In addition to clathrin-mediated endocytosis (CME), cells take up extracellular contents and cell-surface proteins through several routes that are independent of clathrin. The relationship between the various endocytic pathways is not well understood, although it has been suggested that CME components have broader roles than previously thought. Now, Alan Lau and Margaret Chou () provide evidence that the adaptor AP-2 – which is a key mediator of CME – also has a role in a clathrin-independent endocytic pathway. Using HeLa cells, the authors investigate the uptake of β1 integrin and MHC class I, which occurs through the non-clathrin, Arf6-regulated endocytic pathway in this system. Unexpectedly, the intracellular distribution of both proteins is altered if clathrin or an AP-2 subunit is knocked down; moreover, uptake of Arf6 cargo proteins is delayed when AP-2 is depleted. The authors go on to show that depletion of AP-2 promotes the lysosomal degradation of MHC class I, and decreases its half-life. In addition, depleting AP-2 (but not clathrin) enhances the colocalisation of AP-2 with late endosomes and lysosomes. The authors conclude that AP-2 has trafficking functions beyond its role in CME, underscoring the complexity and interdependence of endosomal membrane systems.
