Small GTPases - Ras and Rho

Guanine nucleotide exchange factors (GEFs) activate Ras and Rho by promoting the exchange of bound GDP with GTP. When growth factors bind receptor tyrosine kinases, adapter proteins bring Ras-GEF near membrane-bound Ras to trigger this exchange. For Rho, phosphorylation of Rho-GEF by protein kinases enables it to replace GDP with GTP, converting the GTPase to its active state.

GTPase-activating proteins (GAPs) inactivate Ras and Rho by accelerating GTP hydrolysis, converting the active GTP-bound form back to inactive GDP-bound form. This rapid inactivation prevents uncontrolled cell proliferation and ensures precise temporal control of signaling. GAPs are essential for turning off GTPase activity and terminating downstream signaling cascades.

GDP dissociation inhibitor (GDI) binds to cytosolic Rho and arrests it in its GDP-bound inactive form, preventing premature activation. GDI sequesters Rho in the cytoplasm until stimulation occurs. Upon activation signals, GDI dissociates from Rho through displacement factors like ERM proteins or phosphorylation, allowing Rho to localize to the membrane and interact with activated Rho-GEF.

Ras is constitutively membrane-bound via prenyl chains attached to the plasma membrane. Rho, conversely, is cytosolic when bound to GDI but can translocate to the membrane following activation. Rho's membrane localization is promoted by electrostatic forces between its positively charged C terminus and negatively charged membrane lipids, or through displacement of GDI by regulatory proteins.

Once activated by GTP binding, Ras and Rho act on downstream substrates including MAP kinases and SHP2 proteins to relay signals. Active Ras-GTP recruits and activates the first kinase of the MAPK signaling cascades, such as Raf, triggering cell proliferation. Active Rho-GTP promotes downstream signaling that regulates cell shape, movement, and migration through interactions with effector proteins.

Hyperactive mutant Ras proteins remain locked in the GTP-bound active state, unable to be inactivated by GAPs. This constitutive activation continuously triggers downstream signaling pathways that promote cell proliferation without normal regulatory controls. The persistent activation of growth signals leads to uncontrolled cell division and tumorigenesis, making Ras mutations common in human cancers.

Rho family proteins, including Cdc42, Rho, and Rac, modulate cell shape, movement, and migration. These GTPases regulate the cytoskeleton and cell adhesion dynamics to control morphological changes and directional cell movement. Their activation downstream of receptor tyrosine kinases enables cells to respond to external signals by altering their physical properties and migratory behavior.