In biology, small GTPases are small (20-25 KDa) proteins that bind to GTP. This family of proteins are homologous to Ras GTPases and also called the Ras superfamily GTPases. Small GTPases regulate a wide variety of processes in the cell, including growth, cellular differentiation, cell movement and lipid vesicle transport. Heterotrimeric G-proteins and small GTPases share common futures as GTPases but they have slightly different structures and mechanisms of action.

Active form (ON) and Inactive form (OFF)

Small GTPases act as molecular switches and regulate functions of other proteins. They are active or 'ON' when it is bound to GTP and inactive or 'OFF' when bound to GDP. Inactive form of GTPases (GDP-form) can be activated by another protein called guanine nucleotide exchange factor (GEF). GEF removes GDP from small GTPases and exchanges it with GTP. GTP can be hydrolysed to GDP by small GTPases. Small GTPases hydrolyse GTP quickly when they are bound to GTPase-activating protein (GAP), and slowly when they are not bound to GAP. In short, GEF switches small GTPases ON, and GAP switches them OFF.

The Ras superfamily

There are more than hundred proteins in the Ras superfamily. Based on structure and functions, the Ras superfamily is further divided into five main subfamilies: Ras, Rho, Rab, Arf and Ran.

References

Technical

Science's STKE http://stke.sciencemag.org/cgi/content/full/OC_sigtrans;2001/68/pe1