Component of the adaptor protein complex 4 (AP-4). Adaptor protein complexes are vesicle coat components involved both in vesicle formation and cargo selection. They control the vesicular transport of proteins in different trafficking pathways (PubMed:10066790, PubMed:10436028). AP-4 forms a non clathrin-associated coat on vesicles departing the trans-Golgi network (TGN) and may be involved in the targeting of proteins from the trans-Golgi network (TGN) to the endosomal-lysosomal system. It is also involved in protein sorting to the basolateral membrane in epithelial cells and the proper asymmetric localization of somatodendritic proteins in neurons. AP-4 is involved in the recognition and binding of tyrosine-based sorting signals found in the cytoplasmic part of cargos, but may also recognize other types of sorting signal (Probable). Adaptor protein complex 4 (AP-4) is a heterotetramer composed of two large adaptins (epsilon-type subunit AP4E1 and beta-type subunit AP4B1), a medium adaptin (mu-type subunit AP4M1) and a small adaptin (sigma-type AP4S1) (PubMed:10436028, PubMed:10066790). Interacts with TEPSIN (PubMed:26542808). Interacts with GRIA2; probably indirect it mediates the somatodendritic localization of GRIA2 in neurons (By similarity). Widely expressed. The disease is caused by variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Belongs to the adaptor complexes large subunit family.