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 pla sma membrane protein. The primary structure of rat wild-type DPPIV con tains eight potential N-glycosylation sites. To investigate the role o f N-glycosylation in the function of DPPIV, three of its asparagine re sidues were separately converted to glutamine by site-direcred mutagen esis. The resulting N-glycosylation mutants of rat DPPIV were studied in stable transfected Chinese hamster ovary cells. All three N-glycosy lation mutants of DPPIV showed a reduced half-life, as well as differi ng degrees of inhibition of the processing of their N-glycans. Mutatio n of the first (Asn83 --> Gln) or eighth (Asn686 --> Gln) N-glycosylat ion site had only a small effect on its enzymatic activity, cell-surfa ce 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 cytopla sm and its degration was drastically increased. Our data suggest that the N-glycosylation at Asn319 is involved in protein trafficking and c orrect protein folding.