A BLOOD GROUP-RELATED POLYMORPHISM OF CD44 ABOLISHES A HYALURONAN-BINDING CONSENSUS SEQUENCE WITHOUT PREVENTING HYALURONAN-BINDING

CD44 is a widely expressed integral membrane protein that acts as a re ceptor for hyaluronan (HA) and is proposed to be important to cell-ext racellular matrix interaction. The Indian (In) blood group antigens re side on CD44, and most individuals express the In-b antigen. Homozygos ity for the In-a allele occurs as a rare event and is associated with production of alloantibody to the common In-b antigen after transfusio n or pregnancy. The present study demonstrates that a single point mut ation (G(252) --> C) causes an Arg(46) --> Pro substitution, which is responsible for the In-b/In-a polymorphism. Additional mutations were found in In(a+b-) cDNA but were not necessary to the antigenic phenoty pe as determined in site-directed mutagenesis studies, In studies usin g CD44 chimeric constructs, Arg(46) has previously been shown to be cr ucial for maintenance of HA-binding ability to a CD44 peptide. However , the present study demonstrates that the Arg(46) --> Pro substitution does not reduce HA binding to the intact CD44 protein, which contains two proposed extracellular HA-binding motifs. Down-regulation of HA b inding to In(a+b-) CD44 by anti-CD44 monoclonal antibody (mAb) ligands , however, was weakened, although all mAbs tested bound In(a+b-) and I n(a-b+) CD44 equally well. Competitive inhibition studies using human anti-In-b also showed that some mAbs that inhibit HA binding to CD44 m ay do so by interacting with a domain separate from, but affecting the structure of, the In-b epitope.