Evidence of a defined spatial arrangement of hyaluronate in the central filament of cartilage proteoglycan aggregates

Aggregates of proteoglycans from the Swarm rat chondrosarcoma reassembled in vitro have been studied by rotary-shadowing electron microscopy, and shown to be similar to native structures that have never been dissociated [Morgelin, Engel, Heinegard and Paulsson (1992) J. Biol. Chem. 267, 14275-14284]. A hyaluronate with defined chain length (HAshort) has now been prepared by autoclaving high-Mr hyaluronate and fractionation to a narrow size distribution by gel filtration. Proteoglycan monomers, core protein, hyaluronate-binding region and link protein were combined with HAshort. Free chains of HAshort and reconstituted complexes with proteoglycan, link protein and aggrecan fragments were examined by electron microscopy after rotary shadowing. Length measurements showed that the hyaluronate was condensed to about half of its original length on binding intact aggrecan monomers, any aggrecan fragment or link protein alone. This strongly implies that hyaluronate adopts a defined spatial arrangement within the central filament of the aggregate, probably different from its secondary structure in solution. No differences in length were observed between link-free and link-stabilized aggregates.