Evidence for a Pleckstrin Homology Domain-containing Proteins as Novel Class of Leaderless Bacteriocins Transporters

Abstract

Bacteriocins biosynthetic pathway is initiated from an inactive precursor, which is composed of an N-terminal leader peptide attached to a C-terminal pro-peptide. However, leaderless bacteriocins (LLB) do not possess this N-terminal leader peptide, nor undergo post-translational modifications. These atypical bacteriocins are observed to be immediately active after their translation in the cytoplasm. However, although considered to be simple, the biosynthetic pathway of LLB remains to be fully understood. Enterocin DD14 (EntDD14) is a two-peptide LLB produced by Enterococcus faecalis 14, a strain isolated from meconium. In silico analysis of DNA encoding EntDD14 located a cluster of 10 genes ddABCDEFGHIJ, where ddE and ddF encode the peculiar DdE and DdF proteins, carrying pleckstrin homology (PH) domains. These modules are quite common in Eucarya proteins and are known to be involved in intracellular signalling or cytoskeleton organization. To elucidate their role within the EntDD14 genetic determinant, we constructed deletion mutants of the 2 corresponding genes. As a result, the ddE or ddF mutants are unable to externalize EntDD14 outside of the cytoplasm, even though there was clear expression of structural genes ddAB encoding EntDD14, and genes ddHIJ encoding an ABC transporter. Importantly, in these mutant strains (DddE and DddF), EntDD14 was detected by mass spectrometry in the intracellular soluble fraction exerting, upon its accumulation, a toxic effect on the producing strain as revealed by cell-counting and confocal microscopy analysis. Taken together, these results clearly indicate that PH domain-containing proteins, like DdE and DdF, are acting as transporters of the leaderless two-peptide EntDD14.