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. 2006 Jun 15;90(12):4317-26.
doi: 10.1529/biophysj.105.079905. Epub 2006 Mar 24.

Receptor-receptor coupling in bacterial chemotaxis: evidence for strongly coupled clusters

Monica L Skoge  1 , Robert G Endres, Ned S Wingreen
Affiliations

Receptor-receptor coupling in bacterial chemotaxis: evidence for strongly coupled clusters

Monica L Skoge et al. Biophys J. .

Abstract

Receptor coupling is believed to explain the high sensitivity of the Escherichia coli chemotaxis network to small changes in levels of chemoattractant. We compare in detail the activity response of coupled two-state receptors for different models of receptor coupling: weakly-coupled extended one-dimensional and two-dimensional lattice models and the Monod-Wyman-Changeux model of isolated strongly-coupled clusters. We identify features in recent data that distinguish between the models. Specifically, researchers have measured the receptor activity response to steps of chemoattractant for a variety of engineered E. coli strains using in vivo fluorescence resonance energy transfer. We find that the fluorescence resonance energy transfer results for wild-type and for a low-activity mutant are inconsistent with the lattice models of receptor coupling, but consistent with the Monod-Wyman-Changeux model of receptor coupling, suggesting that receptors form isolated strongly-coupled clusters.

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Figures

FIGURE 1
FIGURE 1
Dose-response curves showing receptor activity as a function of attractant (MeAsp) concentration for various E. coli strains, obtained by Sourjik and Berg using in vivo FRET. Results are shown for wild-type (WT) and cheR strains, which express wild-type levels of both Tar and Tsr receptors (1), and a cheRcheB strain (QEQE), which expresses six times the wild-type level of Tar receptors (2). Receptor activity is shown normalized by the activity at zero MeAsp concentration and is shown unnormalized in the inset (lines are guides to the eye).
FIGURE 2
FIGURE 2
Receptor activity as a function of free-energy difference f between active and inactive states for a single receptor, a one-dimensional lattice of receptors with Ising coupling J = 1.5, a two-dimensional lattice of receptors with Ising coupling J = 0.38, and an MWC cluster of size N = 20. The inset shows the magnitude of the susceptibility χ = dA/df versus f on a semilog scale. Energies are measured in units of the thermal energy kBT.
FIGURE 3
FIGURE 3
Receptor activity as a function of ligand concentration for receptors with various offset energies Δε for (a) a single receptor, (b) a one-dimensional lattice of receptors with Ising coupling J = 1.5, (c) a two-dimensional lattice of receptors with Ising coupling J = 0.38, and (d) an MWC cluster of receptors of size N = 20. Receptor activity is normalized by its value at zero ligand concentration and is shown unnormalized for a single receptor in the inset to panel a. The dissociation constants for receptor-ligand binding are formula image = 0.02mM and formula image = 0.5 mM, and energies are measured in units of the thermal energy kBT.
FIGURE 4
FIGURE 4
Inhibition constant Ki versus receptor offset energy Δε for (a) a one-dimensional lattice of receptors, (b) a two-dimensional lattice of receptors, and (c) MWC clusters, for various values of the Ising coupling J and MWC cluster size N. The dissociation constants for receptor-ligand binding are formula image = 0.02 mM and formula image = 0.5 mM, as in Fig. 3. Insets to panels a and b: correlation length ξ versus free-energy difference f for one-dimensional and two-dimensional lattices, for the same parameters as in the main panels.
FIGURE 5
FIGURE 5
Receptor response R = |dA/d log [L]| for (a) a single receptor, (b) a one-dimensional lattice of receptors with Ising coupling J = 1.5, (c) a two-dimensional lattice of receptors with Ising coupling J = 0.38, and (d) an MWC cluster of receptors of size N = 20, as in Fig. 3. Response curves are shown for different values of the offset energy Δε. The insets show the receptor responses for the low-activity regime (Δε > 0) on a smaller scale with formula image indicated by a dashed line; the receptor response for Δε = 1 in panel d is too low to be visible.
FIGURE 6
FIGURE 6
Receptor-occupancy difference between active and inactive states θ = df/d log [L]. The dissociation constants for receptor-ligand binding are formula image = 0.02 mM and formula image = 0.5 mM, as in Fig. 3.
FIGURE 7
FIGURE 7
Receptor susceptibility χ = dA/df for (a) a single receptor, (b) a one-dimensional lattice of receptors with Ising coupling J = 1.5, (c) a two-dimensional lattice of receptors with Ising coupling J = 0.38, and (d) an MWC cluster of receptors of size N = 20, as in Fig. 3. Susceptibility curves are shown for different values of the offset energy Δε. The insets show the susceptibility for the low-activity regime (Δε > 0) on a smaller scale with formula image indicated by a dashed line; the susceptibility for Δε = 1 in panel d is too low to be visible.
FIGURE 8
FIGURE 8
Receptor activity as a function of ligand concentration for receptors with offset energy Δε = −2 for a one-dimensional lattice with Ising coupling J = 1.5, a two-dimensional lattice with Ising coupling J = 0.38, and an MWC cluster of size N = 20. The curves have approximately the same slope at [L] = Ki, but deviate in the tails of the transition, as indicated by the arrows. (Inset) Effect of having a mixture of receptor modification states. Receptor activity as a function of ligand concentration for MWC clusters of size N = 20 for homogenous clusters with offset energies Δε = −1, Δε = −1.5, and Δε = −2, and for mixed clusters with Δε = −1 and Δε = −2.
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