Structure-function analysis of Tetraspanin CD151

The basement membrane protein laminin-332 (laminin-5) mediates both stable cell adhesion and rapid cell migration, and thus has the potential to either restrain or promote tumor cell metastasis. The major cellular receptors for laminin-332 are integrin α3β1, which mediates rapid tumor cell migration, and integrin α6β4, which often mediates stable cell attachment. Tetraspanin protein CD151 interacts directly with both α3β1 and α6β4 integrins and with other tetraspanins, thereby promoting α3β1 and α6β4 association with tetraspanin-enriched microdomains on the cell surface. To explore the possibility of selectively modulating tumor cell responses to laminin-332, we re-expressed a series of CD151 mutants in epidermoid carcinoma cells with near total, RNAi- mediated silencing of endogenous CD151. CD151's interactions with its integrin partners or its interactions with other tetraspanins were selectively disrupted by specific mutations in the CD151 large extracellular loop (EC2 domain) or in intracellular CD151 palmitoylation sites, respectively. CD151- integrin association and CD151-tetraspanin association were both important for α3β1 integrin- dependent initial adhesion and rapid migration on laminin-332. Remarkably, however, only CD151-integrin association was required for stable, α6β4 integrin-dependent cell attachment on laminin-332. In gap-filling assays, where CD151-silenced cells moved more rapidly than WT cells, again, only CD151-integrin association was required to restrict movement into the gap, suggesting that both α3β1 and α6β4 integrin must be able to associate with CD151 in order restrict group motility. In addition, we found that a QRD amino acid motif in the CD151 EC2 domain that had been thought to be crucial for CD151-integrin interaction is not essential for CD151-integrin association or for CD151's ability to promote several different integrin functions. These new data suggest potential strategies for selectively modulating migratory cell responses to laminin-332, while leaving stable cell attachment on laminin-332 intact.