Severe Acute Respiratory Syndrome Coronavirus Protein 6 Accelerates Murine Hepatitis Virus Infections by More than One Mechanism

Severe acute respiratory syndrome coronavirus (SARS-CoV) is a zoonotic virus that is endemic in bats (33, 34). This virus can infect exotic animals marketed for consumption in southeast China (22, 58), and in doing so can acquire proximity to human populations. SARS-CoV entered human hosts in 2002 to 2003 and disseminated rapidly to more than 30 countries across five continents, killing ∼800 individuals. The SARS-CoV virion contains a ∼30-kb, plus-strand RNA genome enclosed within a pleiomorphic membrane envelope. Many of the features of this virus are known from over 25 years of research on animal coronaviruses. The 5′ ∼2/3 of the genome encodes two very large polyproteins which are rapidly proteolyzed into components functioning in viral RNA synthesis and metabolism, while the 3′ ∼1/3 includes several virion assembly subunits, notably the proteins spike (S), envelope (E), membrane (M), and nucleocapsid (N). Integrated both between and within these genes encoding virion proteins are eight so-called accessory genes (51). These accessory genes, originally designated numerically as open reading frames (ORFs) 3a through 9b, were identified in the original 2003 SARS-CoV isolate (strain Urbani) and are now known to be ubiquitous in SARS-CoVs obtained from infected bats, civit cats, raccoon dogs, and humans (33, 46, 64), suggesting evolutionary conservation of important viral functions. The majority of the accessory genes are expressed in SARS-CoV-infected cells (8, 10, 28, 37, 39, 48, 54, 70). Many are membrane proteins, and as antibodies have become available, several of the accessory proteins have been detected as copurifying with virions (26, 48, 50) and have been structurally resolved by X-ray crystallography (39). Although these studies have provided important information about the SARS-CoV accessory proteins, their central functions remain largely unknown. Indeed, accessory protein functions may be difficult to discern, as elimination of most of the accessory genes through reverse genetics generates recombinant SARS-CoVs with tissue culture growth properties remarkably similar to native SARS viruses (1, 16, 68). Experimental rodent models for SARS-CoV infection and disease have been recently developed (35, 45, 56, 61), and under these experimental conditions, recombinant SARS-CoVs lacking a subset of accessory genes were nearly equivalent to complete SARS-CoV in virulence and pathogenicity (2, 14, 16, 68). Thus, at present there is no satisfying explanation of the roles that these evolutionarily conserved accessory proteins play in natural animal or human SARS-CoV infections. Accessory protein 6 has received significant scrutiny, as this protein does exhibit functions that may relate to its presumed roles in SARS-CoV infections. When expressed alone from plasmid cDNA, protein 6 ablates type I interferon signaling, indicating its potential in thwarting innate immune effectors (15, 31). Notably, several different coronavirus products may have similar immune evasion activities (31), a redundancy that presumably accounts for the inapparent in vivo consequences of ORF6 elimination from SARS coronavirus. However, when evaluated in the context of heterologous murine coronavirus infections, protein 6 has clearly recognized activities. Engineered recombinant murine coronaviruses constructed to express SARS-CoV ORF6, designated rJ.2.2.6 (recombinant JHM strain 2.2-V-1 encoding SARS p6) (41), have been compared with isogenic rJ.2.2.6-KO viruses in which ORF6 is closed via knockout (KO) of the initiation codon. Relative to rJ.2.2.6-KO, the rJ.2.2.6 viruses grow more rapidly, both in tissue culture (55) and in the murine central nervous system, and have increased in vivo lethality (40, 41). These properties of protein 6 may relate to direct effects on the coronavirus replication machinery, as protein 6 colocalizes intracytoplasmically with membrane-proximal sites of viral RNA-dependent RNA synthesis and does indeed provide for earlier viral RNA synthesis in tissue culture contexts (55). Protein 6 is a 63-amino-acid peptide that comprises a relatively hydrophobic N-terminal portion of ∼40 residues and a C-terminal hydrophilic extension. The N-terminal region may function as a signal/anchor for membrane association, as protein 6 has the biochemical characteristics of an integral membrane protein (55). This membrane-associating property of the N-terminal region is presumed to operate in assisting development of viral RNA replication sites, which are well-known to reside on the interfaces between intracellular membranes and cytosol. The C-terminal region extends from membranes into cytosolic environments (40) and displays several motifs that are vital for a separate and novel activity (15). C-terminal residues of protein 6 interact with cellular importins, mediators of protein import into the nucleus (15). Accumulating protein 6 on cytoplasmic membranes titrates importins to cytoplasmic sites and thereby thwarts cellular capacity to transport cytoplasmic cargo into the nucleus. Protein 6 has been shown to impede nuclear import of proteins such as IRF3 and STAT1, key regulators of interferon gene transcription (31). While it is reasonably assumed that this activity of the protein 6 C terminus contributes to virus growth or pathogenesis by thwarting innate immune responses, there are few supporting data. In fact, alanine substitutions engineered into the C terminus did not reduce the murine neurovirulence of rJ.2.2.6 (40), leaving it unclear whether the hydrophilic cytoplasmic tail of protein 6 makes significant contributions to the growth or virulence of this related coronavirus. These indications that unique biological activities might be separately ascribed to the N- and C-terminal regions of protein 6 led us into investigations dissecting the relative contributions of each region to recombinant JHM virus growth. In extending a recently published report (15), we found that protein 6 blocked the nuclear accumulation of proteins relying on classical nuclear import pathways, with the C terminus being vital in blocking nuclear import. More importantly, we correlated this nuclear import blockade with enhanced murine hepatitis virus (MHV) infections, giving credence to the hypothesis that this conserved, 63-amino-acid accessory protein creates superior environments for virus growth by enriching the cytosol with proteins otherwise destined to the nucleus. We found, however, that this impediment to protein import into the nucleus made only a minor contribution to virus growth, as C-terminally truncated forms of protein 6 lacking this activity still retained the majority of virus-augmenting function. These findings indicate that this small protein 6 supports murine coronavirus infections by more than one mechanism.