May be involved in regulating membrane traffic to and from trans-Golgi network. Primarily in trans-Golgi network. Cycles between the trans-Golgi network and the cell surface returning via endosomes. Isoform TGN46 is widely expressed. Isoform TGN51 is more abundant in fetal lung and kidney. Isoform TGN48 is barely expressed in embryonic kidney and promyelocytic cells.