STRUCTURE AND LOCALIZATION OF O-LINKED AND N-LINKED OLIGOSACCHARIDE CHAINS ON BASEMENT-MEMBRANE PROTEIN NIDOGEN

The carbohydrate content of mouse nidogen predicts the occupation of t wo N- and about seven O-linked acceptor sites. The corresponding oligo saccharides were examined by sequential exoglycosidase digestions. The data indicate N-linked substitutions by several bi-, tri- and tetraan tennary complex types of oligosaccharides which are further modified b y additional lactosamines and terminal alpha-galactose and/or sialic a cid. Mannose-rich oligosaccharides were of low abundance. 0-linked str uctures included a di- and tetrasaccharide core structure that were in addition sialylated and may be similar to structures found in fetuin. Evidence is provided that the two sequence-predicted asparagine accep tors are almost fully substituted. Sequence analysis of tryptic peptid es identified Thr-271, Ser-303, Thr-309, Thr-317, Thr-320, Thr-892 and Thr-905 as the most likely sites for galactosamine substitutions. The se residues are located in the flexible link connecting the N-terminal globular domains G1 and G2 of nidogen and at the border between the r od and the C-terminal globe G3. Four of them showed Pro in the - 1 or + 3 position. All these Ser, Thr and Pro residues but not the N-linked attachment sites are identical in human nidogen.