A receptor tyrosine kinase (RTK) is a cell-surface receptor whose cytoplasmic region contains or is associated with tyrosine-kinase activity.

Many RTKs are activated when ligand binding promotes receptor dimerization or rearranges a pre-existing dimer. The receptor kinase domains then phosphorylate tyrosine residues on the receptors themselves.

This autophosphorylation creates docking sites for intracellular signaling proteins.

Receptor phosphorylation recruits signaling machinery

The basic sequence is

ligand binds RTK
      ↓
receptor kinase activity increases
      ↓
tyrosines on receptor become phosphorylated
      ↓
proteins recognize phosphotyrosine docking sites
      ↓
downstream signaling pathways assemble

Phosphorylation therefore changes not only enzyme activity but also which proteins can bind the receptor.

Ras provides a GDP/GTP switch

One common RTK branch recruits proteins that activate the small GTPase Ras. A guanine-nucleotide exchange factor promotes exchange of GDP for GTP on Ras.

$$\mathrm{Ras{-}GDP\rightarrow Ras{-}GTP}$$

Ras-GTP binds and activates downstream effectors. GTP hydrolysis eventually returns Ras toward the GDP-bound state.

A MAP-kinase cascade relays the signal

A canonical downstream route is

Ras-GTP
  ↓
Raf
  ↓ phosphorylation cascade
MEK
  ↓
ERK / MAPK

Raf, MEK and ERK are successive protein kinases in this signaling module. Activated ERK can phosphorylate proteins in the cytoplasm and can also influence transcriptional regulators in the nucleus.

Because each activated kinase can modify many molecules of the next layer, the cascade can amplify a receptor signal.

Signaling must remain conditional

Mutations that keep an RTK, Ras, or downstream kinase active can make signaling less dependent on the extracellular ligand. This illustrates why both activation and shutoff mechanisms are essential for controlled cell growth and differentiation.

The RTK-Ras-MAPK pathway therefore exemplifies a general architecture: ligand-controlled receptor phosphorylation creates docking sites, activates a GTPase switch, and feeds a kinase cascade that changes cell behavior.