CD44 variant exon v5 encodes a tyrosine that is sulphated

Functional differences between members of the CD44 family of cell surface glycoproteins is mediated in part by differential post-translational modification of these proteins and by alternative splicing. Tyrosine sulphation is a secondary modification of the primary amino acid structure of a number of secreted, transmembrane and lysosomal proteins, which is associated with promotion of protein-protein interactions. Here we identify a cannonical tyrosine-sulphation motif within rat and mouse CD44 exon v5. We show that inorganic sulphate is incorporated into the metastasis-associated rat CD44v4-v7 splice variant. The sulphate is not incorporated into sulphated glycosaminoglycan or other sugar modifications of CD44v4-v7. A point mutation of the exon v5 tyrosine to phenylalanine destroys inorganic sulphate incorporation into CD44v4-v7. These results demonstrate that the tyrosine-sulphation motif within rat CD44 exon v5 is used in vivo, and suggest that exon v5 may be involved in mediating CD44 ligand binding-activity by means of its sulphated tyrosine.