Light scattering studies of bovine skin proteodermatan sulfate

Crystal Structure of the Biglycan Dimer and Evidence That Dimerization Is Essential for Folding and Stability of Class I Small Leucine-rich Repeat Proteoglycans

Biglycan and decorin are two closely related proteoglycans whose protein cores contain leucine-rich repeats flanked by disulfides. We have previously shown that decorin is dimeric both in solution and in crystal structures. In this study we determined whether biglycan dimerizes and investigated the role of dimerization in the folding and stability of these proteoglycans. We used light scattering to show that biglycan is dimeric in solution and solved the crystal structure of the glycoprotein core of biglycan at 3.40-angstroms resolution. This structure reveals that biglycan dimerizes in the same way as decorin, i.e. by apposition of the concave inner surfaces of the leucine-rich repeat domains. We demonstrate that low concentrations of guanidinium chloride denature biglycan and decorin but that the denaturation is completely reversible following removal of the guanidinium chloride, as assessed by circular dichroism spectroscopy. Furthermore, the rate of refolding is dependent on protein concentration, demonstrating that it is not a unimolecular process. Upon heating, decorin shows a single structural transition at a T(m) of 45-46 degrees C but refolds completely upon cooling to 25 degrees C. This property of decorin enabled us to show both by calorimetry and light scattering that dimer to monomer transition coincided with unfolding and monomer to dimer transition coincided with refolding; thus these processes are inextricably linked. We further conclude that folded monomeric biglycan or decorin cannot exist in solution. This implies novel interrelated functions for the parallel beta sheet faces of these leucine-rich repeat proteoglycans, including dimerization and stabilization of protein folding.

Characterization of opticin and evidence of stable dimerization in solution

Opticin is a class III member of the extracellular matrix small leucine-rich repeat protein (SLRP) family that was initially identified in the eye in association with the collagen fibrils of the vitreous humor. Recombinant and tissue-extracted forms of bovine opticin were subjected to biochemical and biophysical characterization. Following SDS-PAGE the predominant component produced by both forms was a broad band between 45-52 kDa. There was evidence for two-stage processing and, additionally, a proteolytic cleavage product of approximately 25 kDa. Deconvolution of circular dichroism spectra revealed beta-sheet (41%), beta-turn (21%), and alpha-helix (10%), and thermal denaturation experiments showed a transition with a midpoint of 47 degrees C. Weight-averaged molecular mass measurements using both light scattering and analytical ultracentrifugation demonstrated that opticin exists in solution as a stable dimer of approximately 90 kDa, which can be dissociated into a monomer by denaturation with 2.5 m guanidine hydrochloride or during SDS-polyacrylamide electrophoresis. Opticin remains a dimer after removal of the amino-terminal region by O-sialoglycoprotein endopeptidase digestion, suggesting that dimer formation is mediated by the leucine-rich repeats. Dimerization could have a number of functional consequences, including divalent ligand interactions.

Light and X-ray scattering show decorin to be a dimer in solution

Decorin is a widely distributed member of the extracellular matrix small leucine-rich repeat glycoprotein/proteoglycan family. For investigation of its physical properties, decorin from two sources (young steer skin and a recombinant adenovirus) was used. The first sample was extracted into 7 m urea and purified, while the second was isolated from medium conditioned by 293A cells infected with adenovirus and purified without chaotropes. The only chemical differences detected between these materials were a slightly shorter glycosaminoglycan chain and the retention of the propeptide on the latter. Circular dichroism spectra of the two samples were virtually identical, showing a high proportion of beta-sheet and beta-turn and little alpha-helix. The protein cores were completely denatured in 2.25 m guanidine HCl (GdnHCl) but recovered their secondary structure on removal of chaotrope. Light scattering of material eluted from gel-filtration columns in Tris-buffered saline, pH 7.0, gave molecular mass values of 165 +/- 1 kDa and 84.6 +/- 4 kDa for intact decorin and the glycoprotein core produced by digestion with chondroitin ABC lyase, respectively. Intact recombinant prodecorin had a mass of 148 +/- 18 kDa. These values, which are double those estimated from SDS gel electrophoresis or from the known sequences and compositions, were halved in 2.5 m GdnHCl. Data from solution x-ray scattering of intact decorin and its core in Tris-buffered saline are consistent with a dimeric particle whose protein component has a radius of gyration of 31.6 +/- 0.4 A, a maximum diameter of 98 +/- 5 A, and approximates two intertwined C shapes.

Small proteoglycans from different regions of the fibrocartilaginous temporomandibular joint disc

Proteoglycans were isolated from two zones--the periphery and the inner zone--of bovine temporomandibular joint articular discs and separated into two pools by gel-filtration. Proteoglycans in the low molecular mass pool were further resolved by hydrophobic affinity chromatography into two groups identified by cyanogen bromide peptide analysis, amino acid analysis and amino-terminal sequence analysis as PGI (biglycan) and PGII (decorin). These two proteoglycans were isolated in approximately equal proportions from the 'inner' disc tissue but PGII predominated in the 'outer' tissue. Direct chemical analysis showed that the glycosaminoglycan chains on both PGI and PGII were high in iduronate (64-68% of total uronic acid). The dermatan sulfate chains on proteoglycans from the inner disc tissue were longer than those from the outer tissue. Comparison of the galactosamine contents of the intact proteoglycans with electrophoretic mobilities of the isolated dermatan sulfate chains showed that the PGI from the disc carries two dermatan sulfate chains. Inclusion of disc DS-PGI in a solution of soluble type I collagen lengthened the lag-phase, steepened the turbidity-time curve and increased the final opacity attained during fibril formation in vitro. The median fibril diameter and the range of diameters were both higher in the presence of DS-PGI. By contrast, disc DS-PGII reduced the slope of the turbidity-time curve but had little effect on the final turbidity or the fibril diameter.