Receptor tyrosin kinases

Overview of Protein Kinase–Linked Receptors

  • Shift in focus: after covering G-protein–linked receptors, we now examine receptors that possess (or recruit) protein-kinase activity.

  • Core idea: ligand binding → activation of an intracellular phosphorylation cascade → modulation of target protein activity, ultimately changing cell behavior.

  • Two principal subclasses
    • Tyrosine kinases (TK): add phosphate ((\text{PO}_4^{3-})) groups to the hydroxyl on tyrosine ((\text{Y})) residues.
    • Serine/Threonine kinases (S/TK): phosphorylate serine ((\text{S})) and/or threonine ((\text{T})) residues.
    ➔ The name explicitly tells you which amino acid side chain becomes phosphorylated.

Typical Ligands & Physiological Context

  • Ligands are growth factors or growth-factor–like hormones.
    • Classical growth factors: Epidermal Growth Factor (EGF), Nerve Growth Factor (NGF), Insulin-like Growth Factor-1 (IGF-1).
    • Hormonal exception: Insulin is technically a metabolic hormone yet binds a canonical TK receptor.

  • IGF-1 as a growth-hormone surrogate
    • Growth hormone (GH) → liver & other tissues secrete IGF-1.
    • IGF-1 then drives bone lengthening, muscle protein synthesis, and other anabolic effects attributed to GH.

Receptor Tyrosine Kinases (RTKs)

  • Primary outcomes of RTK activation
    • Cell growth (hypertrophy)
    • Cell proliferation (hyperplasia)
    • Cell differentiation/specialization
    ➔ Dysregulation = hallmark of many cancers.

Structural Hallmarks

  • Monomeric single-pass polypeptide (one (\alpha)-helix crossing the membrane).
    • Smaller than a 7-TM GPCR.

  • Extracellular domain (ECD)
    • Houses ligand-binding site.

  • Intracellular domain (ICD)
    • Contains the kinase catalytic core.
    • Also presents numerous intrinsic tyrosine residues that become auto-phosphorylated.

  • Visualization tip: “lollipop” ECD outside, short TM stem, bulky kinase head inside.

Functional Sequence (canonical RTK logic)

  1. Ligand binds → receptors often dimerize (or rearrange pre-existing dimers).

  2. Trans-phosphorylation: each kinase domain phosphorylates tyrosines on its partner.

  3. Phospho-tyrosines serve as docking sites for SH2 / PTB domain-containing adaptor proteins.

  4. Downstream cascades: MAP-K, PI3-K/AKT, PLC-γ, etc. (details to be expanded in later lectures).

Concrete Examples

  • Insulin Receptor (IR)

  • Nerve Growth Factor Receptor (TrkA)

  • Epidermal Growth Factor Receptor (EGFR)
    • Previously discussed in receptor-mediated endocytosis: cells internalize EGFR to dampen excessive mitogenic signalling.

Non-Receptor (Cytoplasmic) Tyrosine Kinases

  • Some surface receptors lack an intrinsic kinase domain; instead, they recruit a separate protein tyrosine kinase.
    • Receptor + kinase associate to form a functional signalling unit.
    • Only the kinase subunit owns catalytic activity.

  • First discovered member: Src (pronounced “sarc”)
    • Isolated from avian sarcoma virus (v-Src gene).
    • A mutated Src can transform normal cells into malignant ones.
    – Classic demonstration that a single oncogenic kinase can override normal growth control.

Connections to Earlier Material & Broader Significance

  • Complements GPCR pathways: whereas GPCRs often modulate second messengers (cAMP, IP$_3$, Ca$^{2+}$), TK receptors directly phosphorylate protein substrates.

  • Endocytosis linkage: EGFR story illustrates one cellular “brake” on RTK signalling—internalization & degradation.

  • Cancer biology: many oncogenes (EGFR, HER2, Src, Bcr-Abl) are kinases or adaptor proteins downstream of kinases.

  • Pharmacology: numerous anticancer drugs are ATP-competitive TK inhibitors (e.g., imatinib for Bcr-Abl, erlotinib for EGFR).

Ethical, Clinical, & Practical Implications

  • Targeting kinase pathways can save lives (e.g., chronic myeloid leukemia), yet raises questions about
    • Drug affordability & global access.
    • Off-target toxicities (because kinases regulate diverse tissues).
    • Resistance evolution—mutations in kinase domains can render drugs ineffective.

  • Personalized medicine: tumor genotyping to match patients with specific kinase inhibitors is now standard practice.

Key Take-Home Bullets

  • Protein kinase–linked receptors signal via phosphorylation cascades.

  • Two amino-acid specificities: Tyr  (Y)\text{Tyr}\;(Y) vs Ser/Thr  (S/T)\text{Ser/Thr}\;(S/T).

  • RTKs = single-pass membrane proteins; many drive growth & proliferation.

  • Some receptors rely on non-receptor tyrosine kinases (e.g., Src) for catalytic activity.

  • Dysregulation = major route to oncogenesis; therapeutic inhibition is a central theme of modern cancer treatment.