Proteoglycans: cell biologists have had a love-hate relationship with these molecules almost since their discovery. Their biochemical properties, dominated by heterogeneous and highly charged glycosaminoglycan (GAG) chains, can make purification challenging and structural analysis painful. Their ability to bind scores of growth factors, growth factor-binding proteins, extracellular proteases, protease inhibitors, extracellular matrix molecules, and other proteins takes the concept of molecular promiscuity to new heights. On top of this, they seem always to be underfoot, showing up on plasma membranes in hundreds of thousands of copies per cell and in extracellular matrices at milligram per milliliter concentrations.
And yet despite these peculiarities (or perhaps because of them) proteoglycans have inspired an extraordinary range of models, theories, and speculation. Since the 1960s, proteoglycans have been credited, in one system or another, with controlling the following: cell division, adhesion, spreading, migration,...
