Glycosylation efficiency of Asn-Xaa-Thr sequons is independent of distance from the C-terminus in membrane dipeptidase.

In vitro transcription/translation studies with model proteins have shown that glycosylation of Asn-Xaa-Thr sequons is reduced when the sequon is within 60 residues of the C-terminus of the protein. We have previously shown that in living cells N-glycosylation of the prion protein (PrP) is also abolished when its Asn-Ile-Thr and Asn-Phe-Thr sequons are less than 60 residues from the C-terminus (Walmsley and Hooper [2003] Biochemical Journal, 370, 351-355). To investigate whether sequon distance to the C-terminus is a general determinant of N-glycosylation in living cells, Asn-Ile/Phe-Thr sequons were introduced into another glycosylphosphatidylinositol (GPI) anchored protein, membrane dipeptidase (MDP), at similar distances from the C-terminus as those in PrP. When expressed in the human neuroblastoma SH-SY5Y cell line, the introduced sequons were fully N-glycosylated even when they were less than 60 residues from the C-terminus in both GPI-anchored and secreted forms of MDP. These data demonstrate that the utilization of sequons in some proteins is independent of their distance from the C-terminus.