Associates with the adapter-like complex 4 (AP-4) and may therefore play a role in vesicular trafficking of proteins at the trans-Golgi network. Interacts with AP4B1 and AP4E1; the interaction is direct and mediates the association of TEPSIN with the adapter-like complex 4 (AP-4), a heterotetramer composed of AP4B1, AP4E1, AP4M1 and AP4S1. Extensively colocalizes with AP-4 which mediates the recruitment of TEPSIN to the trans-Golgi network.