Key Determinants of Receptor Activation in the agr Autoinducing Peptides of Staphylococcus aureus

Abstract

Staphylococcal pathogenesis is regulated by a two-component quorum-sensing system, agr, activated upon binding of a self-coded autoinducing peptide (AIP) to the receptor-histidine kinase, AgrC. The AIPs consist of a thiolactone macrocyle and an exocyclic "tail", both of which are important for function. In this report, characterization of the unique AIPs from the four known agr specificity groups of Staphylococcus aureus has been completed, along with analysis of cross-group inhibition of AgrC activation by each of the four AIPs. The following conclusions have been drawn: (i) The native thiolactone macrocyle and tail are necessary and sufficient for full activation by the AIPs, whereas the AIP-I macrocycle alone is a partial agonist. (ii) The native N-terminus is less critical, as that of AIP-I can be modified without affecting bioactivity, although that of AIP-III cannot. (iii) The ring and tail may function differently in different AIPs. Thus the group I and IV AIPs differ at a single (endocyclic) residue, which is the determinant of AIP specificity for these two groups and is essential for function. A similarly critical residue in AIP-II, however, is exocyclic. (iv) Cross-inhibition is more tolerant of sequence and structural diversity than is activation, suggesting that the AIPs interact differently with cognate than with heterologous receptors. (v) Chimeric peptides, in which the tails and macrocycles are switched, do not activate and instead inhibit receptor activation. These data suggest a model in which activation and inhibition involves different binding orientations within the ligand binding pocket of each receptor.

Exploiting Quorum Sensing To Confuse Bacterial Pathogens

“…The tail of the AIP molecule is required only for activation, as mutation of the tail amino acid residues or removal of the tail did not alter inhibition of the agr response in heterologous groups despite eliminating self-activation activity (130,219). Numerous SAR studies have mapped amino acid residues critical for AIP activity, and overall the results demonstrate that critical amino acids differ between AIP groups (218)(219)(220). For example, in AIP-I the endocyclic aspartic acid residue adjacent to the cysteine residue is critical, as replacement of this amino acid with alanine converts AIP-I to a potent inhibitor of AgrC of groups I and IV (218,219).…”

“…The AgrC receptor of each group responds to its own AIP molecule by activating the agr regulon, but this activation is inhibited by heterologous AIPs of other groups (217). The exceptions to this are the AIPs of of groups I and IV, which differ by only a single endocyclic residue; AIP-I strongly activates its cognate AgrC receptor while weakly activating AgrC of group IV strains, while AIP-IV strongly activates the agr response in both groups I and IV (218). The molecular determinants for AIP specificity have been shown to reside entirely in the interaction between the AIP signal and the AgrC receptor (130).…”

“…The molecular determinants for AIP specificity have been shown to reside entirely in the interaction between the AIP signal and the AgrC receptor (130). Both cognate activation and heterologous inhibition activities require the ring structure of AIP, as linear AIPs have no detectable activity in any agr group (219,220), but the requirement of the thioester linkage has been debated (218)(219)(220). The tail of the AIP molecule is required only for activation, as mutation of the tail amino acid residues or removal of the tail did not alter inhibition of the agr response in heterologous groups despite eliminating self-activation activity (130,219).…”

Surface-attached molecules control Staphylococcus aureus quorum sensing and biofilm development

“…In the S. aureus agr -I strain that we study, AIP-I is the native autoinducer. TrAIP-II, a truncated AIP-II with the exocyclic tail replaced by an acetyl group, is a universal inhibitor for all four S. aureus Agr quorum-sensing systems 32 . TrAIP-II competes with the cognate AIPs for binding to the receptor 32 .…”

“…TrAIP-II, a truncated AIP-II with the exocyclic tail replaced by an acetyl group, is a universal inhibitor for all four S. aureus Agr quorum-sensing systems 32 . TrAIP-II competes with the cognate AIPs for binding to the receptor 32 . Throughout this work, we focus on S. aureus agr-I because it possesses the most prevalent Agr type found world-wide in nosocomial infections.…”