Figure 1

Retroactivity arises due to enzyme sequestration in covalently modified cascades. A simple signaling cascade is depicted where each sequential cycle represents the activation (denoted by *) and inactivation of protein Y _i . Y₁* serves as the activating enzyme of Y₂ and Y₂* serves as the activating enzyme of Y₃. The cycles can be thought of as modules where each module's substrate sequesters a key component of the previous module, limiting the component's ability to participate in the previous module. This sequestration induces a natural change in the preceding module which may propagate upstream through one or more preceding modules. In this example, a perturbation in the deactivation reaction of cycle 3 induces an effect in cycle 2. If the perturbation takes the form of an increase in the concentration or activity of the enzyme catalyzing the conversion of Y₃* to Y₃, more Y₃ will be available to react with and sequester Y₂*, resulting in less Y₂ substrate availability for the reaction with Y₁*. Thus, a reverse response can propagate upstream to a preceding cycle or cycles. In the schematic, black arrows represent the cell surface to nucleus direction of cellular signaling and red arrows represent the direction of retroactive signaling.