Sequences within the cytoplasmic domain of gp180 carboxypeptidase D mediate localization to the trans-Golgi network

Gp180, a duck protein that was proposed to be a cell surface receptor for d uck hepatitis B virus, is the homolog of metallocarboxypeptidase D, a mamma lian protein thought to function in the trans-Golgi network (TGN) in the pr ocessing of proteins that transit the secretory pathway. Both gp180 and mam malian metallocarboxypeptidase D are type I integral membrane proteins that contain a 58-residue cytosolic C-terminal tail that is highly conserved be tween duck and rat. To investigate the regions of the gp180 tail involved w ith TGN retention and intracellular trafficking, gp180 and various deletion and point mutations were expressed in the AtT-20 mouse pituitary corticotr oph cell line. Full length gp180 is enriched in the TGN and also cycles to the cell surface. Truncation of the C-terminal 56 residues of the cytosolic tail eliminates the enrichment in the TGN and the retrieval from the cell surface. Truncation of 12-43 residues of the tail reduced retention in the TGN and greatly accelerated the turnover of the protein. In contrast, delet ion of the C-terminal 45 residues, which truncates a potential YxxL-like se quence (FxxL), reduced the protein turnover and caused accumulation of the protein on the cell surface. A point mutation of the FxxL sequence to AxxL slowed internalization, showing that this element is important for retrieva l from the cell surface. Mutation of a pair of casein kinase II sites withi n an acidic cluster showed that they are also important for trafficking. Th e present study demonstrates that multiple sequence elements within the cyt oplasmic tail of gp180 participate in TGN localization.