Correct option is A
Explanation:
G-protein-coupled receptors (GPCRs) are a major class of cell surface receptors that activate intracellular G proteins. These G proteins are typically composed of three subunits: α (Gα), β (Gβ), and γ (Gγ). The activation of GPCRs leads to GDP-GTP exchange on the Gα subunit, triggering downstream signaling.
Evaluating the Statements:
A. Gα is membrane-bound, and in an unstimulated state, it binds to GDP. (Correct)
The Gα subunit is anchored to the plasma membrane through lipid modifications.
In its inactive state, it is bound to GDP.
When a ligand binds to the GPCR, the receptor stimulates the exchange of GDP for GTP, activating Gα.
B. Gα is non-membrane bound, and in an unstimulated state, it binds to GDP. (Incorrect)
Gα is membrane-bound, not free in the cytoplasm.
The Gβγ subunits are also membrane-associated and remain bound to Gα in its inactive GDP-bound state.
This statement is incorrect because Gα is membrane-bound.
C. After binding to the ligand, GPCR acts like a guanine nucleotide exchange factor (GEF) and helps in Gα activation. (Correct)
GPCRs function as guanine nucleotide exchange factors (GEFs).
When a ligand binds, the GPCR facilitates the exchange of GDP for GTP on the Gα subunit.
This activation releases Gα from Gβγ, allowing both to mediate downstream signaling.
D. RGS proteins act as α-subunit-specific GTPase-activating proteins (GAPs). (Correct)
Regulator of G protein Signaling (RGS) proteins function as GTPase-activating proteins (GAPs) for the Gα subunit.
They increase the rate of GTP hydrolysis, thereby terminating the signal and returning Gα to its inactive GDP-bound state.
This helps in the regulation and termination of GPCR signaling.
