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Guanine nucleotide binding protein that binds GTP when activated, which it hydrolyses to GDP.  The active form of the protein can interact with a number of down stream effectors such as adenylyl cyclase (producing cAMP from ATP), and phospholipase C (PLC).

The G-proteins can be divided in to two families - the heterotrimeric G-proteins and the small or mono-meric G-proteins.

G-protein coupled receptors (GPCR) upon binding of stimulus activates G-protein.

The structure of a GPCR shows 7 transmembrane spans, with 3 exoloops and 4 cytoloops. The N-terminal is found on the outside of the membrane and the C-terminal found on the cytosolic side of the membrane.


G protein families

There are 4 major families of heterotrimeric G-protein. The G-proteins are divided based on the homology of alpha subunit. Each receptor only affects one type of G-protein [1].

There are 20 alpha subunits, 6 beta subunits and 12 gamma subunits in human that make up the diversity of G-proteins.

Heterotrimeric G-proteins contain three subunits: alpha, beta and gamma. In the inactive form, all three subunits are associated together and GDP binds to the alpha subunit. On activation, there is a conformational change in the alpha subunit, which now binds to GTP and disocciates from the beta-gamma complex. The alpha subunit and beta-gamma complex can now interact with different effectors to bring about a response. The GTPase activity of the alpha subunit causes the loss of GTP from the binding site in the alpha subunit, and another GDP binds, resetting the system [2].G-proteins can be classified as either trimeric, which are seen bound to G protein coupled receptors, or as monomeric, which lack the three different subunits that trimeric G-proteins have.

Family I

Consists of Gs and Golf. The alpha subunit mediates the function in both G-proteins. Gs activates adenylyl cyclase and Calcium ion channels while Golf activates adenylyl cyclase in olfactory sensory neurons.

Family II

Consists of Gi, Go and Gt. The alpha subunit of Gi inhibits adenylyl cyclase while the beta-gamma complex of Gi activates potassium ion channels. For Go, the beta-gamma complex activates Potassium ion channels and at the same time inactivates Calcium ion channel. Both alpha subunit and beta-gamma complex of Go activate phospholipase C-beta. The alpha subunit of Gt activates cyclic GMP phosphodiesterase in vertebrate rod photoreceptors.

Family III

Consists of Gq. The alpha subunit of Gq functions to activate phospholipase C-beta.

Family IV

Consists of G12/13. The action of alpha subunit of G12/13 is to activate Rho family monomeric GTPase.


  1. Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Robers and Peter Walter(2008) Molecular Biology of The Cell, 5th edition, USA: Garland Science. pg 919
  2. Alberts et al., Molecular Biology Of The Cell, Fifth Edition

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