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Fig. 3. Model for the regulation of the CD44-ERM complex by dynamic phosphorylation of the CD44 cytoplasmic tail. The ERM (ezrin, radixin, moesin)-family proteins can function to crosslink transmembrane receptors, including CD44, to the cytoskeleton. Their basic structure consists of the three-lobed N-terminal FERM domain followed by a coiled-coil region and a C-terminal domain that contains an F-actin binding site. In their `inactive' conformation, the C-terminal domain binds to the FERM domain, masking both transmembrane receptors and F-actin interaction sites. Conformational regulation between the `inactive' and `active' forms involves complex mechanisms including phosphorylation and binding to the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PIP<sub>2</sub>) (reviewed by Bretscher et al., 2002). The cytoplasmic tail of CD44 is phosphorylated at Ser325 by CaMKII and this form of the receptor binds to an `active' ERM protein that links CD44 to the actin cytoskeleton. PKC activation results in a concomitant dephosphorylation of Ser325 and phosphorylation of Ser291, resulting in disengagement of the ERM proteins and loss of cytoskeletal association.

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