Bacteria occupy their surfaces with long proteinaceous pili (fimbriae) that extend from the cell surface to mediate adherence, genetic exchange, biofilm formation, and host cell invasion. The Gram-positive pathogen Corynebacterium diphtheriae assembles three distinct sortase-crosslinked pilus structures (SpaA, SpaD, and SpaH pili) that are critical for host colonization. In C. diphtheriae , sortase A (SrtA), a class C enzyme, regulates pilin incorporation into SpaA pili. Activation of SrtA requires displacement of an inhibitory loop, termed the “lid,” which normally occludes the active site. Despite its central role in pilus construction, the molecular basis of lid opening and pilin specificity remains poorly understood. Here, we employ all-atom molecular dynamics simulations of SrtA embedded in membrane bilayers to investigate lid conformational dynamics. We examine transmembrane interactions of SrtA with the shaft pilin SpaA and the minor pilin SpaB, and highlight the SpaA connector region, which lies adjacent to the lid and may stabilize its closed conformation. Our results suggest how membrane environment and pilin contacts modulate lid behavior, providing insight into the regulation of SpaA pilus biogenesis in C. diphtheriae .
Cheatham et al. (Sun,) studied this question.