Direct Extracellular Contact between Integrin α₃β₁ and TM4SF Protein CD151*

Abstract

Previously we established that the α₃β₁ integrin shows stable, specific, and stoichiometric association with the TM4SF (tetraspannin) protein CD151. Here we used a membrane impermeable cross-linking agent to show a direct association between extracellular domains of α₃β₁ and CD151. The α₃β₁−CD151 association site was then mapped using chimeric α₆/α₃ integrins and CD151/NAG2 TM4SF proteins. Complex formation required an extracellular α₃ site (amino acids (aa) 570–705) not previously known to be involved in specific integrin contacts with other proteins and a region (aa 186–217) within the large extracellular loop of CD151. Notably, the anti-CD151 monoclonal antibody TS151r binding epitope, previously implicated in α₃ integrin association, was mapped to the same region of CD151 (aa 186–217). Finally, we demonstrated that both NH₂- and COOH-terminal domains of CD151 are located on the inside of the plasma membrane, thus confirming a long suspected model of TM4SF protein topology.