DEFECTIVE PHOSPHORYLATION AND HYALURONATE BINDING OF CD44 WITH POINT MUTATIONS IN THE CYTOPLASMIC DOMAIN

CD44 is a cell surface adhesion molecule that plays a role in leukocyt e extravasation, leukopoiesis, T lymphocyte activation, and tumor meta stasis. The principal known ligand for CD44 is the glycosaminoglycan h yaluronate, (HA), a major constituent of extracellular matrices. CD44 expression is required but is not sufficient to confer cellular adhesi on to HA, suggesting that the adhesion function of the receptor is reg ulated. We recently demonstrated that CD44 in primary leukocytes is ph osphorylated in a cell type- and activation state-dependent fashion. I n this study we demonstrate that serines 325 and 327 within the cytopl asmic domain of CD44 are required for the constitutive phosphorylation of CD44 in T cells. Furthermore, we demonstrate that cells expressing mutated CD44 containing a serine to glycine substitution at position 325 or a serine to alanine substitution at amino acid 327 are defectiv e in HA binding, CD44-mediated adhesion of T cells to smooth muscle ce lls, as well as ligand-induced receptor modulation. The effect of thes e mutations can be partially reversed by a monoclonal anti-CD44 antibo dy that enhances CD44-mediated HA binding.