The murine cytomegalovirus immune evasion protein m4/gp34 forms biochemically distinct complexes with class I MHC at the cell surface and in a pre-golgi compartment

We have recently demonstrated that the murine CMV (MCMV) gene m4 is an immu ne evasion gene that protects MCMV-infected targets from some virus-specifi c CTL clones. m4 encodes m4/gp34, a 34-kDa glycoprotein that binds to major histocompatibility complex class I in the endoplasmic reticulum and forms a detergent-stable complex that is exported to the surface of the cell. To investigate how m4/gp34 promotes CTL evasion, we analyzed the assembly and export of m4/gp34-K-b complexes. We found that 50-70% of K-b exported over the course of MCMV infection was m4/gp34 associated. Because these complexe s are present at the cell surface, it is possible that m4 mediates CTL evas ion by interfering with contact between class I and receptors on the T cell . In addition. we found that K-b retained by the MCMV immune evasion gene m 152 formed a novel type of complex with Endo H-sensitive m4/gp34; these com plexes are distinguished from the exported complexes by being stable in 1% digitonin and unstable in 1% Nonidet P-40. Because this association occurs in a pre-Golgi compartment, m4/gp34 might also interfere with Ag presentati on by affecting some aspect of class I assembly, such as peptide loading. A lthough m4/gp34 requires beta (2)-microglobulin to bind class I, there was no significant binding of m4/gp34 to beta (2)-microglobulin in the absence of class I H chain, demonstrating that m4/gp34 forms Nonidet P-40-stable co mplexes specifically with folded conformations of class I. We conclude that m4/gp34 promotes immune evasion by a novel mechanism involving altered ass embly and/or T cell recognition of class I molecules.