membrane-bound FtsH protease is involved in osmoregulation in Synechocystis sp. PCC 6803: the compatible solute synthesizing enzyme GgpS is one of the targets for proteolysis

Protein quality control and proteolysis are involved in cell maintenance and environmental acclimatization in bacteria and eukaryotes. The AAA protease FtsH2 of the cyanobacterium Synechocystis sp. PCC 6803 was identified during a screening for mutants impaired in osmoregulation. The ftsH2⁻ mutant was salt sensitive because of a decreased level of the osmoprotectant glucosylglycerol (GG). In spite of wild type-like transcription of the ggpS gene in ftsH2⁻ cells the GgpS protein content increased but only low levels of GgpS activity were observed. Consequently, salt tolerance of the ftsH2⁻ mutant decreased while addition of external osmolyte complemented the salt sensitivity. The proteolytic degradation of the GgpS protein by FtsH2 was demonstrated by an in vitro assay using inverted membrane vesicles. The GgpS is part of a GG synthesizing complex, because yeast two-hybrid screens identified a close interaction with the GG-phosphate phosphatase. Besides GgpS as the first soluble substrate of a cyanobacterial FtsH protease, several other putative targets were identified by a proteomic approach. We present a novel molecular explanation for the salt-sensitive phenotype of bacterial ftsH⁻ mutants as the result of accumulation of inactive enzymes for compatible solute synthesis, in this case GgpS the key enzyme of GG synthesis.