A researcher introduces a mutation that causes a receptor tyrosine kinase (RTK) to dimerize and autophosphorylate even in the absence of its ligand. What is the most likely consequence for the cell?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

Receptor tyrosine kinases (RTKs) such as the epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor (PDGFR) operate through a tightly regulated sequence of molecular events. In their inactive state, RTKs exist as monomeric transmembrane proteins with an extracellular ligand-binding domain, a single alpha-helical transmembrane segment, and an intracellular kinase domain. Ligand binding—by molecules like EGF, PDGF, or insulin—to the extracellular domain induces a conformational change that drives dimerization. This dimerization positions the two intracellular kinase domains in close proximity, enabling each kinase to phosphorylate specific tyrosine residues on the partner receptor's cytoplasmic tail. This reciprocal autophosphorylation creates docking sites containing phosphotyrosine motifs recognized by SH2 (Src Homology 2) domains on downstream adaptor proteins such as GRB2. GRB2 then recruits SOS, a guanine nucleotide exchange factor, which catalyzes the exchange of GDP for GTP on the Ras GTPase. Activated Ras initiates the MAP kinase cascade (Raf → MEK → ERK), ultimately driving transcription of genes promoting cell division, including cyclin D1 and c-Myc. The system's fidelity depends absolutely on ligand-gated dimerization; without the ligand, the kinase domains remain spatially separated, and no autophosphorylation occurs.

Why Other Options Are Wrong

A mutation that forces constitutive dimerization effectively bypasses the ligand requirement entirely. The juxtamembrane and transmembrane domains, when mutated, can lock the receptor in a dimeric conformation through stabilized hydrophobic interactions or disulfide bond formation between cysteine residues. The kinase domains then autophosphorylate regardless of whether EGF, PDGF, or any native ligand occupies the extracellular binding pocket. This creates a ligand-independent signaling state: GRB2 constitutively binds phosphotyrosine residues, SOS continuously loads Ras with GTP, and the Raf-MEK-ERK cascade fires persistently. The cell interprets this unceasing signal as a perpetual mitogenic command, driving unregulated progression through the cell cycle past the G1/S checkpoint even in the absence of any external growth factor stimulus.

PILLAR 2 — STEP-BY-STEP LOGIC

The question specifies a mutation causing dimerization and autophosphorylation in the absence of ligand. Tracing the causal chain: dimerization → kinase domain proximity → autophosphorylation of tyrosine residues → SH2 domain docking by adaptor proteins → Ras activation → MAPK cascade → transcription of proliferation genes. Since the mutation renders the receptor permanently dimerized, every downstream step proceeds constitutively. The phosphotyrosine residues on the RTK cytoplasmic tail are chronically available for GRB2 binding, the Ras-GTP concentration remains elevated, and nuclear transcription factors like ELK1 and c-Fos are continuously phosphorylated and activated. The cell receives an uninterrupted mitogenic signal. Option B correctly identifies this outcome: the signal transduction pathway remains continuously activated, driving unregulated cell proliferation. This mechanism precisely mirrors the pathology of oncogenic RTK mutations, such as HER2/neu amplification in certain breast adenocarcinomas, where receptor overexpression drives spontaneous dimerization and constitutive downstream signaling through the PI3K/Akt and Ras/MAPK pathways.

PILLAR 3 — DISTRACTOR ANALYSIS

Option A suggests that signal transduction would be inhibited or blocked entirely. This reflects a fundamental misunderstanding of RTK activation mechanics. Students selecting this answer conflate "mutation" with "loss of function" and fail to recognize that autophosphorylation is the activating event itself, not a neutral or inhibitory modification. The mutation described produces a gain-of-function phenotype, not a signaling blockade.

Option C proposes that the receptor would be degraded or internalized and rendered nonfunctional. While prolonged RTK activation can trigger clathrin-mediated endocytosis and lysosomal degradation as a negative feedback mechanism, this occurs over extended timeframes and does not represent the immediate or most likely consequence. Furthermore, even during internalization, the receptor often continues signaling from endosomal compartments. Students choosing this option confuse long-term receptor downregulation with the primary direct consequence of constitutive kinase activity.

Option D states that the cell would become unresponsive to all extracellular signals. This incorrectly generalizes the mutation's effect across all receptor systems. A mutated RTK affects only the specific signaling cascade downstream of that particular receptor. The cell retains functional G-protein coupled receptors, ligand-gated ion channels, and other RTKs that respond normally to their respective ligands. This distractor exploits the tendency to overgeneralize a single molecular defect to global cellular dysfunction, a common reasoning error in cell communication questions.