Introduction to Signal Transduction Practice Test

A signaling ligand G binds to a receptor on the plasma membrane and quickly increases cytosolic cGMP. Cells pretreated with a drug that locks heterotrimeric G proteins in the GDP-bound state show no cGMP increase after ligand addition, even though ligand binding is unchanged. When a membrane-permeable cGMP analog is added, the intracellular response occurs despite the drug. Which of the following best explains the early transduction step normally required for cGMP production?

A cell expresses Receptor R, a transmembrane protein. When Ligand L binds, an intracellular protein becomes rapidly phosphorylated on tyrosine residues. If a mutation deletes the receptor’s cytosolic tail but leaves the extracellular ligand-binding domain intact, Ligand L still binds but phosphorylation of the intracellular protein does not occur. Which of the following best explains this result in terms of early signal transduction?