Profiling and Compositional Analysis of the Exoproteome of Synechocystis Sp. PCC 6803

The exoproteome is one of the important components of the extracellular milieu of the unicellular cyanobacterium Synechocystis sp. PCC 6803, a photosynthetic model organism that is emerging as a potential cell factory for producing clean and renewable biofuel. Herein, we identified the exoproteome of this organism using high resolution mass spectrometry and analyzed the sources of origins of the identified extracellular proteins. In total, 201 proteins were identified, including the cell surface protein HlyA and the periplasmic protein FutA2 that are the most abundant in the exoproteome. The combined amount of the two proteins is estimated to constitute more than 50 percent of the total amount of proteins in the extracellular milieu. More than 100 proteins were predicted to be secretory or previously identified as periplasmic proteins, and the total amount of these proteins is nearly 88% of that of all identified extracellular proteins, establishing secretion as the major contributor to the protein population of the exoproteome. In addition, 2 proteins were found to be heavily glycosylated, suggesting that the exoproteome is an enriched pool of a certain subset of glycoproteins. Together, our work provided a comprehensive catalog of the Synechocystis exoproteome and a high valuable resource for the related research fields.