STRUCTURE OF THE VIRAL TAP-INHIBITOR ICP47 INDUCED BY MEMBRANE ASSOCIATION

Herpes simplex virus type I protein ICP47 (IE12) turns off antigen pre sentation by specifically binding to and blocking the major histocompa tibility complex- (MHC-) encoded transporter associated with antigen p rocessing (TAP). Due to the lack of translocated peptides inside the e ndoplasmic reticulum, MHC class I molecules fail to assemble and there fore MHC-peptide complexes do not reach the cell surface for immune re cognition by cytotoxic T-lymphocytes. Here we investigated the structu re of ICP47 representing the first natural inhibitor of an ATP-binding -cassette (ABC) transporter identified so far. First, we demonstrate t hat the N-terminal half of ICP47 is as active in inhibition of human T AP as the full-length protein and therefore serves as an ideal model t o investigate structural and functional aspects of the inhibitor. Seco nd, from circular dichroism analysis, the viral inhibitor of TAP appea rs to be mainly unstructured in aqueous solution. However, in the pres ence of membrane mimetics or lipid membranes an cl-helical structure i s induced. Third, circular dichroism and fluorescence spectroscopy rev eal that membrane adsorption and conformational change of ICP47 are di rectly dependent on the surface charge density of the lipid membrane. Therefore we conclude that docking to membranes induces a conformation al change in ICP47 that may be prerequisite to blocking TAP function.