Dependence of the localization and function of the human cytomegalovirus protein US6 on the transporter associated...

Human cytomegalovirus protein US6 inhibits the transporter associated with antigen processing (TAP), which transports peptides into the endoplasmic reticulum (ER) for binding to major histocompatibility complex (MHC) class I molecules. We demonstrate that, in TAP-deficient cells, US6 is retained in the ER and binds to calnexin, but does not inhibit cell-surface expression of HLA-A201, an MHC class I allele that binds to peptides whose import into the ER is TAP-independent. Furthermore, in TAP-positive cells, US6 reduces the cell-surface expression of HLA-B2705, an MHC class I allele that is dependent on TAP for peptide binding, to a greater extent than that of HLA-A201. These data demonstrate that US6 has differential effects on the cell-surface expression of MHC class I alleles and are consistent with TAP being the sole inhibitory target of US6 in the MHC class I antigen-presentation pathway.