A Short Isoform of Human Cytomegalovirus US3 Functions as a Dominant Negative Inhibitor of the Full-Length Form

Abstract

Human cytomegalovirus encodes four unique short (US) region proteins, each of which is independently sufficient for causing the down-regulation of major histocompatibility complex (MHC) class I molecules on the cell surface. This down-regulation enables infected cells to evade recognition by cytotoxic T lymphocytes (CTLs) but makes them vulnerable to lysis by natural killer (NK) cells, which lyse those cells that lack MHC class I molecules. The 22-kDa US3 glycoprotein is able to down-regulate the surface expression of MHC class I molecules by dual mechanisms: direct endoplasmic reticulum retention by physical association and/or tapasin inhibition. The alternative splicing of the US3 gene generates two additional products, including 17-kDa and 3.5-kDa truncated isoforms; however, the functional significance of these isoforms during viral infection is unknown. Here, we describe a novel mode of self-regulation of US3 function that uses the endogenously produced truncated isoform. The truncated isoform itself neither binds to MHC class I molecules nor prevents the full-length US3 from interacting with MHC class I molecules. Instead, the truncated isoform associates with tapasin and competes with full-length US3 for binding to tapasin; thus, it suppresses the action of US3 that causes the disruption of the function of tapasin. Our results indicate that the truncated isoform of the US3 locus acts as a dominant negative regulator of full-length US3 activity. These data reflect the manner in which the virus has developed temporal survival strategies during viral infection against immune surveillance involving both CTLs and NK cells.Human cytomegalovirus (HCMV), a member of the Betaherpesviridae, leads to lifelong persistence in the infected host, followed by causing opportunistic pathogenesis in immunocompromised individuals such as neonates, organ transplant recipients, and AIDS patients. Both innate and adaptive immune systems participate in host defense against the viral infection in which natural killer (NK) cells and cytotoxic T lymphocytes (CTLs) function as the major effector cells. NK cell response plays an important role in initial defense, and the deficiency of NK cells results in susceptibility to severe herpesvirus infections (8). NK cell activation is controlled by the interaction of inhibitory receptors of NK cells with major histocompatibility complex (MHC) class I antigens on target cells (26). The missing-self hypothesis proposes that NK cells recognize and attack target cells that lack an adequate level of cell surface-expressed self-MHC class I molecules (25). Generally, the immune response of the host against a virus, including HCMV, is mediated by CTLs, which recognize and eliminate infected cells expressing viral peptide-loaded MHC class I molecules (41).Since the MHC class I molecule is a key factor in the modulation of the immune response against viral pathogens, it is not surprising that class I molecules are preferred targets of viruses to avoid immune surveillance and clearance (27,32,40). HCM...

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The Ubiquitin System and Kaposi’s Sarcoma-Associated Herpesvirus

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RNA editing and alternative splicing of the insect nAChR subunit alpha6 transcript: evolutionary conservation, divergence and regulation

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