Some signaling pathways in the cell are chiefly based on inhibition rather than activation. For example, proteins that initiate programmed cell death by apoptosis are inhibited by anti-apoptotic proteins, which can in turn be inhibited by a group of pro-apoptotic proteins, and so on. Consider the following networks of inhibitory interactions, in which each protein is inactive if at least one of its inhibitors is active. In which pathway is the activation of either upstream protein (1 or 2) sufficient to inhibit apoptosis?

Question

Quizplus · Accepted Answer

Pathway A is equivalent to an OR gate; either protein (1 or 2) can trigger apoptosis. Pathway B is equivalent to a NOR gate; either 1 or 2 can inhibit apoptosis. Pathway C is equivalent to a gate that ignores its second input (protein 2). Pathway D is equivalent to an AND gate; both proteins 1 and 2 in their active form are required to trigger apoptosis.

Some Signaling Pathways in the Cell Are Chiefly Based on Inhibition