Shotgun Secretome Analysis of Synechocystis sp. PCC 6803 Response to Phosphate Limitation

Aims: Secretomics will lead to an increase in understanding of how cells combine the concerted action of secreted protein networks with their internal and external environments. Methods: Shotgun proteomics were used to analyze Synechocystis sp. PCC 6803 secreted proteins and results showed the identification of unique proteins, 13 in normal medium and 7 in phosphate-depleted BG-11 medium, respectively. Results: Six proteins were secreted under both normal and phosphate-depleted conditions (slr0513, slr1667, sll0654, sll1694, sll1578, sll1009), but only one secreted protein (Sll1694) was commonly detected by shotgun proteomics in the present study and previous gel-based studies. Both the number of secreted proteins and their classifications decreased in phosphate-depleted conditions. The peptide count of hypothetical protein codified by slr1667 decreased significantly under phosphate-depleted conditions, whereas alkaline phosphatase protein codified by sll0654 increases significantly under phosphate- depleted conditions. Results of quantitative real-time PCR showed that the transcript level of alkaline phosphatases (sll0654) was higher in phosphate-depleted BG-11 medium than in normal medium, and the transcript level of proteins related to twitching motility decreased significantly. Conclusion: This comprehensive data set provides novel insights into the composition and function of the cyanobacterium secretome and improves understanding of the biological processes at work between cyanobacterium and its environment.