FIP, a Putative Regulator of FtsH5 in Chloroplasts, is Involved with Abiotic Stress Response in Arabidopsis

Chloroplast proteases are related to a fine regulation of the intraplastid proteome, which interfere in a wide set of physiological processes. Eukaryotic AAA/FtsH proteases play a key role in protein quality control of membrane-inserted proteins in mitochondria and chloroplasts. In Arabidopsis thaliana, thylakoidal FtsH proteases form a heterohexameric complex made by FtsH1/FtsH5 (type A) and FtsH2/FtsH8 (type B) subunits. This complex is responsible for turnover of the photo-damaged PSII reaction center protein D1. In previous experiments, our group identified a putative interacting protein named FIP (FtsH5 Interacting Protein), which has been proved to interact with FtsH5 and to be located at the thylakoid membranes. Mirror tree analysis showed the existence of coevolution between FIP and type A FtsH proteases, but not to type B. In addition, extensive protein-protein interaction experiments showed that FIP interacts with different chloroplast proteins involved in photosynthesis, suggesting a wide role in plastid biogenesis. Interestingly, FIP is down-regulated in several abiotic stresses. Phenotypic differences among fip knock-down mutants, wild type and FIP-overexpressing (OE) plants were not significant under control conditions. However, under abiotic stress conditions such as, high salt, increased osmotic potential and high light intensity indicated that mutant plants were considerably more tolerant to different stresses. Overall, these results suggest that FIP might be a negative regulator of the FtsH5 activity.