Ligand binding specificity of alternatively spliced CD44 isoforms. Recognition and binding of hyaluronan by CD44R1.

CD44 species of widely differing molecular mass have been identified on various normal and/or transformed cells. Recent studies have demonstrated that much of this heterogeneity is produced as a result of the alternative splicing of a series of 10 exons present within the CD44 gene generating a large number of CD44 isoforms containing additional peptide sequences of varying length inserted into a single site within the extracellular domain of the molecule. At present, the effect of such insertions on the ligand binding specificity of CD44 remains unclear. CD44H, the major CD44 isoform expressed by most resting cell types, has been shown to function as a receptor for the glycosaminoglycan hyaluronan. In contrast, CD44E, the major isoform expressed by the colon carcinoma cell line HT29, which contains a 132-amino acid insert, is unable to recognize and bind this ligand. In the present study we demonstrate that CD44R1, an isoform isolated from the myelomonocytic cell line KG1a, that differs from CD44E by just 3 amino acid substitutions, is fully capable of mediating the attachment of transfected COS7 cells to hyaluronan-coated plastic. In order to confirm that such binding was directly mediated by the introduced CD44 species, chimeric proteins containing the entire extracellular domain of CD44H or CD44R1 fused in-frame to human bone/liver/kidney alkaline phosphatase were prepared and tested for their ability to bind hyaluronan-coated plastic. Both fusion proteins bound equally well to hyaluronan and in each case their attachment could be readily inhibited by monoclonal antibodies directed against the hyaluronan-binding domain of CD44. These data indicate that the 132-amino acid insert present within the extracellular domain of CD44R1 does not interfere with the hyaluronan binding function of the molecule. Since CD44E contains an identically sized insert but is unable to bind hyaluronan, it is likely that mutation of one or more of the 3 amino acid residues that differ between CD44E and CD44R1 is responsible for the altered functional activity of this particular molecule.