Poly-ubiquitylated transmembrane proteins outcompete other cargo for limited space inside clathrin-coated vesicles.

Unlabelled: Endocytic recycling of transmembrane proteins is essential to cell signaling, ligand uptake, protein traffic and degradation. The intracellular domains of many transmembrane proteins are ubiquitylated, which promotes their internalization by clathrin-mediated endocytosis. How might this enhanced internalization impact endocytic uptake of transmembrane proteins that lack ubiquitylation? Recent work demonstrates that diverse transmembrane proteins compete for space within highly crowded endocytic structures, suggesting that enhanced internalization of one group of transmembrane proteins may come at the expense of other groups. Here we show that preferential internalization of poly-ubiquitylated transmembrane proteins results in reduced endocytosis of mono-ubiquitylated and non-ubiquitylated proteins. Using a combination of live-cell imaging and ligand uptake assays, we confirmed that increased ubiquitylation correlates with increased internalization by clathrin-coated vesicles. Further, poly-ubiquitylated receptors significantly outcompeted their mono-ubiquitylated and non-ubiquitylated counterparts for localization to endocytic sites and uptake of extracellular ligands. These findings demonstrate the inherent interdependence of transmembrane protein recycling, suggesting that clathrin-coated vesicles act as selective filters, prioritizing highly ubiquitylated transmembrane proteins for uptake while leaving proteins with little or no ubiquitylation behind. Given that poly-ubiquitylation is thought to signal protein aging and damage, our findings suggest a mechanism for selective internalization of high priority cargo proteins, with simultaneously exclusion and protection of functional proteins that lack poly-ubiquitylation.

Significance: Ubiquitylation is essential for maintaining cellular homeostasis by regulating transmembrane protein trafficking, degradation, and signaling. Traditionally, poly-ubiquitylation has been viewed primarily as a signal for protein degradation, while mono-ubiquitylation is considered sufficient to trigger endocytosis. However, our work reveals a previously unrecognized role for poly-ubiquitylation in clathrin-mediated endocytosis, demonstrating that poly-ubiquitylated transmembrane proteins outcompete their non-ubiquitylated counterparts for incorporation into clathrin-coated vesicles, thereby establishing a competitive framework for endocytic cargo sorting. This mechanism reveals a selective sorting mechanism driven by the extent of ubiquitylation, which could regulate the removal of damaged proteins while protecting functional proteins at the plasma membrane.