Characterization of the HHV-6 U20 immunoevasin

Abstract

Roseoloviruses (HHV-6A, -6B, and -7) infect >90% of the human population during early childhood, and are thought to remain latent or persistent throughout the life of the host. As such, these viruses are among the most pervasive and stealthy of all viruses; they must necessarily excel at escaping immune detection throughout the life of the host, and yet very little is known about how these viruses so successfully escape host defenses. Herein, we characterize the HHV6A and HHV6B U20 gene products, which are encoded within a block of genes unique to the roseoloviruses, and therefore of particular interest. Despite 92% amino acid identity, U20 proteins from HHV6A and 6B have been shown to possess different host evasion functions. Here we characterize expression, trafficking, and post-translational modifications of U20 during HHV6A infection. While U20 localized to lysosomes in HHV-6A-infected cells, HHV-6B U20 trafficked to the cell surface and was rapidly internalized. HHV-6B U20 trafficked slowly through the secretory system, receiving several post translational modifications to its N-linked glycans indicative of surface expressed glycoproteins. Interestingly, U20 is also phosphorylated on at least one Ser, Thr, or Tyr residue. These results provide a framework to understand the role(s) of U20 in evading host defenses. ImportanceHHV6A and HHV6B U20 proteins are virus-encoded integral membrane glycoproteins possessing class I MHC-like folds. As such, it is tempting to speculate that they are involved in host evasion. Indeed, although they share 92% identity, HHV6A U20 has been shown to target NK activating ligands (1) and HHV6B U20 has been shown to inhibit TNF receptor signaling and apoptosis (2). Here, we have performed cell biological and biochemical characterization of the trafficking, glycosylation, and post-translational modifications occurring on HHV6A and -6B U20, and we demonstrate U20 expression in the context of HHV6 infection.