Author SummaryThe bacterial cell–cell communication process known as quorum sensing regulates important social behaviors including antibiotic production, motility, virulence, biofilm formation, sporulation, bioluminescence, and genetic competence. Gram-positive bacteria secrete oligopeptide quorum-sensing signals that bind to membrane-bound and cytosolic receptors. How oligopeptide quorum-sensing signals regulate the activity of their target receptors was previously largely unknown. Here we show that proteins belonging to the family of bacterial quorum-sensing receptors known as the Rap phosphatases undergo a remarkable regulatory conformational change upon binding oligopeptide signals. More specifically, in the absence of the oligopeptide signal, Rap proteins consist of two distinct domains: an N-terminal domain consisting of a three-helix bundle, and a superhelical C-terminal domain comprising an array of seven similar helix-turn-helix repeats. A flexible helix-containing linker region connects these domains. In complex with the regulatory oligopeptide, however, the Rap protein domains and linker region rearrange, merging to form a single continuous superhelical structure consisting of nine helix-turn-helix repeats. To our knowledge, this represents the first example of conformational change-induced repeat domain expansion. The structure-function studies presented here set the stage for the rational development of antimicrobial peptides and peptide-mimetics capable of targeting cell–cell signaling mediated by Rap proteins and similar bacterial receptors.