Component of the adapter complexes which link clathrin to receptors in coated vesicles (PubMed:10564262). Clathrin-associated protein complexes are believed to interact with the cytoplasmic tails of membrane proteins, leading to their selection and concentration (PubMed:10564262). AP19 is probably a subunit of the Golgi membrane adapter (PubMed:10564262). Component of the AP-1-related (AP-1R) complex, an adapter protein complex that mediates sorting of cargo SNARE SNC1 (PubMed:26658609). In contrast to the APM1-containing AP-1 complex, AP-1R is incapable of sorting CHS3 (PubMed:26658609). Adapter 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) (PubMed:10564262). AP-1 interacts with clathrin (PubMed:10564262). Also a component of the AP-1R complex composed of at least APM2, APL4 and APS1 (PubMed:26658609). Present with 6370 molecules/cell in log phase SD medium. Belongs to the adaptor complexes small subunit family.