1. Review question: endocytosis / desensitization of receptors
The recycling of transferrin receptors has been studied by labeling the receptors on the cell surface and following their fate at 0°C and 37°C. A sample of intact cells at 0°C was reacted with radioactive iodine under conditions that label cell- surface proteins. If these cells were kept on ice and incubated in the presence of trypsin, which destroys the receptors without damaging the integrity of the cell, the radioactive transferrin receptors were completely degraded. If the cells were first warmed to 37°C for 1 hour and then treated with trypsin on ice, about 70% of the initial radioactivity was resistant to trypsin.
At both temperatures, most of the receptors were not labeled and most remained intact, as apparent from a protein stain.
When the cells were kept on ice, why did trypsin treatment destroy the labeled transferrin receptors, but not the majority of receptors? Why did most of the labeled receptors become resistant to trypsin when the cells were incubated at 37°C?

2. At 0°C, endocytosis is blocked and the labeled transferrin receptors are trapped on the cell surface and accessible to trypsin treatment. After 1 hour at 37°C, most of the receptors in intact cells (approx. 70%) are not sensitive to trypsin because they are inside the cell (presumably in endosomes) and, therefore, are not accessible. When cells are incubated at 37°C, the labeled receptors are endocytosed and cycle through the endosomal compartment of the cell, thereby becoming inaccessible to trypsin.
trypsin treatment indicate that 30% of the total transferrin receptor is on the cell surface after 1 hour at 37°C. When the transferrin receptors are allowed to recycle by incubation at 37°C, 30% is accessible to trypsin treatment of intact cells; therefore, 30% is on the surface. Recycling of transferrin receptors is very fast, and this distribution between the surface and internal compartments turns out to be the equilibrium distribution for transferrin receptors.