Involved in the regulation of membrane traffic through the trans-Golgi network (TGN). Functions in close cooperation with the GGAs in the sorting of hydrolases to lysosomes. Homodimer. Interacts with GGA1, GGA2 and AP1G1. Colocalizes with GGA1, GGA2 and GGA3. Widely expressed. Binds GGAs. Does not bind GGAs. Truncated N-terminus.