VARIANT EXONS V6 AND V7 TOGETHER EXPAND THE REPERTOIRE OF GLYCOSAMINOGLYCANS BOUND BY CD44

Isoforms of the glycoprotein CD44 are cell surface receptors for the g lycosaminoglycan hyaluronate. They have been implicated in many biolog ical processes, but their function in these is poorly understood and c annot be explained solely by hyaluronate binding. In the present work we examine the ligand binding properties of alternatively spliced CD44 variant isoforms which are functionally involved in the immune system , embryonic development, and tumor behavior. We shaw that these isofor ms bind directly to the purified glycosaminoglycans chondroitin sulfat e, heparin, and heparin sulfate, in addition to being able to bind to hyaluronate. Binding to this extended repertoire of glycosaminoglycans by CD44 depends on the inclusion of peptide sequences encoded by the alternatively spliced exons v6 and v7, and occurs both when the CD44 i s solubilized from the plasma membrane and when it is expressed on int act cells. A single point mutation in the most N-terminal hyaluronate binding motif of CD44 ablates both hyaluronate and chondroitin sulfate binding, suggesting that glycosaminoglycans are bound through a commo n motif, and that only one of the hyaluronate binding motifs is respon sible for the majority of glycosaminoglycan binding by CD44 on the cel l. surface. Taken together, these observations indicate that alternati ve splicing regulates the ligand binding specificity of CD44 and sugge st that structural changes in the CD44 protein have a profound effect on the range of ligands to which this molecule can bind with potential ly wide-ranging functional consequences.