It has been known for a long time  that potential N-glycosylation sites are
specific to the consensus sequence Asn-Xaa-Ser/Thr. It must be noted that the
presence of the consensus tripeptide is not sufficient to conclude that an
asparagine residue is glycosylated, due to the fact that the folding of the
protein plays an important role in the regulation of N-glycosylation . It
has been shown  that the presence of proline between Asn and Ser/Thr will
inhibit N-glycosylation; this has been confirmed by a statistical analysis of
glycosylation sites , which also shows that about 50% of the sites that
have a proline C-terminal to Ser/Thr are not glycosylated.
It must also be noted that there are a few reported cases of glycosylation
sites with the pattern Asn-Xaa-Cys; an experimentally demonstrated occurrence
of such a non-standard site is found in the plasma protein C .
May 1991 / Text revised.
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