  The chemotaxis sensory system allows bacteria such as Escherichia coli to
swim towards nutrients and away from repellents. The underlying pathway is
remarkably sensitive in detecting chemical gradients over a wide range of
ambient concentrations. Interactions among receptors, which are predominantly
clustered at the cell poles, are crucial to this sensitivity. Although it has
been suggested that the kinase CheA and the adapter protein CheW are integral
for receptor connectivity, the exact coupling mechanism remains unclear. Here,
we present a statistical-mechanics approach to model the receptor linkage
mechanism itself, building on nanodisc and electron cryotomography experiments.
Specifically, we investigate how the sensing behavior of mixed receptor
clusters is affected by variations in the expression levels of CheA and CheW at
a constant receptor density in the membrane. Our model compares favorably with
dose-response curves from in vivo F\"orster resonance energy transfer (FRET)
measurements, demonstrating that the receptor-methylation level has only minor
effects on receptor cooperativity. Importantly, our model provides an
explanation for the non-intuitive conclusion that the receptor cooperativity
decreases with increasing levels of CheA, a core signaling protein associated
with the receptors, whereas the receptor cooperativity increases with
increasing levels of CheW, a key adapter protein. Finally, we propose an
evolutionary advantage as explanation for the recently suggested CheW-only
linker structures.
