Signal Transduction

The shape of a signaling pathway

Every signaling pathway follows the same five-step pattern: ligand binds receptor; receptor activates transducer; transducer modifies a messenger; messenger drives a cascade; cascade rewrites behavior. The variations are in the chemistry of each step.

   Ligand                              (outside)
     │
   Receptor ━━━━━━━━━━━━━━━━━ membrane
     │
   Transducer  → G-protein (Gα-GTP)
     │          → autophosphorylation (RTK)
     │          → channel opens
   2nd messenger:  cAMP / IP3 / DAG / Ca²⁺
     │
   Kinase cascade:  PKA, PKC, MAPK, Akt
     │              → ultrasensitive switch
   Effector / output: TF, channel, enzyme,
                      cytoskeleton → expression,
                      metabolism, shape, division

The two big receptor families

Two receptor classes do most of the heavy lifting in animal cells.

G-protein-coupled receptors (GPCRs) are seven-pass transmembrane proteins. ~800 in the human genome — rhodopsin (vision), ~400 odorant receptors, most neurotransmitter and peptide-hormone receptors, plus a long orphan tail. Ligand binds outside; the seventh helix shifts; on the inside, the receptor catalyzes GDP/GTP exchange on the α subunit of a heterotrimeric G-protein. Gα-GTP dissociates from Gβγ, and both halves go on to activate effectors:

• Gαs → adenylyl cyclase → cAMP up. β-adrenergic, glucagon, vasopressin V2.
• Gαi → inhibits adenylyl cyclase → cAMP down. M2 muscarinic, α2-adrenergic, opioids.
• Gαq → phospholipase C → IP3 + DAG → Ca²⁺ release and PKC activation. α1, M1/M3.
• Gα12/13 → Rho-GEFs → cytoskeletal reorganization. LPA receptors.
• Gβγ → activates K⁺ channels (cardiac slowing by ACh), inhibits Ca²⁺ channels, activates PI3K-γ.

Gα has an intrinsic GTPase that resets it to GDP within seconds, accelerated by RGS proteins. Multiple off layers.

Receptor tyrosine kinases (RTKs) are single-pass transmembrane proteins with a cytoplasmic kinase domain. Ligands like EGF, insulin, PDGF, FGF, and VEGF trigger receptor dimerization, and the two cytoplasmic kinase domains phosphorylate each other on tyrosines. The phosphotyrosines become docking sites for SH2/PTB-domain adaptors (Grb2, IRS-1, PLC-γ, PI3K) that assemble a multi-protein signaling complex on the receptor tail. RTKs typically activate Ras-MAPK, PI3K-Akt, and PLC-γ in parallel. ~58 RTKs in the human genome; mutations drive a large fraction of cancer.

Other major classes — ligand-gated ion channels (nicotinic ACh, GABA-A, glutamate), cytokine receptors (use JAK kinases instead of intrinsic kinase activity), nuclear receptors (steroid hormones bind transcription factors directly) — fill in around GPCRs and RTKs.

The MAPK cascade — amplification and switching

The mitogen-activated protein kinase cascade is the textbook signaling pathway. Three kinases phosphorylate each other in series:

   Ras-GTP  →  Raf  →  MEK  →  ERK  →  TFs (Elk-1, Myc, …)
                                       → gene expression, cell-cycle entry

Ras is a small GTPase — same family as Gα but monomeric. Growth factors via RTKs recruit GEFs (SOS) that load Ras with GTP. Ras-GTP binds Raf; Raf phosphorylates MEK; MEK phosphorylates ERK. Each step amplifies; the doubly-phosphorylated requirement makes the overall response ultrasensitive — essentially digital, off below a threshold and fully on above it. The same cascade signals proliferation when ERK activity is brief and differentiation when it stays high for hours: duration and amplitude carry information.

Pathways at a glance

	Gαs / cAMP / PKA	Gαq / IP3-DAG / PKC	RTK / Ras-MAPK	RTK / PI3K-Akt	JAK-STAT	Wnt / β-catenin
Receptor	GPCR (β-adrenergic, glucagon)	GPCR (α1, M1/M3)	EGFR, FGFR, etc.	Insulin receptor, PI3K-coupled RTKs	Cytokine receptors	Frizzled (atypical GPCR)
Transducer	Gαs → adenylyl cyclase	Gαq → PLC-β	Ras → Raf	PI3K → PIP3	JAK kinases	Disheveled → axin/GSK3 disruption
Second messenger	cAMP	IP3 (Ca²⁺) + DAG	None — phosphorylation	PIP3 lipid	None — phosphorylation	None — protein stability
Major effector	PKA	PKC + Ca²⁺ enzymes	ERK	Akt → mTOR	STAT TFs	β-catenin → TCF/LEF
Typical timescale	Seconds	Seconds (Ca²⁺ spike)	Minutes	Minutes	Minutes to hours	Hours
Hallmark output	Glycogen breakdown, lipolysis	Smooth muscle contraction	Proliferation, differentiation	Survival, growth, glucose uptake	Immune gene expression	Embryonic patterning, stem-cell renewal
Disease example	Cholera (constitutive Gαs)	Hypertension	~25% of cancers (Ras, BRAF)	Cancer, diabetes (insulin resistance)	Leukemias (JAK2 V617F)	Colorectal cancer (APC loss)

Real numbers

• Human genome: ~800 GPCRs, ~58 RTKs, ~518 protein kinases total.
• ~30% of FDA-approved drugs target a GPCR — the most successful drug-target class in pharmacology.
• cAMP: ~10⁻⁷ M (rest) → ~10⁻⁵ M within seconds of β-adrenergic stimulation — a 100-fold rise.
• Cytosolic Ca²⁺: ~100 nM rest → ~1 μM during a signaling spike — tenfold change in microdomains.
• MAPK cascade typically amplifies ~10⁴–10⁶ fold from receptor to ERK output.
• Ras-GTP hydrolysis: ~0.02/sec alone, ~5/sec with GAPs. Oncogenic G12V/G13D mutants drop GAP-stimulated rate to ~zero — Ras stays "on."
• Cholera produces stool volumes up to 1 L/hr; untreated mortality ~50%.

Variants and drugs

• β-blockers (propranolol, metoprolol): block β-adrenergic GPCRs. Hypertension, heart failure, anxiety.
• Antihistamines (loratadine, cetirizine), opioids (morphine, fentanyl), SSRIs (fluoxetine): all act through GPCR signaling, directly or indirectly.
• Imatinib (Gleevec): BCR-ABL tyrosine kinase inhibitor for CML — turned a fatal leukemia into a chronic disease.
• Trastuzumab (Herceptin): HER2 RTK antibody for HER2+ breast cancer. Vemurafenib: BRAF V600E inhibitor for melanoma. Sotorasib: covalent KRAS G12C inhibitor, the first direct Ras drug.
• Sildenafil (Viagra): PDE5 inhibitor; raises cGMP in vascular smooth muscle. Developed for angina — the side effects became the indication.
• Cholera toxin: ADP-ribosylates Gαs at Arg201, blocking GTP hydrolysis — Gαs stays GTP-bound, adenylyl cyclase runs continuously, crypt cells dump Cl⁻ and water.
• Pertussis toxin: ADP-ribosylates Gαi, locking it GDP-bound — inhibitory signaling fails.
• Forskolin: directly activates adenylyl cyclase; standard research reagent.

Common misconceptions and failure modes

• "One receptor, one pathway." Most receptors are pleiotropic — β2-AR uses Gαs and Gαi; RTKs activate Ras-MAPK, PI3K-Akt, and PLC-γ in parallel.
• "Pathways are linear." They're networks. Crosstalk is the rule.
• "Stronger signal = stronger response." Often sigmoidal or biphasic — excess signal can desensitize the cell.
• "Phosphorylation always activates." Sometimes activates, sometimes inactivates, sometimes just creates a docking site. Context-dependent.
• "G-proteins are receptors." No — they're separate peripheral membrane proteins. The GPCR is the receptor; the G-protein is its partner.
• "All cancers are DNA repair mutations." A huge fraction are signaling mutations: KRAS/NRAS/HRAS, BRAF, PIK3CA, EGFR, HER2. ~25% of human cancers carry a Ras mutation alone.
• "Cholera kills with a toxin." The toxin locks Gαs ON; the cell dumps water and electrolytes; the patient dies of dehydration. Restore fluids and most survive — basis of oral rehydration therapy.
• "Termination is just the ligand falling off." Active and multilayered: GRK-arrestin desensitization, GTPase hydrolysis, phosphodiesterases, phosphatases, internalization — all parallel. Without them the cell saturates and goes deaf.