The Aer2 receptor from Vibrio cholerae is a dual PAS-heme oxygen sensor

Abstract

The diarrheal pathogen Vibrio cholerae navigates complex environments using three chemosensory systems and 44–45 chemoreceptors. Chemosensory cluster II modulates chemotaxis, whereas clusters I and III have unknown functions. Ligands have been identified for only five V. cholerae chemoreceptors. Here, we report that the cluster III receptor, VcAer2, binds and responds to O2. VcAer2 is an ortholog of Pseudomonas aeruginosa Aer2 (PaAer2) but differs in that VcAer2 has two, rather than one, N-terminal PAS domain. We have determined that both PAS1 and PAS2 form homodimers and bind penta-coordinate b-type heme via an Eη-His residue. Heme binding to PAS1 required the entire PAS core, but receptor function also required the N-terminal cap. PAS2 functioned as an O2-sensor [Kd(o2), 19 μM], utilizing the same Iβ Trp (W276) as PaAer2 to stabilize O2. The crystal structure of PAS2-W276L was similar to that of PaAer2-PAS but resided in an active conformation mimicking the ligand-bound state, consistent with its signal-on phenotype. PAS1 also bound O2 [Kd(o2), 12 μM], although O2 binding was stabilized by either a Trp residue or Tyr residue. Moreover, PAS1 appeared to function as a signal modulator, regulating O2-mediated signaling from PAS2 and resulting in activation of the cluster III chemosensory pathway.