Abstract
Type VII secretion systems (T7SS) are found in many bacteria and secrete proteins involved in virulence and bacterial competition. In Staphylococcus aureus the small ubiquitin-like EsaB protein has been previously implicated as having a regulatory role in the production of the EsxC substrate. Here we show that in the S. aureus RN6390 strain, EsaB does not genetically regulate production of any T7 substrates or components, but is indispensable for secretion activity. Consistent with EsaB being an essential component of the T7SS, loss of either EsaB or EssC are associated with upregulation of a common set of iron acquisition genes. However, a further subset of genes were dysregulated only in the absence of EsaB. Quantitative western blotting indicates that EsaB is present at very low levels in cells. Substitution of a highly conserved threonine for alanine or arginine resulted in a loss of EsaB activity and destabilisation of the protein. Taken together our findings show that EsaB is essential for T7SS activity in RN6390.
Highlights
Protein secretion systems are nanomachines employed by bacteria to transport protein substrates across their cell envelopes
The T7SS was initially described in the pathogenic mycobacteria Mycobacterium tuberculosis and Mycobacterium bovis, where the ESX-1 T7SS was shown to be essential for virulence, due to the secretion of two major T-cell antigens EsxA and EsxB [3,4,5]
We have addressed the role of EsaB in S. aureus T7 secretion using strain RN6390
Summary
Protein secretion systems are nanomachines employed by bacteria to transport protein substrates across their cell envelopes. The T7SS was initially described in the pathogenic mycobacteria Mycobacterium tuberculosis and Mycobacterium bovis, where the ESX-1 T7SS was shown to be essential for virulence, due to the secretion of two major T-cell antigens EsxA (formerly known as ESAT-6) and EsxB (formerly known as CFP-10) [3,4,5]. In Mycobacteria, three further membrane proteins EccB, EccD and EccE assemble with EccC to form a large 1.5 MDa core complex [7, 8]. This complex further associates with a membrane-bound mycosin serine protease, MycP, that is essential for T7 protein secretion and for stability of the membrane complex [9]
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