Signal transduction pathways Original Scenario-Based Download PDF

Questions

1. 1.

   Synthetic biologists engineered a bioluminescent cell line called Astrocyte-L that emits light in response to an extracellular signaling peptide, Starlight-A. When Starlight-A binds to its membrane receptor, it initiates a multi-step phosphorylation cascade through relay kinases, ending in the activation of the light-producing enzyme luciferase. When the researchers introduced a chemical inhibitor that specifically blocks intracellular protein phosphatases, the cells continued to emit light persistently even after all extracellular Starlight-A was removed. What underlying mechanism of signal transduction explains this sustained activation?

   • A) Phosphatases normally degrade extracellular ligands, so blocking them causes Starlight-A to re-bind continuously to the cell-surface receptors.
   • B) Phosphatases produce cyclic AMP, so their inhibition forces kinases to directly phosphorylate transcription factors inside the nucleus.
   • C) Phosphatases normally remove phosphate groups from active kinases, so inhibiting them leaves the relay proteins in an active state despite ligand removal.
   • D) Phosphatases act as membrane transport channels, so blocking them traps the primary ligand inside the cytoplasm where it sustains signaling.