Structure of the viral TAP-inhibitor ICP47 induced by membrane association.

Herpes simplex virus type I protein ICP47 (IE12) turns off antigen presentation by specifically binding to and blocking the major histocompatibility complex- (MHC-) encoded transporter associated with antigen processing (TAP). Due to the lack of translocated peptides inside the endoplasmic reticulum, MHC class I molecules fail to assemble and therefore MHC-peptide complexes do not reach the cell surface for immune recognition by cytotoxic T-lymphocytes. Here we investigated the structure of ICP47 representing the first natural inhibitor of an ATP-binding-cassette (ABC) transporter identified so far. First, we demonstrate that the N-terminal half of ICP47 is as active in inhibition of human TAP as the full-length protein and therefore serves as an ideal model to investigate structural and functional aspects of the inhibitor. Second, from circular dichroism analysis, the viral inhibitor of TAP appears to be mainly unstructured in aqueous solution. However, in the presence of membrane mimetics or lipid membranes an alpha-helical structure is induced. Third, circular dichroism and fluorescence spectroscopy reveal that membrane adsorption and conformational change of ICP47 are directly dependent on the surface charge density of the lipid membrane. Therefore we conclude that docking to membranes induces a conformational change in ICP47 that may be prerequisite to blocking TAP function.