Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
12.4 Receptor Tyrosine Kinases
This chapter explains how receptor tyrosine kinases (RTKs), especially the insulin receptor, convert extracellular signals into intracellular phosphorylation cascades that regulate metabolism, glucose uptake, gene expression, growth, and cell division. RTK signaling differs fundamentally from GPCR signaling but shares the general principles of amplification, branching, and signal integration.
What Are Receptor Tyrosine Kinases?
• Receptor tyrosine kinases (RTKs) are plasma membrane receptors that possess intrinsic protein kinase activity.
• They have an extracellular ligand-binding domain.
• They have a single transmembrane segment.
• They have a cytoplasmic catalytic domain that phosphorylates Tyr residues.
• This arrangement is fundamentally different from GPCRs, which signal indirectly through G proteins.
Examples of RTKs
• The insulin receptor and epidermal growth factor receptor are classic RTK prototypes.
• Humans have 58 receptor tyrosine kinases.
General Function of RTKs
• RTKs bind extracellular ligands.
• Ligand binding activates the intracellular Tyr kinase domain.
• The receptor then phosphorylates Tyr residues on itself and on target proteins.
• These phosphotyrosines become docking sites for signaling proteins.
• This initiates signaling cascades that affect cytosol and nucleus.
The Insulin Receptor as the Prototype
• The insulin receptor is the best-studied RTK in this section.
• It regulates both metabolic enzymes and gene expression.
• It sends signals from the plasma membrane to enzymes in the cytosol and transcriptional regulators in the nucleus.
Structure of the Insulin Receptor
• The active insulin receptor consists of two extracellular α subunits and two transmembrane β subunits.
• The α subunits contain the insulin-binding region.
• The intracellular domains of the β subunits contain Tyr kinase activity.
• The insulin receptor is already a dimer before insulin binds.
• This is unusual, because many other RTKs dimerize only after ligand binding.
Activation by Insulin Binding
• One insulin molecule binds between the two α subunits on the extracellular side.
• This causes structural movement that brings the intracellular kinase domains together.
• Each β subunit phosphorylates Tyr residues near the carboxyl terminus of the other β subunit.
• This is receptor autophosphorylation.
Role of the Activation Loop
• In the inactive receptor, the activation loop blocks the active site.
• After autophosphorylation, the activation loop moves away.
• This exposes the active site for substrate binding.
• The mechanism resembles activation of other protein kinases such as PKA and PKC.
Autophosphorylation Consequences
• Autophosphorylation activates the receptor’s Tyr kinase domain.
• It creates a receptor capable of phosphorylating additional intracellular targets.
• This begins downstream signaling cascades.
IRS1 Is a Key Early Target
• One major target of the activated insulin receptor is insulin receptor substrate 1 (IRS1).
• IRS1 is phosphorylated on several Tyr residues.
• These phosphotyrosines form nucleation points for assembly of signaling complexes.
• IRS1 becomes a major branching platform in insulin signaling.
SH2 Domains Recognize Phosphotyrosine
• Many signaling proteins contain SH2 domains.
• SH2 domains bind specifically to phosphotyrosine-containing sequences in other proteins.
• Phosphorylated IRS1 recruits SH2-containing proteins.
• This is a common mechanism throughout RTK signaling.
Adaptor Protein Grb2
• One protein recruited to phospho-IRS1 is Grb2.
• Grb2 binds IRS1 through its SH2 domain.
• Grb2 is an adaptor protein and has no intrinsic catalytic activity.
• Its role is to connect other proteins that need to interact.
SH3 Domain and Sos Recruitment
• Grb2 also contains an SH3 domain.
• SH3 binds a proline-rich region of Sos.
• Sos is thereby recruited to the receptor complex.
• Sos stands for son of sevenless.
Sos Activates Ras
• Sos acts as a guanine nucleotide exchange factor (GEF).
• It promotes exchange of GDP for GTP on Ras.
• Ras is a small G protein.
• Ras becomes active in its GTP-bound state.
Ras Triggers the MAPK Cascade
• Activated Ras stimulates Raf-1.
• Raf-1 activates MEK.
• MEK activates ERK.
• These three kinases form a signaling cascade.
• Each kinase activates the next by phosphorylation.
MAPK Family Terminology
• ERK belongs to the MAPK family (mitogen-activated protein kinases).
• MEK belongs to the MAPKK family (MAP kinase kinase).
• Raf-1 belongs to the MAPKKK family (MAP kinase kinase kinase).
• MAPKKs such as MEK phosphorylate both Thr and Tyr residues in MAPKs.
• MAPKs and MAPKKKs generally phosphorylate Ser or Thr residues.
Nuclear Effects of ERK
• Activated ERK enters the nucleus.
• It phosphorylates transcription factors such as Elk1.
• These factors regulate transcription of insulin-responsive genes.
• Some target genes are essential for cell division.
• Therefore insulin also acts as a growth factor, not just a metabolic hormone.
Signal Amplification
• The insulin receptor pathway is an example of a kinase cascade that amplifies the signal.
• One activated receptor can activate many IRS1 molecules.
• Each signaling step can activate multiple downstream proteins.
• This results in a large cellular response to a relatively small extracellular signal.
MAPK Cascades Are Widely Used
• Similar MAPK cascades are triggered by many growth factors.
• Examples include platelet-derived growth factor (PDGF) and epidermal growth factor (EGF).
• These growth factors also act through receptor tyrosine kinases.
Adaptor Proteins Organize Divergent Pathways
• Nonenzymatic adaptor proteins, often containing intrinsically disordered regions, organize signaling complexes.
• They bring pathway components together.
• This helps one receptor activate multiple downstream branches.
The PI3K Branch of Insulin Signaling
• IRS1 does not recruit only Grb2.
• Another major binding partner is phosphoinositide 3-kinase (PI3K).
• PI3K binds IRS1 through its SH2 domain.
• This initiates a second major branch of signaling.
PI3K Changes a Membrane Lipid
• PI3K phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2).
• It produces phosphatidylinositol 3,4,5-trisphosphate (PIP3).
• The phosphoryl group is transferred from ATP.
• PIP3 accumulates on the cytoplasmic side of the plasma membrane.
PIP3 as a Docking Site
• The highly negatively charged head group of PIP3 serves as a nucleation point for signaling proteins.
• It recruits proteins involved in another kinase cascade.
• Thus a membrane phospholipid becomes a critical branch point in insulin signaling.
Activation of PKB/Akt
• Protein kinase B (PKB), also called Akt, binds to PIP3.
• PKB is then phosphorylated and activated by PDK1.
• Activated PKB phosphorylates downstream target proteins on Ser or Thr residues.
Regulation of GSK3
• One key PKB target is glycogen synthase kinase 3 (GSK3).
• Active, nonphosphorylated GSK3 phosphorylates glycogen synthase.
• This inactivates glycogen synthase.
• Therefore active GSK3 tends to slow glycogen synthesis.
Insulin Stimulates Glycogen Synthesis
• PKB phosphorylates GSK3.
• Phosphorylated GSK3 becomes inactive.
• Glycogen synthase is no longer inhibited by GSK3.
• Glycogen synthesis therefore increases in liver and muscle.
• This explains part of insulin’s anabolic effect on glycogen storage.
A Third Branch: GLUT4 Movement
• In muscle and fat tissue, insulin also stimulates movement of GLUT4 transporters to the plasma membrane.
• This branch also depends on PI3K and PKB.
• PKB acts through the small G proteins RAC1 and Rab.
• These proteins promote clathrin-aided movement of GLUT4-containing vesicles.
• The vesicles fuse with the plasma membrane.
• Glucose uptake from blood increases.
Metabolic Importance of GLUT4 Trafficking
• By increasing plasma membrane GLUT4, insulin strongly enhances glucose uptake into muscle and adipose tissue.
• This has major metabolic and medical importance.
Timing of Insulin Responses
• Different parts of the pathway act on different time scales.
• Insulin receptor and IRS1 autophosphorylation occur within seconds.
• Protein kinase Cβ phosphorylation occurs within about 15 seconds.
• Sos and Gab proteins are phosphorylated within about 0.5 to 1 minute.
• ERK1 reaches maximal phosphorylation within about 3 minutes.
• GLUT4 translocation takes roughly 15 minutes.
• Changes in gene expression occur over several hours.
Insulin Can Also Act in the Nucleus
• A slower, recently described mechanism allows insulin to enter the cell and nucleus in some cases.
• There, with the help of nuclear proteins, insulin can regulate gene expression by binding promoter regions of DNA.
Termination of the PI3K-PKB Pathway
• Like all signaling pathways, the PI3K-PKB branch must be shut off.
• PTEN is a PIP3-specific phosphatase.
• PTEN removes the phosphate at the 3 position of PIP3.
• This converts PIP3 back to PIP2.
• Without PIP3, PKB can no longer dock and remain activated.
• The signaling chain is broken.
PTEN and Cancer
• PTEN acts as a tumor suppressor.
• In many cancers, PTEN is mutated.
• This causes excessive PI3K pathway activity.
• Persistent signaling promotes cell division and tumor growth.
Other Receptor Tyrosine Kinases
• The insulin receptor is one RTK prototype, but many others exist.
• Examples include receptors for:
• EGF
• PDGF
• VEGF
• TrkA
• FGF
• These all possess cytoplasmic Tyr kinase domains.
• Their extracellular domains differ and determine ligand specificity.
RTK Dimerization
• Most RTKs are monomeric until ligand binding induces dimerization.
• Dimerization activates their Tyr kinase domains.
• The insulin receptor is the major exception because it is already dimeric.
Why RTK Systems Are So Complex
• This architecture allows amplification.
• It allows integration of signals from several receptors, such as EGFR and PDGFR, which can all phosphorylate IRS1.
• It allows one receptor to trigger several different pathways through IRS1.
• Closely related IRS proteins such as IRS2 and IRS3 have distinct tissue distributions and functions, increasing regulatory flexibility.
Cross Talk between Signaling Pathways
• Signaling pathways do not act independently.
• They communicate extensively through cross talk.
• Metabolic control is therefore highly interwoven and multilayered.
Insulin Opposes Epinephrine
• Insulin generally opposes the metabolic effects of epinephrine.
• Insulin signaling directly reduces β-adrenergic signaling.
Mechanism of Insulin–β-Adrenergic Cross Talk
• The insulin receptor directly phosphorylates two Tyr residues on the cytoplasmic tail of the β₂-adrenergic receptor.
• PKB, activated by insulin, phosphorylates two Ser residues in the same region.
• These four phosphorylations trigger clathrin-aided internalization of the β-adrenergic receptor.
• This lowers the cell’s sensitivity to epinephrine.
Cross Talk Can Also Enhance Signaling
• In another form of cross talk, phosphotyrosine residues on the β-adrenergic receptor can recruit SH2-containing proteins such as Grb2.
• This can enhance MAPK activation by insulin.
• ERK activation by insulin can be 5- to 10-fold greater in the presence of the β-adrenergic receptor.
Other Layers of Signaling Complexity
• cAMP and Ca²⁺ signaling systems also interact extensively.
• Metabolic intermediates such as fatty acids, ceramides, amino acids, and bile acids can influence insulin signaling.
• Systems biology must account for all of these interactions to explain tissue-wide metabolic regulation.
In a Nutshell
Receptor tyrosine kinases such as the insulin receptor signal by ligand-induced activation of intrinsic Tyr kinase domains, followed by autophosphorylation and recruitment of adaptor and effector proteins. IRS1 serves as a central branching platform that activates Ras-MAPK pathways for gene expression and PI3K-PKB pathways for glycogen synthesis and GLUT4 translocation. RTK signaling is highly amplified, branched, and deeply integrated with other pathways such as β-adrenergic signaling.