Phorbol myristate acetate stimulates the dimerization of CD44 involving a cysteine in the transmembrane domain

Receptor oligomerization is important for ligand binding and signal transduction. CD44 is a transmembrane protein present on many cell types. One of the ligands for CD44 is hyaluronic acid (HA). HA binding activity of CD44 is linked to cellular activation in some cell types. Clustering of CD44 has been speculated to be important for binding of HA. However, the molecular mechanisms for converting CD44 from an inactive receptor to an active receptor are not known. Here we report that PMA stimulates the binding of CD44 to HA by inducing clustering of CD44 followed by covalent homodimerization of CD44 on the cell surface. Covalent dimerization involves a cysteine (Cys286) in the transmembrane domain of CD44 and is essential for binding of high levels of fluorescein-conjugated HA. Activation-induced clustering followed by disulfide bond-mediated dimerization of CD44 represents an additional signal transduction mechanism for regulating receptor-ligand interactions.