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Signaling

Cell fate decisions

Cells make decisions to:

  • die (default)
  • differentiate
  • grow and divide
  • survive

Types of signaling

Endocrine

  • long distance signaling via bloodstream

Autocrine

  • self-signaling (single cell)

Paracrine

  • signaling to adjacent cells

Generic signaling pathway structure

  1. signal

  2. reception

  3. ligand-binding domain

  4. signaling domain
  5. integration (often to nucleus)
  6. amplification
  7. target response (turns processes on/off)

Receptor classes

Ion channel receptors

  • receptor itself is an ion channel

GPCR (G-protein–coupled receptors)

  • uses G-nucleotides for internal signaling/integration

Enzyme-linked receptors

  • often tyrosine kinase receptors
  • enzyme activity is in the receptor (faster signaling)

Intracellular receptors

  • bind lipid-soluble ligands (e.g., steroids)

Integration mechanisms

G-proteins (general)

  • bind G-nucleotides
  • ON when bound to GTP, OFF when bound to GDP

Monomeric G-proteins

  • do not bind directly to the signaling domain of receptors
  • require a G-nucleotide exchange factor (GEF) to convert GDP → GTP

Heterotrimeric G-proteins (G\(\alpha\), G\(\beta\), G\(\gamma\))

  • receptor activation can drive GDP → GTP exchange without a separate GEF

Activation cycle :

  1. receptor activates; C-terminal tail activates heterotrimeric G-proteins

  2. multiple G\(\alpha\) subunits can be activated

  3. G\(\alpha\) loses GDP binding and gains GTP binding; binds GTP
  4. G\(\alpha\) dissociates from receptor and separates from G\(\beta\gamma\)
  5. G\(\beta\gamma\) can alter the shape/activity of enzyme targets

  6. termination:

  7. GTP → GDP via intrinsic GTPase activity of G\(\alpha\)

  8. subunits reassociate and signaling stops

Key principle:

  • the specific activated pathway is determined by the G\(\alpha\) subtype

GPCR downstream pathways

G\(_s\)

  • activates adenylyl cyclase

  • can generate many cAMP molecules

  • opens Ca\(^{2+}\) channels

cAMP (second messenger)

  • adenylyl cyclase converts ATP → cAMP

  • phosphate connects to the 3' OH

  • activates protein kinase A (PKA)

PKA mechanism :

  • cAMP binds regulatory subunits
  • releases/activates catalytic subunits
  • catalytic subunits transfer phosphate groups to targets (on/off switch)

G\(_i\)

  • deactivates adenylyl cyclase
  • closes Ca\(^{2+}\) channels
  • G\(\beta\) subunit activates K\(^+\) channels

G\(_q\)

  • activates phospholipase C (PLC)

Phospholipase C (PLC)

  • membrane-associated enzyme
  • cleaves phosphatidylinositol 4,5-bisphosphate (PIP\(_2\))

  • produces:

  • DAG

  • IP\(_3\)

IP\(_3\):

  • opens Ca\(^{2+}\) channels (typically ER Ca\(^{2+}\) release)

DAG:

  • stays in membrane (lipid-associated)

Protein kinase C (PKC)

  • has a masking sequence blocking the active site when inactive
  • activated when Ca\(^{2+}\) is present and signaling inputs are sufficient

G\(_{12}\)

  • affects other GPCR pathways by influencing GEFs

Signal termination

Early initiation of termination

  • termination mechanisms begin alongside reception to prevent overamplification

Second messenger degradation

  • phosphodiesterase converts cAMP → AMP

Receptor desensitization

  • GPCR kinase phosphorylates the receptor C-terminus
  • phosphorylated tail binds arrestins
  • arrestin binding stops signaling

Enzyme-linked receptors (tyrosine kinase receptors)

Domain architecture

  • ligand-binding domain
  • transmembrane domain
  • tyrosine kinase domain
  • C-terminal tail

Activation

  1. ligand binding causes receptors to move laterally and dimerize
  2. receptors cross-phosphorylate each other on tyrosines
  3. phosphorylated tyrosines recruit relay proteins with SH2 domains

Ras signaling (monomeric G-protein)

  • Ras has low intrinsic GTPase activity

Pathway :

  1. relay protein GRB2 binds phosphorylated tyrosine residues (via SH2 interactions)
  2. GRB2 is associated with SOS (a GEF)
  3. SOS activates Ras by converting Ras-GDP → Ras-GTP
  4. Ras-GTP activates Raf (MAPKKK)
  5. Raf phosphorylates MEK
  6. MEK phosphorylates ERK
  7. ERK activates transcription factors

Ras termination

  • GTPase-activating protein (GAP) accelerates Ras GTP hydrolysis:

  • Ras-GTP → Ras-GDP

Notes :

  • 2012 Nobel Prize recognized GPCR processes
  • ~35% of cancers involve Ras mutations that disrupt GTPase activity (functional knockout)

Example: cholera toxin

  • exotoxin
  • enters the cell and modifies the G\(_s\) subunit so it cannot deactivate
  • causes excessive vomiting and diarrhea

Extracellular signaling molecules

  • hormones
  • neurotransmitters (NTs)
  • cytokines
  • growth factors