IDENTIFICATION OF THE N-LINKED OLIGOSACCHARIDE SITES IN CHICK CORNEALLUMICAN AND KERATOCAN THAT RECEIVE KERATAN SULFATE

Corneal proteoglycans have chondroitin/dermatan and keratan sulfate (K S) chains and belong to the leucine-rich proteoglycan gene family. Cor neal KS is N-linked to Asn of an NX(S/T) site through a complex oligos accharide linkage region. Only some sites receive KS, whereas others r emain in a high mannose form. To determine whether the attachment of K S was biased toward specific sites, we isolated trypsin-digested KS-co ntaining fragments of chick corneal proteoglycans and sequenced the pe ptides, Results showed that all of the peptides sequenced aligned to t he deduced amino acid sequence of either chick lumican or chick kerato can at the first, third, and fourth potential N-linked sites. Sites 1 and 4 in lumican and keratocan are in a homologous location. By analog y with the structure of ribonuclease inhibitor (a Leu-rich repeat cont aining protein), the KS chains would extend outward on the outer face of a horseshoe-like structure. The amino acid sequences surrounding th e potential N-linked sites were also compared. Sites receiving KS tend to have a higher occurrence of aromatic residues, in particular Phe, located within 3 amino acids of NX(SPT), These conserved Phe residues may have a role in the conversion of high mannose N-linked oligosaccha rides to polylactosamine and/or keratan sulfate.