An association between peptidoglycan synthesis and organization of the Streptococcus pyogenes ExPortal

The ExPortal of Streptococcus pyogenes is a focal microdomain of the cytoplasmic membrane that clusters the translocons of the general secretory pathway with accessory factors to facilitate the maturation of secreted polypeptides. While it is known that the ExPortal is enriched in anionic lipids, the mechanisms that organize the ExPortal are poorly understood. In the present study, we examined the role of the cell wall in organizing and maintaining the ExPortal. Removal of the cell wall resulted in a loss of ExPortal focal integrity accompanied by the circumferential redistribution of ExPortal lipid and protein components. A similar loss occurred upon treatment with gallidermin, a nonpermeabilizing lantibiotic that targets the lipid II precursor of peptidoglycan synthesis, and this treatment disrupted the secretion of several ExPortal substrates. Furthermore, several enzymes involved in the membrane-associated steps of lipid II synthesis, including MraY and MurN, were found to localize to a single discrete focus in the membrane that was coincident with the focal location of the secretory translocons and the anionic lipid microdomain. These data suggest that the ExPortal is associated with the site of peptidoglycan precursor synthesis and that peptidoglycan biogenesis influences ExPortal organization. These data add to an emerging literature indicating that cell wall biogenesis, cell division, and protein secretion are spatially coorganized processes.

Since Gram-positive bacteria lack a periplasmic space, they lack a protected compartment to spatially coordinate interaction between newly secreted proteins and the factors required to process them. This represents a significant problem for pathogens that depend on the secretion of toxins and cell wall-associated adhesins to cause disease. Streptococci solve this dilemma by restricting secretion and processing factors to a defined region of the membrane. However, the mechanisms that promote restriction are not understood. In this study, we show that restriction of these factors in the pathogen Streptococcus pyogenes is intimately linked with the presence of the cell wall and its synthesis. Furthermore, several cell wall synthesis proteins are also restricted to the site of protein secretion. This study contributes to our understanding of how the Gram-positive cell is organized to coordinate protein secretion and biogenesis with cell wall synthesis and to the ongoing development of antibiotics that target these processes.