Figure 1.

Digital signaling of Ras clusters at the cell membrane. (A) Schematic illustration of the expected effects of digitization and noise on the fidelity of signal transmission. As in information theory, we define fidelity as the number of input signals that can be uniquely and reliably mapped to an output. An analog (smooth) response with low noise has the highest fidelity (upper left). Increasing noise decreases fidelity (upper right). A digital (steplike) response also has reduced fidelity because nearby inputs lead to redundant outputs (lower left). A digital response with high noise has the lowest fidelity (lower right). Here we explore how clustering can turn a noisy analog signal (like the upper right) into a less-noisy digital signal (like the lower left), and the associated benefits for signal fidelity. (B) Model of Ras cluster formation. N active Ras molecules form clusters by sequential and cooperative monomer addition, up to a maximal size ν. The m clusters of size ν signal downstream. To see this figure in color, go online.