AP2 clathrin adaptor complex, but not AP1, controls the access of the major histocompatibility complex (MHC) class II to endosomes

Newly synthesized MHC II alpha- and beta-chains associated with the invariant chain chaperone (Ii) enter the endocytic pathway for Ii degradation and loading with peptides before transport to the cell surface. It is unclear how alpha beta Ii complexes are sorted from the Golgi apparatus and directed to endosomes. However, indirect evidence tends to support direct transport involving the AP1 clathrin adaptor complex. Surprisingly, we show here that knocking down the production of AP1 by RNA interference did not affect the trafficking of alpha beta Ii complexes. In contrast, AP2 depletion led to a large increase in surface levels of alpha beta Ii complexes, inhibited their rapid internalization, and strongly delayed the appearance of mature MHC II in intracellular compartments. Thus, in the cell systems studied here, rapid internalization of alpha beta Ii complexes via an AP2-dependent pathway represents a key step for MHC II delivery to endosomes and lysosomes.