IDENTIFICATION OF HYALURONIC-ACID BINDING-SITES IN THE EXTRACELLULAR DOMAIN OF CD44

CD44 is a polymorphic glycoprotein expressed on the surface of many ti ssues and cell lines which has been implicated in a number of cellular functions including lymphocyte homing to mucosal lymphoid tissue (Pey ers patches), leukocyte activation, lymphopoiesis, and tumor metastasi s. The predominant isoform found on human leukocytes, CD44H, is a rece ptor for hyaluronic acid. Because of the prominent role CD44 plays in diverse biological processes, we set out to identify the hyaluronic ac id binding site(s) in the extracellular domain of CD44H. Using truncat ion and site-directed mutagenesis we identified two regions containing clusters of conserved basic residues which are important in hyaluroni c acid binding. One of these regions is situated near the NH2 terminus and is homologous to other hyaluronic acid binding proteins including cartilage link protein. The other more membrane proximal region lies outside the link protein homologous domain. Mutagenesis of basic resid ues within these regions established their role as determinants in hya luronic acid binding. Mutation of Arg 41, a position where a basic res idue is conserved in all hyaluronic acid binding proteins, completely abolished binding suggesting that this residue plays a critical role i n hyaluronic acid binding.