The gelatinase biosynthesis-activating pheromone binds and stabilises the FsrB membrane protein in Enterococcus faecalis quorum sensing

Quorum-sensing mechanisms regulate gene expression in response to changing cell-population density detected through pheromones. In Enterococcus faecalis, Fsr quorum sensing produces and responds to the gelatinase biosynthesis-activating pheromone (GBAP). Here we establish that the enterococcal FsrB membrane protein has a direct role connected with GBAP by showing that GBAP binds to purified FsrB. Far-UV CD measurements demonstrated a predominantly alpha-helical protein exhibiting a small level of conformational flexibility. Fivefold (400 mu m) GBAP stabilised FsrB (80 mu m) secondary structure. FsrB thermal denaturation in the presence and absence of GBAP revealed melting temperatures of 70.1 and 60.8 degrees C, respectively, demonstrating GBAP interactions and increased thermal stability conferred by GBAP. Addition of GBAP also resulted in tertiary structural changes, confirming GBAP binding.