Structural Characterization of Native Autoinducing Peptides and Abiotic Analogues Reveals Key Features Essential for Activation and Inhibition of an AgrC Quorum Sensing Receptor in Staphylococcus aureus

Staphylococcus aureus is a major
human pathogen that uses quorum sensing (QS) to control virulence.
Its QS system is regulated by macrocyclic peptide signals (or autoinducing
peptides (AIPs)) and their cognate transmembrane receptors (AgrCs).
Four different specificity groups of S. aureus have been identified to date (groups I–IV), each of which
uses a different AIP:AgrC pair. Non-native ligands capable of intercepting
AIP:AgrC binding, and thereby
QS, in S. aureus have attracted considerable
interest as chemical tools to study QS pathways and as possible antivirulence
strategies for the treatment of infection. We recently reported a
set of analogues of the group-III AIP that are capable of strongly
modulating the activity of all four AgrC receptors. Critical to the
further development of such ligands is a detailed understanding of
the structural features of both native AIPs and non-native analogues
that are essential for activity. Herein, we report the first three-dimensional
structural analysis of the known native AIP signals (AIPs-I–IV)
and several AIP-III analogues with varied biological activities using
NMR spectroscopy. Integration of these NMR studies with the known
agonism and antagonism profiles of these peptides in AgrC-III revealed
two key structural elements that control AIP-III (and non-native peptide)
activity: (1) a tri-residue hydrophobic “knob” essential
for both activation and inhibition and (2) a fourth anchor point on
the exocyclic tail needed for receptor activation. These results provide
strong structural support for a mechanism of AIP-mediated AgrC activation
and inhibition in S. aureus, and should
facilitate the design of new AgrC ligands with enhanced activities
(as agonists or antagonists) and simplified chemical structures.