts and in C-terminal processing of the Dl protein. Moreover, there are an increasing number of proteases in chloroplasts which have been discovered and identified as bacterial homologues. These include a Clp-type protease, a homologue of the bacterial protease FtsH and the cyanobacterial PcrA protease, all of which have a specific location in the chloroplast but their definite physiological substrates are still missing. Attempts are made to bring together the recent progress in the identification of proteases and characterisation of proteolytic events in chloroplasts. [ABSTRACT FROM AUTHOR]