As part of AP-5, a probable fifth adaptor protein complex it may be involved in endosomal transport. According to PubMed:18395520, it may play a role in cell death. Probably part of the adaptor protein complex 5 (AP-5) a tetramer composed of AP5B1, AP5M1, AP5S1 and AP5Z1. May cycle on and off membranes. Expressed in various tumor cell lines including Jurkat, Hep-G2 and HeLa. May be due to an intron retention. Belongs to the adaptor complexes medium subunit family. Truncated N-terminus.