Proteomic analysis of Haloferax volcanii reveals salinity-mediated regulation of the stress response protein PspA

A proteomic survey of the halophilic archaeon Haloferax volcanii was performed by comparative two-dimensional gel electrophoresis in order to determine the molecular effects of salt stress on the organism. Cells were grown under optimal (2.1 M) and high (3.5 M) NaCl conditions. From this analysis, over 44 protein spots responsive to these conditions were detected. These spots were excised, digested in-gel with trypsin, subjected to OSTAR tandem mass spectrometry (LC/MS/MS) analysis, and identified by comparing the MS/MS-derived peptide sequence to that deduced from the H. volcanii genome. Approximately 40% of the proteins detected (18 in total) displayed differential abundance based on the detection of at least two peptide fragments per protein and overall MOWSE scores of >= 75 per protein. All of these identified proteins were either uniquely present or 2.3- to 26-fold higher in abundance under one condition compared to the other. The majority of proteins identified in this study were preferentially displayed under optimal salinity and primarily involved in translation, transport and metabolism. However, one protein of interest whose transcript levels were confirmed in these studies to be upregulated under high salt conditions was identified as a homologue of the phage shock protein PspA. The pspA gene belongs to the psp stress-responsive regulon commonly found among Gram-negative bacteria where its transcription is stimulated by a wide variety of stressors, including heat shock, osmotic shock and prolonged stationary-phase incubation. Homologues of PspA are also found among the genomes of cyanobacteria, higher plants and other Archaea, suggesting that this protein may retain some aspects of functional conservation across the three domains of life. Given its integral role in sensing a variety of membrane stressors in bacteria, these results suggest that PspA may play an important role in hypersaline adaptation in H. volcanii.