Domain‐Specific N‐Glycosylation of the Membrane Glycoprotein Dipeptidylpeptidase IV (CD26) Influences its Subcellular Trafficking, Biological Stability, Enzyme Activity and Protein Folding

Dipeptidyl peptidase IV (DPPIV, CD26) is an N-glycosylated type II plasma membrane protein. The primary structure of rat wild-type DPPIV contains eight potential N-glycosylation sites. To investigate the role of N-glycosylation in the function of DPPIV, three of its asparagine residues were separately converted to glutamine by site-directed mutagenesis. The resulting N-glycosylation mutants of rat DPPIV were studied in stable transfected Chinese hamster ovary cells. All three N-glycosylation mutants of DPPIV showed a reduced half-life, as well as differing degrees of inhibition of the processing of their N-glycans. Mutation of the first (Asn83-->Gln) or eighth (Asn686-->Gln) N-glycosylation site had only a small effect on its enzymatic activity, cell-surface expression and dimer formation, whereas the mutation of the sixth N-glycosylation site (Asn319-->Gln) abolished the enzymatic activity, eliminated cell-surface expression and prevented the dimerization of the DPPIV protein. The mutant [Gln319]DPPIV is retained in the cytoplasm and its degradation was drastically increased. Our data suggest that the N-glycosylation at Asn319 is involved in protein trafficking and correct protein folding.