Type VIIb secretion system recruits a dedicated cell wall hydrolase EssH to enable effector secretion by Staphylococcus aureus

Abstract

Staphylococcus aureus is a pervasive human pathogen that heavily relies on protein secretion to exert its virulence strategies. S. aureus encodes a specialized type VIIb secretion system (T7SSb) that contributes to virulence and persistence in the host. T7SSb supports secretion of small proteins belonging to a WXG100 family as well as larger LXG polymorphic toxins. Secretion of these proteins is facilitated by the T7SSb complex that assembles in the cell envelope from core components EsaA, EssA, EssB, and the conserved ATPase EssC. T7SSb-mediated secretion also requires the cell wall hydrolase EssH that bears a cystine histidine-dependent amidase/peptidase (CHAP) domain. Hereby, we show that the N-terminal domain of EssH functions together with the CHAP domain to support T7SSb secretion. We find that EssH copurifies with the T7SSb core components and is required for the assembly of EsaA across the cell wall. Finally, secreted EssH is released into the extracellular milieu and degraded by the secreted staphylococcal protease, staphopain A. We propose a model to capture the function of the dedicated cell wall hydrolase EssH for T7SSb in S. aureus. IMPORTANCEStaphylococcus aureus is a leading cause of infections worldwide. S. aureus utilizes a specialized type VIIb secretion system (T7SSb) to persist in the infected host tissues as well as target competitor bacteria to establish its niche. T7SSb assembles into a multiprotein translocation complex and facilitates secretion of a set of small proteins and larger polymorphic toxins across the cytosolic membrane. Beyond the membrane, secreted proteins were thought to diffuse through the thick yet porous cell wall and release into the environment. Here, we demonstrate for the first time that S. aureus T7SSb extends across the cell wall via its EsaA subunit. Furthermore, accommodation of EsaA within the cell wall requires an associated cell wall hydrolase EssH and is essential for protein secretion via T7SSb. Thus, our findings provide a mechanistic insight for a coordinated cell wall processing and T7SSb assembly to support specialized protein secretion in S. aureus.