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D. Heinegård investigated the molecular organization of cartilage proteoglycans. A 1974 study with colleagues dissected large proteoglycan assemblies using controlled proteolysis, gel filtration and gel electrophoresis.^[ref]

Digestion yielded a fraction derived from the proteoglycan core protein and a distinct linking protein fraction. Both retained binding to hyaluronic acid. Separating components while testing which interaction survives is a useful way to infer how a large molecular assembly is held together: the original aggregate's size alone does not identify its binding partners.^[ref]

A proteoglycan consists of a core protein bearing one or more covalently attached glycosaminoglycan chains. These chains are polysaccharides containing amino sugars. Thus the molecule combines a sequence-defined polypeptide with long carbohydrate components, whose number and composition can vary.^[ref]

An illuminating example comes from cartilage, where individual proteoglycans can associate with hyaluronic acid into larger aggregates. A proteoglycan molecule and an aggregate of many such molecules are different organizational levels: a change in aggregate size need not mean that every core protein has changed length.

In 1974, researchers digested bovine cartilage aggregates with enzymes and separated the resulting components by chromatography. They isolated a core-protein-derived fraction and a smaller linking-protein fraction that both retained binding to hyaluronic acid. Disassembly and selective recovery of binding components helped distinguish the carbohydrate-rich molecule from the interactions holding a larger aggregate together.^[ref]