Component of the AP-1 complex which links clathrin to receptors in coated vesicles. Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration. Adaptor protein complex 1 (AP-1) is a heterotetramer composed of two large adaptins (gamma-type subunit apl4 and beta-type subunit apl2), a medium adaptin (mu-type subunit apm1) and a small adaptin (sigma-type subunit aps1). AP-1 interacts with clathrin (By similarity). Leads to hypersensitivity to valproic acid. Belongs to the adaptor complexes small subunit family.