Early region 3 of adenovirus type 19 (subgroup D) encodes an HLA-binding protein distinct from that of subgroups B and C

Early region 3 (E3) of human adenoviruses (Ads) codes for proteins that appear to control viral interactions with the host. For example, the most abundant E3 protein, E3/19K, inhibits the transport of newly synthesized class I major histocompatibility molecules to the cell surface, thereby interfering with antigen presentation. So far, the E3 regions of Ad subgroups A, B, C, and F have been characterized. We have cloned the E3A region of Ad type 19a (Ad19a), which belongs to the largest subgroup, D, and causes epidemic keratoconjunctivitis in humans. The sequence reveals five open reading frames (ORFs) with the potential to encode the Ad19 equivalent of pVIII, as well as proteins 12.2K, 16.2K, and 18.6K. The last ORF predicts a novel 49K protein which has no counterpart in other subgroups. Both the sequence and the overall organization of the E3 region from Ad19a shows a closer relationship to group B than to group C Ads. The 18.6K ORF represents the Ad19 homolog of the Ad2 E3/19K protein. By using 293 cells stably transfected with the Ad19a E3A region, we showed by immunoprecipitation, pulse-chase experiments, and fluorescence-activated cell sorter analysis that the Ad19 E3/19K protein binds to and prevents the transport of major histocompatibility complex molecules to the cell surface. The similar but distinct functional activity of the Ad19 E3/19K protein, combined with the new sequence which differs from those of subgroup B and C proteins, allows a more precise definition of amino acids essential for HLA binding.