RAS and Small GTPases

Small GTPases, including the RAS proteins, are monomeric guanine-nucleotide-binding proteins that act as molecular switches in intracellular signaling. They cycle between an active GTP-bound state and an inactive GDP-bound state, and in the active state they engage effectors that drive processes such as proliferation, vesicle traffic, and cytoskeletal dynamics.

Definition

A small GTPase is a single-subunit (~20-25 kDa) guanine-nucleotide-binding protein that signals when bound to GTP and is switched off by hydrolysis of GTP to GDP, with its nucleotide state set by accessory regulatory proteins.

Scope

This topic covers the GTPase switch cycle, the regulators that control it (GEFs, GAPs, and GDIs), the organization of the Ras superfamily into functional branches, and the canonical RAS effector pathways. It treats small GTPases as a signaling mechanism; disease associations are described as reference context only.

Core questions

• How does the GTPase switch convert a transient signal into a controlled output?
• What proteins regulate the rate of nucleotide exchange and hydrolysis?
• How is the Ras superfamily organized into functional branches?

Key concepts

• GTP/GDP conformational switch
• Switch I and switch II regions
• Guanine-nucleotide exchange factors (GEFs)
• GTPase-activating proteins (GAPs)
• Guanine-nucleotide dissociation inhibitors (GDIs)
• Ras superfamily branches (Ras, Rho, Rab, Ran, Arf)
• Effector engagement and signal amplification

Mechanisms

Small GTPases work as binary switches. In the GTP-bound state, two flexible regions known as switch I and switch II adopt an ordered conformation that creates the binding surface for downstream effectors; hydrolysis of GTP to GDP relaxes these regions and turns the switch off (Vetter & Wittinghofer, 2001). Because intrinsic exchange and hydrolysis are slow, the cycle is controlled by accessory proteins: guanine-nucleotide exchange factors (GEFs) promote release of GDP so GTP can load and activate the switch, while GTPase-activating proteins (GAPs) accelerate hydrolysis to inactivate it; guanine-nucleotide dissociation inhibitors (GDIs) sequester some GTPases in the cytosol (Bos et al., 2007). The Ras superfamily comprises functional branches — Ras, Rho, Rab, Ran, and Arf — that govern proliferation signaling, cytoskeletal organization, vesicular transport, and nucleocytoplasmic and membrane traffic, respectively (Wennerberg et al., 2005).

Clinical relevance

RAS proteins are among the most frequently studied signaling molecules in oncology because activating mutations that impair GTP hydrolysis lock the switch in its active state and sustain proliferative signaling; this biology underlies extensive research into RAS-pathway-directed therapeutics (Downward, 2003). The entry describes this mechanism as reference knowledge and is not a basis for diagnosis or treatment decisions.

Evidence & guidelines

The topic rests on structural and biochemical reviews of the GTPase cycle and superfamily organization (Vetter & Wittinghofer, 2001; Wennerberg et al., 2005; Bos et al., 2007) rather than on clinical practice guidelines.

History

RAS genes were identified as transforming oncogenes from retroviruses and human tumors around 1982, and subsequent structural studies of the GTPase fold and the discovery of GEFs and GAPs established the molecular-switch model that now applies across the Ras superfamily (Vetter & Wittinghofer, 2001; Bos et al., 2007).

Key figures

• Alfred Wittinghofer
• Channing Der
• Johannes Bos
• Julian Downward

Seminal works

• vetter-2001
• bos-2007
• wennerberg-2005

Frequently asked questions

Why is a GTPase called a molecular switch?

Because it has two stable states — active when bound to GTP and inactive when bound to GDP — and toggling between them turns downstream signaling on or off.

What do GEFs and GAPs do?

GEFs promote loading of GTP to activate the GTPase, and GAPs accelerate GTP hydrolysis to switch it off; together they set how long the switch stays on.