The hydroxy amino acid in an Asn-X-Ser/Thr sequon can influence N-linked core glycosylation efficiency and the level of expression of a cell surface glycoprotein

N-Linked glycosylation usually occurs at the sequon, Asn-X-Ser/Thr. In this sequon, the side chain of the hydroxy amino acid (Ser or Thr) may play a direct catalytic role in the enzymatic transfer of core oligosaccharides to the Asn residue. Using recombinant variants of rabies virus glycoprotein (RGP), we examined the influence of the hydroxy amino acid on core glycosylation efficiency. A variant of RGP containing a single Asn-X-Ser sequon at Asn³⁷ was modified by site-directed mutagenesis to change the sequon to either Asn-X-Cys or Asn-X-Thr. The impact of these changes on core glycosylation efficiency was assessed by expressing the variants in a cell- free transcription/translation/glycosylation system and in transfected tissue culture cells. Substitution of Cys at position 39 blocks glycosylation, whereas substitution of Thr dramatically increases core glycosylation efficiency of Asn³⁷ in both membrane-anchored and secreted forms of RGP. The substitution of Thr for Ser also dramatically enhances the level of expression and cell surface delivery of RGP when the sequon at Asn³⁷ is the only sequon in the protein. Novel forms of membrane-anchored and secreted RGP which are fully glycosylated at all three sequons were also generated by substitution of Thr at position 39.