Signal transduction protein, which is a major regulator of staphylococcal pathogenesis. Phosphorylated TRAP leads to the activation of agr system and consequent RNAIII synthesis resulting in the expression of several virulence factors. Up-regulates the expression of most toxins and genes known to be necessary for biofilm formation. Membrane-associated. Constitutively expressed. Each of the three conserved histidine residues contributes to TRAP phosphorylation. Phosphorylation is essential for TRAP activity. Phosphorylation of TRAP is activated by RAP and necessary for the induction of RNAIII gene expression but not for ongoing transcription. TRAP is dephosphorylated from the mid-exponential phase of growth, which is when agr is activated and AIP is produced. RIP acts by inhibiting TRAP phosphorylation. Belongs to the TRAP family.