A Mutated Receptor and a Checkpoint
A cultured cell line carries a mutation that removes the intracellular phosphatase domain from a receptor tyrosine kinase (RTK). In normal cells, ligand binding causes receptor dimerization and autophosphorylation, followed by docking of signaling proteins and eventual transcription of a proliferation gene. In the mutant line, ligand binding still causes dimerization, but the receptor stays phosphorylated far longer than normal after ligand is removed. The cells also fail to arrest at the G1/S checkpoint when DNA damage is induced, and they divide in the absence of added ligand after a period in culture.
1. Explain how loss of the phosphatase domain alters the duration and magnitude of the signal.
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The phosphatase domain normally dephosphorylates the receptor, terminating signaling. Without it, autophosphorylation persists even after ligand is removed, so docking sites remain active and downstream signaling (e.g., MAPK pathway) stays on longer. This prolongs and amplifies the transcriptional response - the signal is not shut off, so the cell behaves as if ligand were continuously present.
2. Account for the observation that cells eventually divide without added ligand.
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Prolonged signaling can drive autocrine loops or accumulate enough activated downstream components that the pathway becomes ligand-independent. Additionally, if the mutation allows receptor activation to persist, the cell may produce its own ligand or the pathway may become constitutively active. The result is loss of dependence on external growth factor - a hallmark of dysregulated proliferation.
3. The cells also fail to arrest at G1/S after DNA damage. Explain how this connects to the RTK mutation and what the combined phenotype implies.
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Growth-factor signaling drives progression past the G1/S restriction point (e.g., via cyclin D/CDK4-6 and Rb phosphorylation). Constitutive RTK signaling can override the DNA-damage checkpoint (which normally acts through p53/p21 to inhibit CDKs). With both unchecked proliferation signals and a disabled checkpoint, damaged DNA is replicated and passed on - a combination that promotes genomic instability and uncontrolled division. The two defects act synergistically rather than independently.