Inhibits peptide loading of MHC class I molecules by transporters associated with antigen processing (TAP). Does not prevent peptide binding to TAP, but binds to the lumenal side of the TAP complex and inhibits peptide translocation by specifically blocking ATP-binding to TAP1, but not TAP2. Also prevents the conformational rearrangement of TAP induced by peptide binding. In consequence, infected cells are masked for immune recognition by cytotoxic T-lymphocytes. Interacts with UL18. Expressed at early period of virus infection. The ER-lumenal domain is responsible for TAP inhibition. It is sufficient to inhibit ATP binding to TAP. Belongs to the cytomegalovirus US6 family.