Probable component of an adaptor protein complex. 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. May be part of the adaptor protein complex 4 (AP-4), a heterotetramer composed of two large adaptins (epsilon-type subunitand beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type). Belongs to the adaptor complexes small subunit family.