B-152. Plasma Membrane Sphingomyelin Functions as a Novel Raft Receptor for the Helicobacter pylori VacA

V. R. Gupta1, H. K. Patel1, S. R. Blanke1,2;
1Univ. of Illinois, Urbana, IL, 2Inst. for Genomic Biology, Urbana, IL.

The vacuolating toxin (VacA) is an important virulence factor secreted by Helicobacter pylori, an etiologic agent of peptic ulcer disease and gastric cancer in humans. However, the mechanism underlying VacA intoxication of host cells remains poorly understood. Although several host cell proteins have been identified that increase cell sensitivity to VacA, these factors have not been demonstrated to be important for toxin binding to the cell surface. Because VacA has been previously demonstrated to interact with artificial membranes, we screened plasma membrane lipids as potential receptors for VacA, and identified sphingomyelin as a receptor for VacA. Cellular sphingomyelin levels correlated strongly to sensitivity to VacA. Modulation of sphingomyelin levels specifically at the plasma membrane altered VacA-mediated intoxication. Cells with prebound VacA were no longer sensitive to alterations in cellular sphingomyelin levels, suggesting the importance of this lipid in an early step in the intoxication pathway. VacA binding to the cell surface, as well as subsequent internalization, correlated to sphingomyelin levels. VacA binding and function was dependent on sphingomyelin specifically localized to lipid rafts. Finally, VacA was demonstrated to bind sphingomyelin specifically in vitro, supporting the idea that VacA binds sphingomyelin directly. These results identify, for the first time, sphingomyelin as a receptor for VacA that acts to target the toxin to lipid rafts as the critical first step in the intoxication pathway. VacA is the first bacterial virulence factor that exploits sphingomyelin as a cellular receptor, which because of the role of this sphingolipid in cellular signaling, has important implications for the mechanism by which VacA enters cells.