Opticin is a class III member of the extracellular matrix small
leucine-rich repeat protein/
proteoglycan (SLRP) family found in vitreous humour and cartilage. It was first identified associated with the surface of vitreous
collagen fibrils and several other SLRPs are also known to bind
collagen fibrils and it some cases alter fibril morphology. The purpose of this study was to investigate the binding of opticin to the
collagen II-containing fibrils found in vitreous and cartilage. Electron microscopic studies using
gold labelling demonstrated that opticin binds vitreous and thin
cartilage collagen fibrils specifically at a single site in the gap region of the
collagen D-period corresponding to the e2
stain band; this is the first demonstration of the binding site of a class III SLRP on
collagen fibrils. Opticin did not bind thick
cartilage collagen fibrils from cartilage or tactoids formed in vitro from
collagen II, but shows high specificity for thin, heterotypic
collagen fibrils containing
collagens II, and XI or V/XI. Vitreous
collagen fibrils from opticin null and wild-type mice were compared and no difference in fibril morphology or diameter was observed. Similarly, in vitro fibrillogenesis experiments showed that opticin did not affect fibril formation. We propose that when opticin is bound to
collagen fibrils, rather than influencing their morphology it instead hinders the binding of other molecules to the fibril surfaces and/or act as an intermediary bridge linking the
collagen fibrils to other non-collagenous molecules.