Mechanisms of ligand sensing in the newly characterized, dominant family of prokaryotic receptors
Monash University, Melbourne, VIC 3800, Australia.
Chemotaxis, mediated by membrane-embedded chemotaxis receptors, plays an important role in bacterial ecology and pathogenesis. We investigate the structural basis of how the dominant family of bacterial chemoreceptors with an extracytoplasmic double Cache sensing domain (dCache SD) recognize chemical cues, and how they discriminate between attractants and repellents. We have determined the first representative crystal structures of the characterised dCache SDs of chemoreceptors from a broad range of bacteria of medical and biotechnological importance, including carcinogenic bacterium Helicobacter pyloriand important human and animal pathogen Campylobacter jejuni. Analysis of these structures, in conjunction with mutagenesis, biophysical and molecular simulation studies, provided an insight into diverse mechanisms of ligand recognition by this protein fold. In all previously characterised dCache SDs, direct sensing involved binding of the signal molecule to the membrane-distal, rather than membrane-proximal, subdomain. I will present data that changes this paradigm and reveal the first example of a chemoreceptor that directly recognises its ligand via the membrane-proximal subdomain, helping H. pylori to seek out lactate. In addition, I will present the results of our systematic study of the attractant-bound and repellent bound structures and discuss implications for the mechanism of discrimination between atrractants and repellents. I will then describe examples of very specific (Pseudomonas fluorescens CtaB) versus rather promiscuous (P. fluorescens CtaA, C. jejuni Tlp3) chemoreceptors with the same overall fold and discuss the structural basis behind this phenomenon. Finally, I will present an example of a dCache chemoreceptor (C. jejuni Tlp1) that recognises its signal molecule indirectly.