Axial structure of the heterotypic collagen fibrils of vitreous humour andcartilage

We have compared the axial structures of negatively stained heterotypic, ty pe II collagen-containing fibrils with computer-generated staining patterns . Theoretical negative-staining patterns were created based upon the "bulki ness" of the individual amino acid side-chains in the primary sequence and the D-staggered arrangement of the triple-helices. The theoretical staining pattern of type II collagen was compared and cross-correlated with the exp erimental staining pattern of both reconstituted type II collagen fibrils, and fibrils isolated from adult and foetal cartilage and vitreous humour. T he isolated fibrils differ markedly in both diameter and composition. Corre lations were significantly improved when a degree of theoretical hydroxylys ine glycosylation was applied, showing for the first time that this type of glycosylation influences the negative-staining pattern of collagen fibrils . Increased correlations were obtained when contributions from types V/XI a nd IX collagen were included in the simulation model. The N-propeptide of c ollagen type V/XI and the NC2 domain of type TX collagen both contribute to prominent stain-excluding peaks in the gap region. With decreasing fibril diameter, an increase of these two peaks was observed. Simulations of the f ibril-derived staining patterns with theoretical patterns composed of propo rtions of types II, V/XI and IX collagen confirmed that the thinnest fibril s (i.e. vitreous humour collagen fibrils) have the highest minor collagen c ontent. Comparison of the staining patterns showed that the organisation of collagen molecules within vitreous humour and cartilage fibrils is identic al. The simulation model for vitreous humour, however, did not account for all stain-excluding mass observed in the staining pattern; this additional mass may be accounted for by collagen-associated macromolecules. (C) 2001 A cademic Press.