Receptors Utilize Coated Vesicle Heterogeneity to Evade Competition during Endocytosis

Abstract

Endocytic uptake of receptors is a fundamental process that underlies multiple cellular events from signal transduction and nutrient import to recycling transmembrane proteins and lipids. Since receptor recycling plays a role in most signaling pathways, it is important to understand how receptors are selected for internalization. In order to be internalized, receptors must bind to protein components of the endocytic coat, such as the major adaptor protein, AP2. Receptors with greater affinities for AP2 are more strongly internalized compared to receptors with lower affinity. How do receptors with different affinities compete for space within crowded endocytic structures? To address this question, we constructed receptors with varying affinities for AP2 and allowed them to compete against each other during internalization. Internalization of a receptor with high affinity for AP2 was reduced when co-expressed with a competing receptor of high affinity. Based on these results, we expected that the weaker the affinity of a receptor, the easier it would be for a strongly internalized receptor to compete it out of coated pits. Instead, we found that the fractional change in internalization of our model receptors upon competition with a strongly internalized receptor decreased with decreasing affinity for AP2. These observations suggest that weakly internalized receptors are protected from competition with more strongly internalized competitors. This protection could be due to a small fraction of the coated vesicle capacity being occupied by weakly internalized receptors. Therefore, to drive them out, a strongly internalized competitor has to saturate nearly the entire coated vesicle. Given the critical role that internalization plays in receptor signaling, this effect may serve as a protection mechanism that prevents overexpression of strongly internalized receptors from interfering with the cell's ability to process signals from weakly internalized receptors.