Ras

The Ras protein is a monomeric G-protein with weak GTPase enzyme activity. It is a signal-transducer protein for growth factors and therefore it helps to transfer signals from cell-surface Receptor Tyrosine Kinase (RTK) receptors to the nucleus .

Active and Inactive forms of Ras

Ras is in its inactive form when it is bound to GDP, and becomes active once GDP has been exchanged for a GTP molecule. This reaction is initiated by the action of a specific signal molecule binding to a RTK receptor on the cell membrane. The activated RTK has intrinsic kinase activity, and this leads to phosphorylation of docking proteins at the RTK domains. The adaptor protein Grb-2 mediates the reaction between the RTK and Ras GEF, which exchanges GDP for GTP, acvtivating Ras which can transmit of a signal along multiple pathways. The inactivation of Ras is controlled by GTP hydrolysis.

References:

Alberts, B, et al, 2008. Molecular Biology of the Cell. 5th ed. United States of America: Garland Science.