The hyaluronate-binding site from the plasma membrane is distinct from the binding protein present in brain

To determine whether the hyaluronate-binding protein from brain is similar or identical to the hyaluronate-binding site from the cell surface, the two molecules were compared with respect to their physical and binding properties. The hyaluronate-binding protein was purified from mouse brains by lectin-affinity chromatography, and then analyzed by molecular-sieve chromatography and rate-zonal centrifugation, which showed that it has a Stokes radius of 6.3 nm, and a sedimentation coefficient of 4.8 S. These values are remarkably close to those obtained previously for the membrane-associated binding site (a = 6.5 nm, s20,w = 4.8 S), indicating that the two molecules have similar shapes and sizes. Binding studies of the semi-purified proteins showed that the dissociation constant for the brain derived binding protein (Kd = 270 ng/0.5 ml) was similar to that of the cell-surface binding site (Kd = 350 ng/0.5 ml). However, when the two molecules were compared with respect to oligosaccharide inhibition of binding, significant differences were observed. The hexasaccharide significantly inhibited the binding of [3H] hyaluronate to the cell-surface binding site but had only a small effect on the binding to the brain derived protein. Differences were also found between the two molecules with respect to the effects of a monoclonal antibody (K-3). This antibody blocked most of the binding activity of the membrane-associated binding site, but had no effect on the protein from brain. Taken together, these results indicate that although the hyaluronate-binding protein derived from brain and the cell-surface binding site are physically similar, they are distinct proteins.