Upon activation of a G protein-coupled receptor, phospholipase C is activated, leading to the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). This process generates two key second messengers, diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). Which of the following best describes the primary role of IP3 in cellular signaling?
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The answer
Correct answer: C. IP3 directly binds to receptors on the endoplasmic reticulum, triggering the release of intracellular calcium ions.
Tested concept: Second messengers: cAMP and IP3
Explanation
Inositol 1,4,5-trisphosphate (IP3) is a crucial second messenger that diffuses into the cytoplasm and binds to specific IP3-gated calcium channels located on the membrane of the endoplasmic reticulum. This binding event causes the rapid release of stored calcium ions from the ER into the cytoplasm, which then acts as another second messenger to activate various downstream cellular processes. Diacylglycerol (DAG), the other product of PIP2 hydrolysis, remains in the plasma membrane and activates protein kinase C (PKC). IP3 does not catalyze the formation of cAMP, nor does it directly phosphorylate proteins; its primary action is to mobilize intracellular calcium.
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