23.2: Small GTPases - Ras and Rho
Ras and Rho are small monomeric GTPases that act downstream of receptor tyrosine kinase (RTK) and regulate various cellular processes. These GTPases switch between active and inactive states by binding to guanine nucleotides.
Three regulatory proteins control their activity:
- Guanine nucleotide exchange factors or GEF,
- GTPase-activating proteins or GAPs, and
- Guanine nucleotide-dissociation inhibitors or GDIs.
The GEF activates the GTPase by exchanging the bound-GDP with GTP. The GTPases’ activity is turned off by GAP. They hydrolyze the bound GTP to GDP. The GDI further arrests the GTPase in its GDP-bound inactive form.
Ras is a membrane-bound GTPase and attaches to the plasma membrane via prenyl chains. There are three types of Ras present in mammals: H-Ras, K-Ras, and N-Ras. The binding of growth factor ligand activates RTKs and triggers Ras-GEF to displace GDP with GTP. The active Ras-GTP can now recruit and activate the first kinase of the MAPK signaling cascade, such as MAPKKK/Raf, triggering MAPK signaling, thereby initiating cell proliferation. However, it quickly hydrolyzes the GTP and is turned off, preventing uncontrolled cell proliferation. Hyperactive mutant forms of Ras stay in the GTP-bound state and often lead to tumorigenesis.
Rho family proteins such as Cdc42, Rho, and Rac modulate cell shape, movement, and migration. Rho can be soluble or membrane-bound. Soluble RhoGDP binds GDI and stays inactive. Following stimulation, GDI dissociates, facilitating membrane localization of Rho in one of the following ways:
- Electrostatic attractive forces between positively charged C termini of Rho and the negatively charged plasma membrane lipids promote its localization.
- The truncated tropomyosin-related kinase B receptor or T1 binds RhoGDI and sequesters it on the plasma membrane, allowing Rho to escape to the membrane for activation. Alternatively, displacement factors such as the ezrin (E), radixin (R), and moesin (M) or ERM family proteins interact with the RhoGTPases-GDI complex, subsequently displacing the GDI from Rho.
- Alternatively, phosphorylation of RhoGDI by protein kinases destabilizes the RhoGTPases-GDI complex, releasing Rho for membrane localization.
Once localized to the membrane, Rho interacts with a nearby activated RhoGEF and undergoes GDP/GTP exchange. Activated RhoGTP now promotes downstream signaling and regulates various cellular processes.