Proteomic Study of the Impact of Hik33 Mutation in <i>Synechocystis</i> sp. PCC 6803 under Normal and Salt Stress Conditions

Cyanobacteria are the only prokaryotes possessing plasma, thylakoid, and outer membranes. The plasma membrane of a cyanobacterial cell is essential for the biogenesis of cyanobacterial photosystems and serves as a barrier against environmental stress. We previously identified dozens of salt-responsive proteins in the plasma membrane of <i>Synechocystis</i> sp. PCC 6803. Five histidine kinases (Hiks) including Hik33 were also proposed to be involved in the perception of salt stress in <i>Synechocystis</i>. In this study, we analyzed proteomic profiles of the plasma membrane from a <i>hik33</i>-knockout mutant (ΔHik33) under normal and salt-stress conditions. Using 2D-DIGE followed by mass spectrometry analysis, we identified 26 differentially expressed proteins in ΔHik33 mutant cells. Major changes, due to the Hik33 mutation, included the substrate-binding proteins of ABC transporters, such as GgtB and FutA1, regulatory proteins including MorR and Rre13, as well as several hypothetical proteins. Under salt-stress conditions, the Hik33 mutation reduced levels of 7 additional proteins, such as NrtA, nitrate/sulfonate/bicarbonate-binding protein and LexA, and enhanced levels of 9 additional proteins including SphX. These observations suggest a substantial rearrangement in the plasma membrane proteome of <i>Synechocystis</i> due to the loss of <i>hik33</i>. Furthermore, a comprehensive molecular network was revealed in ΔHik33 mutant coping with salt stress