a protein of the adaptor complexes medium subunit family. A subunit of the heterotetrameric coat assembly protein complex 2 (AP2) which links clathrin to receptors in coated vesicles. Interacts with the cytoplasmic tails of membrane proteins and polyphosphoinositide-containing lipids. Is not required for clathrin-coated vesicle formation at the plasma membrane, but that it is one of several endocytic adaptors required for the uptake of certain cargo proteins. Is required for the activity of a vacuolar ATPase, which is responsible for proton pumping occurring in the acidification of endosomes and lysosomes. AP-2 is a heterotetramer composed of two large chains (alpha and beta), a medium chain (AP50) and a small chain (AP17). Note: This description may include information from UniProtKB.