What would be indicated if EPO is bound to EPOR found on the plasma membrane?

• A. EPO is able to enter the cell
• B. EPOR is a transcription factor
• C. EPOR facilitates signaling from outside the cell
• D. EPOR is not functional

Answer: (C)

When erythropoietin (EPO) binds to its receptor, erythropoietin receptor (EPOR), located on the plasma membrane of a target cell, it triggers a series of signaling events. This interaction does not require EPO to enter the cell; instead, it functions at the cell membrane to send signals into the cell's interior, thereby facilitating important cellular responses.

The binding of EPO to EPOR activates various intracellular signaling pathways, including those involved in the regulation of gene expression necessary for erythropoiesis (the production of red blood cells). This function signifies that EPOR operates as a mediator of signaling rather than directly acting as a transcription factor. Instead, it initiates cascades that ultimately may influence transcription in the nucleus.

Thus, the binding of EPO to EPOR demonstrates that EPOR is essential for translating the extracellular signal (the binding of EPO) into a physiological response within the cell, confirming that it indeed facilitates signaling from outside the cell to produce critical biological effects.