Endocytosis, Exocytosis, and Vesicular Trafficking (2A) Practice Test

In a single-cell assay, cells internalize transferrin (Tf) via receptor-mediated endocytosis. After 10 minutes, most fluorescent Tf is in early endosomes; after 30 minutes, Tf signal returns to the plasma membrane while the transferrin receptor remains detectable at the surface. A mutation disrupts a small GTPase required for endosomal sorting, and Tf remains trapped in intracellular vesicles with reduced return to the surface. Which process is most likely affected by the mutation described?

In cultured hepatocytes, a fluorescently labeled ligand (L*) binds a transmembrane receptor (R) at 4°C, then cells are warmed to 37°C to permit uptake. Within 5 minutes, punctate L* signal appears in intracellular vesicles; after 20 minutes, L* signal colocalizes with an early endosome marker, and by 60 minutes most L* signal colocalizes with a lysosomal marker. When cells are treated with a drug that prevents clathrin coat assembly, surface binding of L* to R is unchanged, but internal puncta are markedly reduced. In this single-cell context, which cellular component is primarily responsible for the uptake step that is blocked by the drug?