Binds and retains class I heavy chains in the endoplasmic reticulum during the early period of virus infection, thereby impairing their transport to the cell surface. Also delays the expression of class I alleles that it cannot affect by direct retention. Binds transporters associated with antigen processing (TAP) and acts as a tapasin inhibitor, preventing class I/TAP association. In consequence, infected cells are masked for immune recognition by cytotoxic T-lymphocytes (By similarity). Expressed at early period of virus infection. The lumenal domain binds directly to the peptide-binding domain of class I molecules. The di-lysine motif confers endoplasmic reticulum localization for type I membrane proteins. Both disulfide bonds are absolutely critical for the interaction with MHC antigens. N-glycosylated; high-mannose. Belongs to the adenoviridae E19 family.