In which direction does the activation of a receptor by its ligand propagate a signal?

In which direction does the activation of a receptor by its ligand propagate a signal?

• A. From the intracellular domain to the extracellular domain
• B. Within the cytosol
• C. From the extracellular domain to the intracellular domain
• D. To the cell nucleus

Answer: (C)

Signal propagation starts when a ligand binds to the extracellular part of a receptor, causing a change that is transmitted across the membrane to the intracellular domain. That conformational shift on the inside then engages cytosolic signaling proteins and sets off cascades that relay and amplify the signal within the cell. In short, the message moves from outside the cell to inside, across the membrane, rather than starting inside or moving toward the nucleus as the initial step. The nucleus may be affected later as transcription factors are activated, but the first propagation direction is from the extracellular domain to the intracellular domain.

Signal propagation starts when a ligand binds to the extracellular part of a receptor, causing a change that is transmitted across the membrane to the intracellular domain. That conformational shift on the inside then engages cytosolic signaling proteins and sets off cascades that relay and amplify the signal within the cell. In short, the message moves from outside the cell to inside, across the membrane, rather than starting inside or moving toward the nucleus as the initial step. The nucleus may be affected later as transcription factors are activated, but the first propagation direction is from the extracellular domain to the intracellular domain.