Question

how to activate a small G protein? how to inactivate a small G protein?

Answer 1

G-proteins are a family of small proteins with GTPase activity that act as molecular switches inside the cell. They are called so because of their ability to bind guanine nucleotides GTP and GDP and are involved in transduction of signal between activated receptor and various intracellular effectors. They may be present as high molecular weight trimeric G-proteins or as small low molecular weight monomeric G-proteins.    The examples of later are Ras, Rho, Rab, Ran etc.

In absence of stimuli, G-proteins are bound to GDP and are in inactive OFF state. On receiving the external stimuli by the cell, guanine nucleotide exchange (GEF) factor brings about conformational change in the monomeric G-protein that reduces its affinity for GDP. This results in release of GDP from G-protein which is subsequently replaced by GTP. This activates the G-protein by inducing the active site to carry out downstream signal transduction. After some time of activation, another protein, GTPase activating protein (GAP) acts on G-protein and accelerates its GTPase activity to bring in back to GDP state. This results in deactivation of G-protein and thus termination of the signalling response from it.