Figure 2.
Signaling network with shared components. (A) The ‘basic architecture'. Component x1 is common to pathways X and Y. Although the desired route of signaling is for x0 to activate x2 and not y2, and y0 to activate y2 and not x2, this cannot be achieved with specificity and fidelity for this network. (B, C) Numerical simulations of signaling through this network under various sets of parameter values. Values that increase SX reciprocally decrease SY, and values that increase FX reciprocally decrease FY. Shown are the values of outputs x2 and y2 in response to inputs x0 and y0, applied separately as square pulses of magnitude 1 and duration 1. In panel B, both the specificity and fidelity of cascade X are larger than those of cascade Y. Parameter values are a1=2, b1=1, a2=2, b2=1, d1=d2x=d2y=1. We have SX=2, FX=2, SY=0.5, FY=0.5. In panel C, the specificity of cascade X is higher than that of cascade Y whereas the opposite holds for fidelity values: FX<FY. Parameter values are a1=1, b1=2, a2=2, b2=1, d1=d2x=d2y=1. This yields SX=2, FX=0.5, SY=0.5, FY=2. (D, E) Two insulating mechanisms that can augment specificity: (D) compartmentalization; (E) the action of a scaffold protein.