Abstract
In vivo fluorescence microscopy and electron cryo-tomography have revealed that chemoreceptors self-assemble into extended honeycomb lattices of chemoreceptor trimers with a well-defined relative orientation of trimers. The signaling response of the observed chemoreceptor lattices is remarkable for its extreme sensitivity, which relies crucially on cooperative interactions among chemoreceptor trimers. In common with other membrane proteins, chemoreceptor trimers are expected to deform the surrounding lipid bilayer, inducing membrane-mediated anisotropic interactions between neighboring trimers. Here we introduce a biophysical model of bilayer-chemoreceptor interactions, which allows us to quantify the role of membrane-mediated interactions in the assembly and architecture of chemoreceptor lattices. We find that, even in the absence of direct protein-protein interactions, membrane-mediated interactions can yield assembly of chemoreceptor lattices at very dilute trimer concentrations. The model correctly predicts the observed honeycomb architecture of chemoreceptor lattices as well as the observed relative orientation of chemoreceptor trimers, suggests a series of “gateway” states for chemoreceptor lattice assembly, and provides a simple mechanism for the localization of large chemoreceptor lattices to the cell poles. Our model of bilayer-chemoreceptor interactions also helps to explain the observed dependence of chemotactic signaling on lipid bilayer properties. Finally, we consider the possibility that membrane-mediated interactions might contribute to cooperativity among neighboring chemoreceptor trimers.
Highlights
The chemotaxis signal transduction pathway [1] allows bacteria to respond to minute relative changes in chemical concentration over several orders of magnitude in ambient chemical concentration [2], and ranks among the most studied signaling pathways in biology
The sensitivity of the chemotaxis system relies crucially on cooperative interactions among chemoreceptor trimers, which are organized into intricate honeycomb lattices
Using a biophysical model of bilayerchemoreceptor interactions we show that the membranemediated interactions induced by chemoreceptor trimers provide a mechanism for the observed self-assembly of chemoreceptor lattices
Summary
The chemotaxis signal transduction pathway [1] allows bacteria to respond to minute relative changes in chemical concentration over several orders of magnitude in ambient chemical concentration [2], and ranks among the most studied signaling pathways in biology. Fluorescence resonance energy transfer (FRET) experiments [8,9] have revealed that a crucial step in signal amplification occurs at the level of chemoreceptors: chemoreceptors signal in cooperative teams [8,9,10]. Ising [11,12,13] and Monod-Wyman-Changeux [9,14,15,16] models of coupled teams of chemoreceptors both achieve quantitative agreement with the FRET data. The fundamental assumption underlying these models of signaling teams is that chemoreceptors do not respond independently to changes in the external ligand concentration, but rather each receptor influences the collective state of a team of neighboring receptors. The observed functional characteristics of chemotactic signaling rely on cooperative local interactions among chemoreceptors, and suggest a well-defined spatial organization of chemoreceptors
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