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. 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. Within AP-4, the mu-type subunit AP4M1 is directly involved in the recognition and binding of tyrosine-based sorting signals found in the cytoplasmic part of cargos (By similarity). The adaptor protein complex 4 (AP-4) may also recognize other types of sorting signal (PubMed:14572453). 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). Interacts with tyrosine-based sorting signals on the cytoplasmic tail of cargo proteins such as APP, ATG9A, LAMP2 and NAGPA (By similarity). Interacts with the C-terminal domain of GRID2 (PubMed:14572453). Interacts with GRIA1 and GRIA2; the interaction is indirect via CACNG3. Interacts with CACNG3; CACNG3 associates GRIA1 and GRIA2 with the adaptor protein complex 4 (AP-4) to target them to the somatodendritic compartment of neurons (By similarity). Interacts with HOOK1 and HOOK2; the interactions are direct, mediate the interaction between FTS-Hook-FHIP (FHF) complex and AP-4 and the perinuclear distribution of AP-4 (By similarity). Found in soma and dendritic shafts of neuronal cells. High levels in the olfactory bulb, the cerebral cortex, the granule and Purkinje cell layers of the cerebellar cortex and the CA3 region of the hippocampus. Low levels found in molecular layer of cerebellum. Belongs to the adaptor complexes medium subunit family.