The heterotetrameric coat assembly protein complex, also known as the adaptor-related protein complex 2 (AP-2), belongs to the adaptor complexes medium subunits family. The mu 1 subunit of the AP-2 complex (AP2M1) is required for the activity of a vacuolar ATPase, which is responsible for proton pumping occurring in the acidification of endosomes and lysosomes.  AP2M1 has also been shown to associate with the HIV-1 protein Nef, suggesting that Nef may use AP-2 complex to enhance the rate of endocytosis of both CD4 and class I MHC.  AP2M1 may also play an important role in regulating the intracellular trafficking and function of cytotoxic T-lymphocyte associated (CTLA)-4 protein.  At least two isoforms of AP2M1 are known to exist.