Nonenzymatic Protein Functions (1A) Practice Test

In cultured neurons, investigators expressed a membrane receptor whose cytosolic tail contains a short motif that binds an intracellular scaffold protein (Scaffold S). When the motif was deleted, ligand binding to the receptor at the cell surface was unchanged, but downstream signaling readouts (calcium transients) became smaller and more variable across trials. No enzymatic domains were altered in either protein. Which statement best describes the protein’s role in this system?

A study of skeletal muscle cells examined oxygen delivery during repeated contractions. Researchers compared wild-type myoglobin to a mutant myoglobin with a reduced heme-pocket complementarity for O$_2$ (lower affinity) but unchanged expression level. Under identical perfusion, mutant cells developed earlier fatigue and showed a larger drop in intracellular O$_2$ near mitochondria during stimulation. Based on the scenario, which interaction is likely based on the protein’s known properties?