  Many membrane-bound molecules in cells form small clusters. It has been
hypothesized that these clusters convert an analog extracellular signal into a
digital intracellular signal and that this conversion increases signaling
fidelity. However, the mechanism by which clusters digitize a signal and the
subsequent effects on fidelity remain poorly understood. Here we demonstrate
using a stochastic model of cooperative cluster formation that sufficient
cooperation leads to digital signaling. We show that despite reducing the
number of output states, which decreases fidelity, digitization also reduces
noise in the system, which increases fidelity. The tradeoff between these
effects leads to an optimal cluster size that agrees with experimental
measurements.
