Advanced Placement (AP) Biology Practice Exam

What is the function of a G-protein in cell signaling?

1. It serves as a receptor on the cell membrane

2. It interacts with and activates enzymes or ion channels

3. It binds to DNA to influence gene expression

4. It transports molecules across the membrane

The correct answer is: It interacts with and activates enzymes or ion channels

The function of a G-protein in cell signaling is primarily to interact with and activate enzymes or ion channels, making it a crucial component of many signaling pathways. When a signaling molecule, such as a hormone or neurotransmitter, binds to a G-protein-coupled receptor (GPCR) on the cell's surface, it triggers a conformational change in the receptor. This change activates the associated G-protein by facilitating the exchange of GDP for GTP on its alpha subunit. Once activated, the G-protein can dissociate into its subunits, and these subunits can then interact with various downstream effectors, including enzymes like adenylyl cyclase or phospholipase C. This interaction leads to the production of second messengers, such as cyclic AMP or inositol trisphosphate, which propagate the signal within the cell and can influence various cellular responses, such as metabolic changes, gene expression, or ion flux across the membrane. In contrast, the role of G-proteins does not include serving as receptors, as that function is specific to GPCRs. Additionally, while they are involved in signaling pathways that may ultimately influence gene expression, G-proteins themselves do not bind to DNA. Lastly,