Structural foundation for the role of enterococcal PrgB in conjugation, biofilm formation and virulence

Abstract

AbstractType 4 Secretion Systems are a main driver for the spread of antibiotic resistance genes and virulence factors in bacteria. In Gram-positives, these secretion systems often rely on surface adhesins to enhance cellular aggregation and mating pair formation. One of the best studied adhesins is PrgB from the conjugative plasmid pCF10 ofEnterococcus faecalis, which has been shown to play major roles in conjugation, biofilm formation and importantly also in bacterial virulence. SinceprgBorthologs exist on a large number of conjugative plasmids in various different species, this makes PrgB a model protein for this widespread virulence factor. Here we report structures for almost the entire PrgB, in the presence or absence of DNA, using a combination of X-ray crystallography and cryo-EM. These reveal that PrgB undergoes a large conformational change upon DNA-binding and that it contains four immunoglobulin-like domains. We re-evaluate previously studied variants and present newin vivodata where specific domains or conserved residues have been mutated. For the first time we can show a decoupling of cellular aggregation from biofilm formation and conjugation inprgBmutant phenotypes. Based on the presented data, we propose a new functional model to explain how PrgB mediates its different functions. We hypothesize that the Ig-like domains act as a rigid stalk that both protect the previously studied polymer adhesin domain from proteolysis, as well as presenting it at the right distance from the cell wall.