Molecular Mechanism of Peptide-Specific Pheromone Signaling in Enterococcus faecalis: Functions of Pheromone Receptor TraA and Pheromone-Binding Protein TraC Encoded by Plasmid pPD1

Conjugative transfer of the Enterococcus faecalis plasmid pPD1 is activated by cPD1, one of several peptide sex pheromones secreted by plasmid-free recipient cells, and is blocked by a donor-produced peptide inhibitor, iPD1. Using a tritiated pheromone, [(3)H]cPD1, we investigated how pPD1-harboring donor cells receive these peptide signals. Donor cells rapidly incorporated [(3)H]cPD1. The cell extract but not the membrane fraction of the donor strain exhibited significant [(3)H]cPD1-binding activity. On the basis of these data and those of tracer studies, it was demonstrated that cPD1 was internalized, where it bound to a high-molecular-weight compound. The cell extract of a strain carrying the traA-bearing multicopy plasmid (pDLHH21) also exhibited high [(3)H]cPD1-binding activity. A recombinant TraA exhibited a dissociation constant of 0.49 ± 0.08 nM against [(3)H]cPD1. iPD1 competitively inhibited [(3)H]cPD1 binding to TraA, whereas pheromones and inhibitors relating to other plasmid systems did not. These results show that TraA is a specific intracellular receptor for cPD1 and that iPD1 acts as an antagonist for TraA. A strain carrying the traC-bearing multicopy plasmid (pDLES23) exhibited significant [(3)H]cPD1-binding activity. A strain carrying traC-disrupted pPD1 (pAM351CM) exhibited lower [(3)H]cPD1-binding activity as well as lower sensitivity to cPD1 than a wild-type donor strain. Some of the other pheromones and inhibitors inhibited [(3)H]cPD1 binding to the traC transformant like cPD1 and iPD1 did. These results show that TraC, as an extracellular less-specific pheromone-binding protein, supports donor cells to receive cPD1.