Identification of TAP2 protein variants resistant to inhibition by the HSV1 ICP47 protein
Herpes Simplex Virus 1 evades the host immune system by expressing a protein, ICP47, that binds to and inhibits the heterodimeric Transporter Associated with Antigen Processing (TAP). We screened a library of 1786 variants in TAP2, one of the components of the TAP heterodimer, and identified 39 variants that were resistant to inhibition by ICP47. Of these 39 variants, five were individually tested, and three (T257I, S274H, and T244R) were confirmed to be significantly resistant to inhibition by ICP47. This resistance to inhibition did not extend to the Epstein Barr Virus BNLF2a protein, another viral factor known to inhibit antigen presentation by targeting TAP. These three residues localize close to the binding site of ICP47, on the 3D structure of TAP, but only Ser274 is spatially close to the antigenic peptide binding site of TAP. These results functionally resolve the TAP2 residues required for peptide binding from those required for ICP47 binding and identify TAP2 residues whose targeting with small molecule inhibitors could effectively prevent Herpes virus downregulation of antigen processing. AUTHOR SUMMARYT cells detect virus infected cells via viral peptides in complex with the major histocompatibility complex type I (MHC-I) on the cell surface. Herpes Simplex Virus 1 expression of the ICP47 protein allows the virus to evade detection and clearance of infected cells by host T cells. ICP47 binds to the TAP1/TAP2 protein complex and prevents the loading of antigenic peptides onto the MHC-I protein complex and thereby blocks the antigen processing. In this study, we screened for and identified variants of TAP2 with single amino acid substitutions that are no longer inhibited by ICP47. TAP2 proteins with the identified amino acid substitutions functioned normally in antigen processing even in the presence of ICP47. Results from this study create the potential strategy for drugs that target the identified portions of the TAP2 protein and block ICP47 inhibition of antigen processing while retaining normal TAP1/TAP2 function and thus suggest a new antiviral therapeutic approach for herpes virus infections.