As part of AP-5, a probable fifth adaptor protein complex it may be involved in endosomal transport. 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. Widely expressed, including in small intestine and testis. In small intestine, highly expressed in cytoplasm of villi epithelial cells and internal glands. In testis, selectively expressed in maturing sperm cells (at protein level). May be due to intron retention. May be due to intron retention. Belongs to the adaptor complexes medium subunit family.