Viperin is an interferon-inducible protein that is directly induced in cells by human cytomegalovirus (HCMV) infection. Why HCMV would induce viperin, which has antiviral activity, is unknown. We show that HCMV-induced viperin disrupts cellular metabolism to enhance the infectious process. Viperin interaction with the viral protein vMIA resulted in viperin relocalization from the endoplasmic reticulum to the mitochondria. There, viperin interacted with the mitochondrial trifunctional protein that mediates β-oxidation of fatty acids to generate adenosine triphosphate (ATP). This interaction with viperin, but not with a mutant lacking the viperin iron-sulfur cluster-binding motif, reduced cellular ATP generation, which resulted in actin cytoskeleton disruption and enhancement of infection. This function of viperin, which was previously attributed to vMIA, suggests that HCMV has coopted viperin to facilitate the infectious process.

Summary Viperin is an interferon-inducible protein that inhibits the replication of a variety of viruses by apparently diverse mechanisms. In some circumstances it also plays a role in intracellular signaling pathways. Its expression in mitochondria, revealed by infection with human cytomegalovirus, also affects cellular metabolic pathways. We review here the current status of our understanding of this unusual molecule.

The human cytomegalovirus UL99 open reading frame encodes a 190-amino-acid (aa) tegument protein, pp28, that is myristoylated and phosphorylated. pp28 is essential for assembly of infectious virus, and nonenveloped virions accumulate in the cytoplasm of cells infected with recombinant viruses with a UL99 deletion. pp28 is localized to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC) in transfected cells, while in infected cells, it is localized together with other virion proteins in a juxtanuclear compartment termed the assembly compartment (AC). We investigated the sequence requirements for pp28 trafficking to the AC and assembly of infectious virus. Our studies indicated that the first 30 to 35 aa were required for localization of pp28 to the ERGIC in transfected cells. Mutant forms of pp28 containing only the first 35 aa localized with other virion structural proteins to cytoplasmic compartments early in infection, but localization to the AC at late times required a minimum of 50 aa. In agreement with previous reports, we demonstrated that the deletion of a cluster of acidic amino acids (aa 44 to 59) prevented wild-type trafficking of pp28 and recovery of infectious virus. A recombinant virus expressing only the first 50 aa was replication competent, and this mutant, pp28, localized to the AC in cells infected with this virus. These findings argued that localization of pp28 to the AC was essential for assembly of infectious virus and raised the possibility that amino acids in the amino terminus of pp28 have additional roles in the envelopment and assembly of the virion other than simply localizing pp28 to the AC.Human cytomegalovirus (HCMV) is the largest and most complex member of the family of human herpesviruses. The virion of HCMV consists of three distinct structures, a nucleocapsid containing a double-stranded linear DNA genome, an envelope including an as-yet-undefined number of viral glycoproteins, and a tegument layer located between the capsid and the envelope (27,40,44,45). HCMV assembly is a multistage and poorly understood process. Although all proposed models include well-studied mechanisms of capsid assembly within the nucleus of infected cells, final tegumentation and envelopment in the cytoplasm of infected cells remain poorly understood (25). The assembly pathway and protein interactions that are required for formation of the tegument layer are not defined. As an example, the tegument protein ppUL69 is expressed only in the nucleus, whereas some tegument proteins such as pp150 (ppUL32) and pp28 (ppUL99) are expressed only in the cytoplasm during the replication of HCMV (33). Other tegument proteins such as ppUL53 and pp65 (ppUL83) are expressed in the nucleus of cells early in infection but are localized predominantly in the cytoplasm late in infection (33). Thus, it is unclear whether tegument proteins associate with the capsid in the nucleus or in the cytoplasmic assembly compartment at a later step (25,33). Electron microscopic studies have revealed that both nuclear and c...