Clustering of oligodendrocyte cell surface proteoglycans within lipid raft microdomains initiates many of the same signaling events as integrin clustering (Simons and Horowitz, 2001) . For example, syndecan-4 is a transmembrane heparan sulfate proteoglycan that is recruited to focal adhesions in response to extracellular matrix-dependent clustering or antibody-induced clustering (Lim et al., 2003; Tkachenko and Simons 2002) . Syndecan-4 signals through protein kinase Cα (PKCα) by potentiating its response to phosphatidylinositol 4,5-bisphosphate (PIP 2 ).