Question

immediately after a membrane receptor protein binds to its ligand, which of the following processes occurs?
cytoplasmic enzymes are activated by phosphorylation.
an intracellular g protein is activated.
the membrane receptor protein undergoes a conformational change.
the receptor/ligand complex enters the cytoplasm by endocytosis.

Explanation:

When a ligand binds to a membrane receptor protein, the immediate event is a conformational change in the receptor. This conformational change is the first step in signal transduction. Activation of cytoplasmic enzymes by phosphorylation (often seen in enzyme - linked receptors) and activation of intracellular G - proteins (in G - protein - coupled receptors) occur after the conformational change. Endocytosis of the receptor/ligand complex is a later event in some receptor - mediated processes and not the immediate consequence of ligand binding.

Answer:

The membrane receptor protein undergoes a conformational change.