Non-classical Protein Excretion Is Boosted by PSMα-Induced Cell Leakage

Release of cytoplasmic proteins into the supernatant occurs both in bacteria and eukaryotes. Because the underlying mechanism remains unclear, the excretion of cytoplasmic proteins (ECP) has been referred to as non-classical protein secretion.'' We show that none of the known specific protein transport systems of Gram-positive bacteria are involved in ECP. However, the expression of the cationic and amphipathic alpha-type phenol-soluble modulins (PSMs), particularly of PSM alpha 2, significantly increase ECP, while PSM beta peptides or delta-toxin have no effect on ECP. Because psm expression is strictly controlled by the accessory gene regulator (agr), ECP is also reduced in agr-negative mutants. PSM alpha peptides damage the cytoplasmic membrane, as indicated by the release of not only CPs but also lipids, nucleic acids, and ATP. Thus, our results show that in Staphylococcus aureus, PSM alpha peptides non-specifically boost the translocation of CPs by their membrane-damaging activity.