Department of Biomedical Engineering, The University of Texas at Austin, 1 University Station C0800, Austin, Texas 78712, United States.
Institute for Cellular and Molecular Biology, The University of Texas at Austin, 1 University Station C0800, Austin, Texas 78712, United States.
Biochemistry. 2021 Jul 13;60(27):2195-2205. doi: 10.1021/acs.biochem.1c00268. Epub 2021 Jun 25.
The uptake of receptors by clathrin-mediated endocytosis underlies signaling, nutrient import, and recycling of transmembrane proteins and lipids. In the complex, crowded environment of the plasma membrane, receptors are internalized when they bind to components of the clathrin coat, such as the major adaptor protein, AP2. Receptors with higher affinity for AP2 are known to be more strongly internalized compared to receptors with lower affinity. However, it remains unclear how 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 one another during internalization. As expected, the internalization of a receptor with high affinity for AP2 was reduced when it was coexpressed with a competing receptor of similar affinity. However, receptors of low affinity for AP2 were surprisingly difficult to displace from endocytic structures, even when expressed alongside receptors with much higher affinity. To understand how these low-affinity receptors are protected from competition, we looked at AP2 heterogeneity across clathrin-coated structures. When we examined structures with lower-than-average AP2 content, we found that they were relatively enriched in cargo of low affinity for AP2 and depleted of cargo with high affinity. These findings suggest that the heterogeneity of adaptor protein content across the population of endocytic structures enables the internalization of diverse receptors. Given the critical role that internalization plays in signaling, this effect may help to prevent strongly internalized receptors from interfering with the cell's ability to process signals from weakly internalized receptors.
网格蛋白介导的胞吞作用通过网格蛋白包被中的主要衔接蛋白 AP2 等组分来内化与配体结合的受体，从而启动信号转导、营养物质摄取以及跨膜蛋白和脂质的循环。在质膜这种复杂、拥挤的环境中，与网格蛋白包被结合的配体能够内化受体。具有更高的 AP2 亲和力的受体比具有较低亲和力的受体更容易被内化。然而，受体的亲和力不同时，它们如何在拥挤的胞吞结构内竞争空间仍然不清楚。为了解决这个问题，我们构建了具有不同亲和力的受体，让它们在内化过程中相互竞争。正如预期的那样，当与具有相似亲和力的竞争受体共表达时，具有高亲和力的 AP2 受体的内化会减少。然而，具有低亲和力的 AP2 受体出人意料地难以从胞吞结构中置换，即使与具有更高亲和力的受体一起表达也是如此。为了了解这些低亲和力受体如何免受竞争的影响，我们研究了网格蛋白包被结构中 AP2 的异质性。当我们检查 AP2 含量低于平均水平的结构时，我们发现它们相对富含低亲和力的 AP2 货物，而缺乏高亲和力的 AP2 货物。这些发现表明，胞吞结构群体中衔接蛋白含量的异质性能够内化各种受体。鉴于内化在信号转导中起着关键作用，这种效应可能有助于防止强烈内化的受体干扰细胞处理弱内化受体信号的能力。
