Interactions of neural glycosaminoglycans and proteoglycans with protein ligands: assessment of selectivity, heterogeneity and the participation of core proteins in binding

The method of affinity coelectrophoresis was used to study the binding ofnine representative glycosaminoglycan (GAG)-binding proteins, all thoughtto play roles in nervous system development, to GAGs and proteoglycansisolated from developing rat brain. Binding to heparin and non-neuralheparan and chondroitin sulfates was also measured. All nineproteins-laminin-1, fibronectin, thrombospondin-1, NCAM, L1, proteasenexin-1, urokinase plasminogen activator, thrombin, and fibroblast growthfactor-2-bound brain heparan sulfate less strongly than heparin, but thedegree of difference in affinity varied considerably. Protease nexin-1bound brain heparan sulfate only 1.8- fold less tightly than heparin(Kdvalues of 35 vs. 20 nM, respectively), whereas NCAM and L1 bound heparinwell (Kd approximately 140 nM) but failed to bind detectably to brainheparan sulfate (Kd>3 microM). Four proteins bound brain chondroitinsulfate, with affinities equal to or a few fold stronger than the sameproteins displayed toward cartilage chondroitin sulfate. Overall, thehighest affinities were observed with intact heparan sulfate proteoglycans:laminin-1's affinities for the proteoglycans cerebroglycan (glypican-2),glypican-1 and syndecan-3 were 300- to 1800-fold stronger than its affinityfor brain heparan sulfate. In contrast, the affinities of fibroblast growthfactor-2 for cerebroglycan and for brain heparan sulfate were similar.Interestingly, partial proteolysis of cerebroglycan resulted in a >400-fold loss of laminin affinity. These data support the views that (1)GAG-binding proteins can be differentially sensitive to variations in GAGstructure, and (2) core proteins can have dramatic, ligand-specificinfluences on protein-proteoglycan interactions.