Signaling mechanism of the transmembrane energy receptor Aer.

Abstract

The Escherichia coli aerotaxis receptor Aer is a bacterial chemoreceptor that senses intracellular redox changes via an N-terminal PAS domain bound to a flavin adenine dinucleotide (FAD) cofactor. Distinct from canonical methyl-accepting chemotaxis proteins (MCPs) such as Tar/Tsr, Aer lacks a periplasmic ligand-binding domain and adaptive methylation, transmitting conformational signals laterally from the PAS domain to the HAMP domain and the methylation helix-like cap (MHL-cap) of the kinase control domain (KCD). To elucidate the Aer signaling mechanism, we determined cryoelectron microscopy structures of full-length Aer in oxidized flavin quinone (kinase-on) and reduced semiquinone (kinase-off) states. Structural comparison reveals redox-linked rearrangements of the FAD-binding pocket, reorientation of PAS-HAMP interactions, and strikingly altered MHL-cap stability. PAS-MHL-cap interactions in the oxidized state compress the receptor and stabilize the proximal KCD helices, whereas reduction disrupts these contacts, increasing KCD flexibility. To probe distal effects on KCD architecture, we performed nanodisc reconstitution and pulse dipolar ESR spectroscopy on spin-labeled positions along the four-helix bundle. Distance distributions indicate redox-dependent changes in helix separation, particularly at the C-terminal MHL2 region, consistent with PAS-driven loosening of KCD packing in kinase-off states. These data support a model in which FAD redox chemistry reorganizes flavin pocket residues that in turn subtly alter PAS conformation to influence PAS-HAMP and PAS-MHL-cap packing and hence KCD conformational stability. The findings reveal an Aer-specific signaling axis distinct from periplasmic-ligand binding MCPs that has adapted MCP architecture for lateral PAS input and cytoplasmic redox sensing.

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