The type VII protein secretion system (T7SS) is found in many Gram-positive bacteria and in pathogenic mycobacteria. All T7SS substrate proteins described to date share a common helical domain architecture at the N-terminus that typically interacts with other helical partner proteins, forming a composite signal sequence for targeting to the T7SS. The C-terminal domains are functionally diverse and in Gram-positive bacteria such as Staphylococcus aureus often specify toxic anti-bacterial activity. Here we describe the first example of a new class of T7 substrate, TslA, that has an unexpected reverse domain organisation. TslA is widely found across Bacillota including Staphylococcus, Enterococcus and Listeria. We show that the S. aureus TslA N-terminal domain is a phospholipase A with anti-staphylococcal activity that is neutralised by the immunity lipoprotein TilA. Two small helical partner proteins, TlaA1 and TlaA2 are essential for T7-dependent secretion of TslA and at least one of these interacts with the TslA C-terminal domain to form a helical stack. Cryo-EM analysis of purified TslA complexes indicate that they share structural similarity with canonical T7 substrates. Our findings suggest that the T7SS has the extraordinary feature of recognising a secretion signal present at either end of a substrate.

The type VIIb secretion system (T7SSb) is a multisubunit protein export machine found in Gram-positive Firmicute bacteria which plays a key role in interbacterial competition. The T7SSb secretes a variety of toxic effector proteins targeting closely related strains, however the mechanism of secretion and the roles of numerous conserved genes within T7SSb gene clusters remain unknown. EsaD is a nuclease toxin secreted by the Staphylococcus aureus T7SSb, which forms a pre-secretion complex with its cognate immunity protein, EsaG, and chaperone EsaE. Encoded upstream of EsaD are three small secreted proteins of unknown function, EsxB, EsxC and EsxD. Here we show that these three proteins bind to EsaD and function as EsaD export factors and we report the first structural information for a complete T7SSb substrate pre-secretion complex. Cryo-EM of the EsaDEG trimer and the EsaDEG-EsxBCD hexamer shows that incorporation of EsxBCD confers an elongated conformation comprising a flexible globular cargo domain attached to a long narrow shaft that is likely to be crucial for efficient toxin export.

Successful colonisation by the opportunistic pathogen Staphylococcus aureus depends on its ability to interact with other microorganisms. S. aureus strains harbour a T7b-subtype type VII secretion system (T7SSb), a protein secretion system found in a wide variety of Firmicutes which functions in bacterial antagonism and virulence. Traditional assays for monitoring protein secretion by the T7SSb rely on concentrating cell culture supernatants for immunoblot analysis, a procedure which is time consuming, low throughput, non-quantitative and has low sensitivity. To overcome some of these disadvantages we have utilised NanoLuc Binary Technology to develop a simple assay to monitor protein secretion via detection of bioluminescence in 384-well plates. High-throughput analysis of secretion in replicate cultures reveals overlapping distributions for T7+ and T7- populations highlighting a significant limitation of lower-throughput methods. We also demonstrate the utility of the NanoLuc assay in comparing T7-dependent protein secretion across different strains and find differences in the temperature dependence of T7 secretion across S. aureus isolates.