Conformational Switch to a β‑Turn in a Staphylococcal Quorum Sensing Signal Peptide Causes a Dramatic Increase in Potency

We report the solution-phase structures
of native signal peptides
and related analogs capable of either strongly agonizing or antagonizing
the AgrC quorum sensing (QS) receptor in the emerging pathogen Staphylococcus epidermidis. Chronic S. epidermidis infections are often recalcitrant to traditional therapies due to
antibiotic resistance and formation of robust biofilms. The accessory
gene regulator (agr) QS system plays an important
role in biofilm formation in this opportunistic pathogen, and the
binding of an autoinducing peptide (AIP) signal to its cognate transmembrane
receptor (AgrC) is responsible for controlling agr. Small molecules or peptides capable of modulating this binding
event are of significant interest as probes to investigate both the agr system and QS as a potential antivirulence target. We
used NMR spectroscopy to characterize the structures of the three
native S. epidermidis AIP signals and five non-native
analogs with distinct activity profiles in the AgrC-I receptor from S. epidermidis. These studies revealed a suite of structural
motifs critical for ligand activity. Interestingly, a unique β-turn
was present in the macrocycles of the two most potent AgrC-I modulators,
in both an agonist and an antagonist, which was distinct from the
macrocycle conformation in the less-potent AgrC-I modulators and in
the native AIP-I itself. This previously unknown β-turn provides
a structural rationale for these ligands’ respective biological
activity profiles. Development of analogs to reinforce the β-turn
resulted in our first antagonist with subnanomolar potency in AgrC-I,
while analogs designed to contain a disrupted β-turn were dramatically
less potent relative to their parent compounds. Collectively, these
studies provide new insights into the AIP:AgrC interactions crucial
for QS activation in S. epidermidis and advance the
understanding of QS at the molecular level.