Which are the major G-protein families?

• A. Transducin, Gi, Gt, and G12
• B. Gs, Gi, Gq, and Gt
• C. Gs (stimulatory), Gi (inhibitory), Gq, and G12/13
• D. Gq, Gi, Gs, and G11

Answer: (C)

G-proteins are grouped into major families based on their alpha subunits and the main signals they trigger after a GPCR is activated. The four widely recognized major families are Gs, Gi/o, Gq/11, and G12/13. Gs stimulates adenylyl cyclase to raise cAMP levels; Gi inhibits adenylyl cyclase to lower cAMP; Gq activates phospholipase Cβ, producing IP3 and DAG and increasing intracellular calcium; G12/13 activates RhoGEFs, driving Rho signaling that affects the cytoskeleton and cell behavior. Some G proteins, like transducin, are more tissue-specific (vision) and aren’t counted among the broad four. G11 is generally grouped with Gq/11 rather than as a separate major family. So, the four listed—Gs, Gi, Gq, and G12/13—best represent the major G-protein families.