The four main classes of G proteins are Gs, Gi, Gq, and G12. They are distinguished by their alpha subunits (Gαs, Gαi, Gαq, and Gα12 respectively) and each class plays a distinct role in cellular signaling pathways.
Here's a breakdown of each type:
-
Gs:
- Primary Function: Activates the cAMP pathway.
- Mechanism: Gαs stimulates adenylyl cyclase, an enzyme that increases the production of cyclic AMP (cAMP). cAMP then activates protein kinase A (PKA), leading to various downstream effects.
- Examples: Includes Gαs and Gαolf.
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Gi:
- Primary Function: Inhibits the cAMP pathway and regulates ion channel function.
- Mechanism: Gαi inhibits adenylyl cyclase, reducing cAMP levels. The Gβγ subunits released from Gi can also directly modulate ion channels.
- Examples: Includes Gαi1, Gαi2, Gαi3, Gαo1, Gαo2, Gαt, Gαgus, and Gαz.
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Gq:
- Primary Function: Activates the inositol phosphate pathway.
- Mechanism: Gαq activates phospholipase C (PLC), which hydrolyzes phosphatidylinositol bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 releases calcium from intracellular stores, and DAG activates protein kinase C (PKC).
- Examples: Includes Gαq, Gα11, Gα14, and Gα16.
-
G12:
- Primary Function: Modulates the cytoskeleton.
- Mechanism: Gα12 activates Rho-GEF (Rho guanine nucleotide exchange factor), which in turn activates Rho, a small GTPase that regulates the actin cytoskeleton.
- Examples: Includes Gα12 and Gα13.
| G Protein Type | Alpha Subunit(s) | Primary Function | Mechanism |
|---|---|---|---|
| Gs | Gαs, Gαolf | Activates cAMP pathway | Stimulates adenylyl cyclase, increasing cAMP levels. |
| Gi | Gαi1, Gαi2, Gαi3, Gαo1, Gαo2, Gαt, Gαgus, Gαz | Inhibits cAMP pathway and regulates ion channels | Inhibits adenylyl cyclase, reduces cAMP levels; Gβγ regulates ion channels. |
| Gq | Gαq, Gα11, Gα14, Gα16 | Activates inositol phosphate pathway | Activates phospholipase C (PLC), increasing IP3 and DAG levels. |
| G12 | Gα12, Gα13 | Activates Rho-GEF, modulates cytoskeleton | Activates Rho-GEF, which activates Rho and affects the actin cytoskeleton. |
