Analysis of Protein-Glycosaminoglycan Interactions by Affinity Co-Electrophoresis

Abstract

Glycosaminoglycans (GAGs) bind a wide variety of cell surface and secreted proteins, including extracellular matrix molecules, polypeptide growth factors, secreted enzymes and enzyme inhibitors, and cell adhesion molecules (1, 2). Such binding is thought to underlie the functions of proteoglycans (PGs). The affinities of physiologically relevant GAG-protein interactions typically range from moderately strong (K_d ∼ 10^-9 M) to rather weak (K_d > 10^-6 M). In addition, variations in GAG structure may give rise to marked heterogeneity in the affinities of GAG populations for particular proteins, heterogeneity that can be of substantial physiological importance (3). Affinity co-electrophoresis (ACE) is a simple and rapid gel electrophoretic technique that allows quantitative measurement of equilibrium binding between GAGs (or PGs) and GAG-binding proteins (4, 5). Because the technique sets up equilibrium conditions for binding, K ^’ _d s across a broad range (picomolar to micromolar) may be measured accurately. The method can also be used both to detect heterogeneity in GAG-protein affinities, and to isolate GAG sub- populations containing specific high- or low-affinity ligand-binding sequences.