May function in protein sorting in late endosomes or multivesucular bodies (MVBs). (Microbial infection) Involved in MVB-assisted maturation of hepatitis B virus (HBV). May interact with AP1S1/Sigma1A-adaptin and AP1S2/Sigma1B-adaptin (PubMed:9733768). Probably does not interact with APB1 (PubMed:9733768). Interacts (via GAE domain) with RABEP1, NECAP1, CLINT1 and AFTPH/aftiphilin (PubMed:14665628). (Microbial infection) Interacts with HBV major surface antigen L. Interacts with HBV core protein C in a ubiquitin-dependent manner. Mainly localized to perinuclear vesicular structures (PubMed:9733768). Colocalizes with HBV major surface antigen L and HBV core protein C in CD63-containing compartments (PubMed:16867982). Colocalizes with HBV major surface antigen L to cis-Golgi-like structures (PubMed:11333915). Expressed in all but one (skeletal muscle) tissues examined. Belongs to the adaptor complexes large subunit family. Does not appear to be a subunit of the clathrin-associated adaptor protein complex 1 (AP-1).