effector

If a signaling pathway is a chain of command, the effector is the worker who actually does the job at the end. It is the protein the whole pathway exists to control — typically an enzyme that makes a second messenger or an ion channel that changes the cell's electrical or chemical state.

Classic effectors include adenylyl cyclase (made by Gs, blocked by Gi), phospholipase C (turned on by Gq), and ion channels such as the potassium and calcium channels regulated by G-protein beta-gamma subunits. The receptor and G protein upstream are the relays; the effector is the output device whose change in activity produces the cellular response. Some effectors generate a further second messenger, extending the cascade; others act directly.

Pinpointing the effector clarifies a drug's true action. Two drugs hitting the same receptor can still differ if the receptor is coupled to different effectors in different tissues, and some agents act on the effector directly — forskolin on adenylyl cyclase, for instance — bypassing the receptor altogether. Naming the effector is therefore part of stating a precise mechanism of action.