Asn-X-Thr/Ser

Gavel Y, von Fleijne G (1990), Sequence differences between glycosylated and non-glycosylated Asn-X-Thr/Ser acceptor sites implications for protein engineering, Protein Eng. 3 433-442. 

Replacement of the asparaginyl group by aspartyl or glutaminyl residues led to a total loss of acceptor activity, as did the replacement of the threonyl residue by valine. This accords with the idea that the sequence Asn-X-Thr/ Ser is essential for transfer. The exchange of the leucyl group for lysine or glycine did not impair acceptor function in any way, but the insertion of an aspartyl residue reduced the effectiveness of the peptide in the transfer reaction by 7 5 per cent. If proline replaced leucine, the peptide would not function at all in the glycosyl transfer. 

None of these studies shows that the sequence Asn-X-Thr/Ser is the initiator of saccharide transfer in the formation of A-glycosidic glycopeptides. In fact, nothing is known of the initiation process. This coding sequence is, on present evidence, simply a landmark for the position of attachment of sugar. Without it, transfer cannot occur, but its presence is not, in itself, sufficient to cause a transfer. Other, sterochemical features of the polypeptide are probably involved, but the triggering of the whole process may be wholly external to the acceptor protein. 

Asn-X-Thr/Ser, is involved in the carbohydrate-peptide bond, and second, O-glycosidic linkage of galactosamine, where serine and/or threonine, as well as hydroxylysine in no apparent signal sequence, are involved in the carbohydrate-peptide bond. The constituents of carbohydrate chains include galactose, mannose, fucose, A -acetylglucosamine, A-acetylgalactosamine, and sialic acid. 

N-linked glycosylation is sequence specific, involving the transfer of a pre-synthesized oligosaccharide chain to an asn residue found in a characteristic sequence Asn-X-Ser, or Asn-X-Thr or Asn-X-Cys, where X represents any amino acid residue, with the exception of proline. An additional glycosylation determinant must also apply, as not all potential N-linked sites are glycosylated in some proteins. The pre-synthesized oligosaccharide side chain then undergoes... 

Not all sequences -Asn-X-Thr( or Ser)- are glycosylated in proteins, however. Hunt and Dayhoff (168) searched the amino acid sequences of 264 proteins. In only 101 cases did they find the occurrence of the sequence -Asn-X-Thr(or Ser)-, and in only 20 cases did the sequence occur in glycosylated form. They did conclude that this sequence is probably the key recognition sequence of the glycosyltransferase. Glyco-sylation may not occur because of the absence of one or more of the required enzymes, because the protein becomes folded into tertiary structure before glycosylation can occur or the carbohydrate groups have been removed prior to isolation. 

Transfer occurs to specific Asn residues in the sequence Asn-X-Ser/Thr, where X is any residue except Pro, Asp, or Glu. 

There are two conserved A-linked sites (based on the Asn-X-Ser/Thr consensus sequence) in all of the PGIPs that have been studied [10], but while there are four sites of potential A-linked glycosylation in bean PGIP, there are seven such sites in the pear and tomato PGIPs (Fig. 8). 

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