Final answer:
The alpha subunit of the G-protein confers its specificity of action, determining the cellular response by interacting with different effector proteins when activated by a G-protein-coupled receptor.
Step-by-step explanation:
The specificity of action of a G-protein is conferred by its alpha (a) subunit. G-proteins are heterotrimeric molecules composed of three subunits: alpha (a), beta (B), and gamma (y). When a signaling molecule binds to a G-protein-coupled receptor (GPCR) on the plasma membrane, an intracellular exchange of GDP for GTP on the alpha subunit occurs, leading to the dissociation of the alpha subunit from the beta and gamma subunits. This activated alpha subunit can then interact with various effectors in the cell, such as adenylyl cyclase or phospholipase C, determining the specific cellular response.
For instance, activation of adenylyl cyclase leads to the production of cAMP, which can then trigger a phosphorylation cascade affecting gene expression. Similarly, activation of phospholipase C results in the release of calcium ions from intracellular stores. The specificity of the response is, therefore, directed by which alpha subunit isoform is present, as different isoforms can bind to distinct sets of effector proteins.