Subunit of clathrin-associated adaptor protein complex 1 that plays a role in protein sorting at the trans-Golgi network and early endosomes (TGN/EE). The AP complexes mediate the recruitment of clathrin to membranes and the recognition of sorting signals within the cytosolic tails of transmembrane cargo molecules (By similarity). Adaptor protein complex 1 (AP-1) is a heterotetramer composed of two large adaptins (gamma-type subunit and beta-type subunit), a medium adaptin (mu-type subunit) and a small adaptin (sigma-type subunit). Expressed in seedlings, roots, stems, leaves, flowers and siliques (developing fruits and seeds). Belongs to the adaptor complexes small subunit family.