Molecular dynamics insights into the membrane-dependent folding of phenol-soluble modulin monomers

Phenol-soluble modulins (PSMs) are critical virulence factors in Staphylococcus aureus. Although the aggregation of multiple αPSMs and their effects on cellular metabolism have been studied, the initial folding process of individual αPSM monomers, which is the fundamental prerequisite for aggregation, is uninvestigated from the mechanism of molecular interactions. Molecular dynamics (MD) studies, which have primarily centered on multi-PSM complexes, leaving the single-monomer folding process in lipid environments uninvestigated. To address this gap, this study shifts the focus from aggregation to folding initiation by employing extensive MD simulations to systematically investigate how single αPSM monomers fold into stable α-helices and maintain this secondary structure in different membrane environments. Fourteen distinct MD simulations were performed for two folded αPSMs (αPSM1 and αPSM3) and their linear counterparts (αPSM1_lin and αPSM3_lin) in aqueous solution and in two membrane lipid bilayers (individually composed of DOPC and DOPG, respectively), which have been employed in prior experimental work. Using DSSP, secondary structure relative metrics, native contact (NC), and MMGBSA analyses, we characterized the folding properties of αPSMs and their interactions with the membranes. Our results demonstrate that both DOPC and DOPG lipids decelerate the folding kinetics of αPSM1 and αPSM3, with DOPG exerting a more pronounced influence on the folding energetics than DOPC. These MD findings are consistent with experimental observations and provide novel mechanistic insights into the crucial early stage of αPSM folding, which modulates the subsequent virulence-related aggregation processes.