Synthetic Effects of secG and secY2 Mutations on Exoproteome Biogenesis in Staphylococcus aureus

The Gram-positive pathogenStaphylococcus aureussecretes various proteins into its extracellular milieu. Bioinformatics analyses have indicated that most of these proteins are directed to the canonical Sec pathway, which consists of the translocation motor SecA and a membrane-embedded channel composed of the SecY, SecE, and SecG proteins. In addition,S. aureuscontains an accessory Sec2 pathway involving the SecA2 and SecY2 proteins. Here, we have addressed the roles of the nonessential channel components SecG and SecY2 in the biogenesis of the extracellular proteome ofS. aureus. The results show that SecG is of major importance for protein secretion byS. aureus.Specifically, the extracellular accumulation of nine abundant exoproteins and seven cell wall-bound proteins was significantly affected in ansecGmutant. No secretion defects were detected for strains with asecY2single mutation. However, deletion ofsecY2exacerbated the secretion defects ofsecGmutants, affecting the extracellular accumulation of one additional exoprotein and one cell wall protein. Furthermore, ansecG secY2double mutant displayed a synthetic growth defect. This might relate to a slightly elevated expression ofsraP, encoding the only known substrate for the Sec2 pathway, in cells lacking SecG. Additionally, the results suggest that SecY2 can interact with the Sec1 channel, which would be consistent with the presence of a single set ofsecEandsecGgenes inS. aureus.