Component of the AP-1-related (AP-1R) complex, an adapter protein complex that mediates of cargo protein sorting in clathrin-coated vesicles (PubMed:26658609). AP-1R has a specific role in SNARE SNC1 sorting (PubMed:26658609). In contrast to the APM1-containing AP-1 complex, AP-1R is incapable of sorting CHS3 (PubMed:26658609). Component of the AP-1R complex composed of at least APM2, APL4 and APS1 (PubMed:26658609). Interacts with MIL1 (PubMed:26658609). Interacts with APL2 (PubMed:10564262). Leads to sensitivity to cationic amphiphilic drugs (CADs) such as sertraline. Present with 2890 molecules/cell in log phase SD medium. Belongs to the adaptor complexes medium subunit family.