AP Biology: G?Protein?Coupled Receptors, Receptor Tyrosine Kinases, Ligand?Gated Ion Channels

G?Protein?Coupled Receptors, Receptor Tyrosine Kinases, Ligand?Gated Ion Channels

Concept Summary

• G-Protein-Coupled Receptors (GPCRs): Seven-transmembrane domain receptors that activate heterotrimeric G-proteins upon ligand binding, initiating signal transduction cascades (e.g., cAMP, IP?/DAG pathways).
• Receptor Tyrosine Kinases (RTKs): Single-transmembrane receptors that dimerize and autophosphorylate tyrosine residues upon ligand binding, triggering downstream signaling (e.g., MAPK, PI3K pathways).
• Ligand-Gated Ion Channels (LGICs): Multimeric receptors that open/close ion-permeable pores in response to ligand binding, enabling rapid changes in membrane potential (e.g., neurotransmitter receptors).
• Signal Amplification: All three systems convert extracellular signals into intracellular responses, but GPCRs/RTKs use second messengers (amplification), while LGICs directly alter ion flow (no amplification).
• Desensitization: Prolonged ligand exposure can reduce receptor responsiveness (e.g., GPCR phosphorylation, RTK internalization, LGIC inactivation).

Core Questions

WHAT (definitional)

Q: What is a G-protein? A: A heterotrimeric (?) GTP-binding protein that acts as a molecular switch, activated by GPCRs to relay signals to downstream effectors. Trap/Clarification: G-proteins are not kinases—they activate enzymes (e.g., adenylyl cyclase) but do not phosphorylate targets themselves.

Q: What distinguishes RTKs from other receptors? A: RTKs possess intrinsic tyrosine kinase activity, autophosphorylating their own cytoplasmic tails to create docking sites for SH2-domain proteins. Trap/Clarification: Not all receptors with "kinase" in the name are RTKs (e.g., serine/threonine kinases like TGF-? receptors).

WHY (causal/explanatory)

Q: Why do GPCRs require G-proteins? A: G-proteins transduce the signal from the receptor to intracellular effectors (e.g., enzymes, ion channels), enabling signal amplification and diversification. Trap/Clarification: GPCRs themselves do not catalyze reactions—they rely on G-proteins to propagate the signal.

Q: Why is RTK dimerization critical? A: Dimerization brings kinase domains into proximity, enabling cross-phosphorylation of tyrosine residues and activation of downstream pathways. Trap/Clarification: Monomeric RTKs are inactive; ligand binding induces dimerization, not the reverse.

HOW (process/application)

Q: How does a GPCR activate a G-protein? A: Ligand-bound GPCR acts as a GEF (guanine nucleotide exchange factor), catalyzing GDP?GTP exchange on the G? subunit, causing G?-GTP and G to dissociate and activate effectors. Trap/Clarification: GPCRs do not hydrolyze GTP—the G? subunit has intrinsic GTPase activity to terminate the signal.

Q: How do ligand-gated ion channels differ from voltage-gated channels? A: LGICs open/close in response to ligand binding (e.g., neurotransmitters), while voltage-gated channels respond to changes in membrane potential. Trap/Clarification: LGICs are not selective for all ions (e.g., nAChRs are Na?/K? permeable, GABA?Rs are Cl? selective).

CAN (conditions/possibilities)

Q: Can RTKs activate G-proteins? A: No—RTKs signal via phosphorylation cascades (e.g., Ras/MAPK), while GPCRs exclusively use G-proteins (though crosstalk exists via scaffold proteins). Trap/Clarification: RTKs and GPCRs are distinct pathways, but some cells integrate signals from both (e.g., via ?-arrestin).

Q: Under what conditions does a ligand-gated ion channel close? A: Channels close when the ligand dissociates or the receptor undergoes desensitization (e.g., phosphorylation, conformational change). Trap/Clarification: Some LGICs (e.g., NMDA receptors) require both ligand binding and membrane depolarization to open.

Quick Facts & Traps

• Fact: GPCRs are the largest family of cell-surface receptors (~800 human genes) and targets of ~35% of FDA-approved drugs.
• Trap: "G-proteins are always stimulatory"-Reality: G? subunits can be stimulatory (G) or inhibitory (G).
• Fact: RTKs often activate the Ras/MAPK pathway, a key regulator of cell division (mutations-cancer).
• Trap: "All RTKs are monomers before ligand binding"-Reality: Some RTKs (e.g., insulin receptor) are pre-dimerized but inactive.
• Fact: LGICs mediate fast synaptic transmission (milliseconds), unlike GPCRs/RTKs (seconds to minutes).
• Trap: "Ion channels always depolarize the cell"-Reality: Cl? influx (e.g., GABA?Rs) hyperpolarizes neurons, inhibiting firing.

Rapid-Fire True/False

• Statement: GPCRs and RTKs both use second messengers to amplify signals. Answer: TRUE Why the common mistake happens: Students forget that RTKs (e.g., via PI3K) generate second messengers (PIP?), while LGICs do not.

• Statement: Ligand-gated ion channels require ATP to function. Answer: FALSE Why the common mistake happens: Confusion with ATP-dependent pumps (e.g., Na?/K? ATPase); LGICs are passive.

• Statement: Phosphorylation of GPCRs always enhances signaling. Answer: FALSE Why the common mistake happens: Phosphorylation (e.g., by GRKs) often desensitizes GPCRs by recruiting ?-arrestin.