aureus USA300 cells (Figure 5C, white arrow). Interestingly, its production caused a reduction of wild-type EssB (Figure 5C, blue arrow). EssB was also unstable in the merodiploid strain expressing EssBMC (Figure

5C; purple arrow). Not surprisingly, destabilization of EssB by either EssBN or EssBMC led to altered expression and secretion of EsxA (Figure 5D). Sedimentable variants encompassing the PTMD, EssBNM and EssBMC, caused a dominant-negative phenotype on the activity of wild-type ATM/ATR cancer EssB and as a result expression or secretion of EsxA were altered. On the contrary, EssBΔM lacking PTMD remained soluble and did not interfere with EssB function. Taken together, these data suggest that EssB variants that sediment with staphylococcal membranes interfere with the stability or function of endogenous EssB and as a consequence EsxA production and secretion are also affected. Thus, EssB is part of the secretion machine and its multimerization and possible association with other Ess components 17DMAG enables the secretion of EsxA. Discussion Secreted proteins are generally tagged with topogenic sequences for recognition by a specific secretion machine and transport across the plasma membrane. Over a third of all proteins synthesized by a bacterial cell carry leader peptides, the topogenic signal for recognition by the Sec machine

[24]. The corresponding sec genes are scattered on the chromosome although their gene products assemble specifically at the membrane to mediate the faithful secretion of a variety of polypeptides. Bacteria have also evolved highly specialized secretion systems for the transport of specific proteins across lipid bilayers and organized the genes encoding machine components and their Carnitine palmitoyltransferase II substrates into clusters whose expression is controlled by adjacent transcriptional units [25, 26]. The S. aureus ESS cluster represents one such SCH772984 dedicated

secretion pathway. ESS genes are encoded within an eleven gene cluster and when deleted impair the production or secretion of small proteins with the WXG amino acid signature. Here, we have begun the characterization of EssB, one of the proteins of the staphylococcal ESS cluster (Figure 1). Bioinformatic searches revealed that EssB is found in Gram-positive bacteria that harbor ESS gene clusters closely related to the staphylococcal ESS pathway (Figure 1). The protein belongs to the Cluster of Orthologous Groups of protein COG4499 and is annotated as a predicted membrane protein homologous to B. subtilis YukC (Figure 1). COG4499 protein members are all arranged in a single architecture meaning that the entire protein defines a single domain that is never truncated nor fused with another protein domain.