Functional organization of MIR2, a novel viral regulator of selective endocytosis

Kaposi's sarcoma-associated herpesvirus encodes two related proteins, MIR1. and MIR2, that lead to reduction of the cell surface levels of major histocompatibility complex class 1 and other polypeptides involved in immune recognition. MIR1 and MIR2 do not affect the assembly or transport of their target proteins through the secretory pathway; rather, they act to enhance the selective endocytosis of target chains from the cell surface. Sequence inspection reveals that the modulator of immune recognition (MIR) proteins contain an NH2-terminal zinc finger of the plant homeodomain (PHD) subfamily, two transmembrane (TM) domains, and a C-terminal conserved region (CR). Here we examine the transmembrane topology and functional organization of MIR2. Both the PHD domain and the CR are disposed cytosolically and are essential for MIR-mediated endocytosis. MIR proteins form homo-oligomers; this activity is independent of the PHD and CR elements and maps instead to the TM regions. Analysis of chimeras between MIR1 and MIR2 reveals that the TM regions also mediate target selectivity. Mutations that ablate the PHD or CR regions generate dominant negative phenotypes for major histocompatibility complex class 1 endocytosis. These findings suggest a domain organization for the MIR proteins, with the TM regions involved in target selection and the cytosolic PHD and CR domains involved in the possible recruitment of cellular machinery that directly or indirectly regulates internalization of target molecules.