RetroCHMP3 blocks budding of enveloped viruses without blocking cytokinesis

Many enveloped viruses require the endosomal sorting complexes required for transport (ESCRT) pathway to exit infected cells. This highly conserved pathway mediates essential cellular membrane fission events, which restricts the acquisition of adaptive mutations to counteract viral co-option. Here, we describe duplicated and truncated copies of the ESCRT-III factor CHMP3 that block ESCRT-dependent virus budding and arose independently in New World monkeys and mice. When expressed in human cells, these retroCHMP3 proteins potently inhibit release of retroviruses, paramyxoviruses, and filoviruses. Remarkably, retroCHMP3 proteins have evolved to reduce interactions with other ESCRT-III factors and have little effect on cellular ESCRT processes, revealing routes for decoupling cellular ESCRT functions from viral exploitation. The repurposing of duplicated ESCRT-III proteins thus provides a mechanism to generate broad-spectrum viral budding inhibitors without blocking highly conserved essential cellular ESCRT functions.

Paramyxovirus assembly and budding: building particles that transmit infections

The paramyxoviruses define a diverse group of enveloped RNA viruses that includes a number of important human and animal pathogens. Examples include human respiratory syncytial virus and the human parainfluenza viruses, which cause respiratory illnesses in young children and the elderly; measles and mumps viruses, which have caused recent resurgences of disease in developed countries; the zoonotic Hendra and Nipah viruses, which have caused several outbreaks of fatal disease in Australia and Asia; and Newcastle disease virus, which infects chickens and other avian species. Like other enveloped viruses, paramyxoviruses form particles that assemble and bud from cellular membranes, allowing the transmission of infections to new cells and hosts. Here, we review recent advances that have improved our understanding of events involved in paramyxovirus particle formation. Contributions of viral matrix proteins, glycoproteins, nucleocapsid proteins, and accessory proteins to particle formation are discussed, as well as the importance of host factor recruitment for efficient virus budding. Trafficking of viral structural components within infected cells is described, together with mechanisms that allow for the selection of specific sites on cellular membranes for the coalescence of viral proteins in preparation of bud formation and virion release.

Phosphorylation of paramyxovirus phosphoprotein and its role in viral gene expression

Paramyxoviruses include many important human and animal pathogens such as measles virus, mumps virus, human parainfluenza viruses, and respiratory syncytial virus, as well as emerging viruses such as Nipah virus and Hendra virus. The paramyxovirus RNA-dependent RNA polymerase consists of the phosphoprotein (P) and the large protein. Both of these proteins are essential for viral RNA synthesis. The P protein is phosphorylated at multiple sites, probably by more than one host kinase. While it is thought that the phosphorylation of P is important for its role in viral RNA synthesis, the precise role of P protein phosphorylation remains an enigma. For instance, it was demonstrated that the putative CKII phosphorylation sites of the P protein of respiratory syncytial virus play a role in viral RNA synthesis using a minigenome replicon system; however, mutating these putative CKII phosphorylation sites within a viral genome had no effect on viral RNA synthesis, leading to the hypothesis that P protein phosphorylation, at least by CKII, does not play a role in viral RNA synthesis. Recently, it has been reported that the phosphorylation state of the P protein of parainfluenza virus 5, a prototypical paramyxovirus, correlates with the ability of P protein to synthesize viral RNA, indicating that P protein phosphorylation does in fact play a role in viral RNA synthesis. Furthermore, host kinases PLK1, as well as AKT1 have been found to play critical roles in paramyxovirus RNA synthesis through regulation of P protein phosphorylation status. Beyond furthering our understanding of paramyxovirus RNA replication, these recent discoveries may also result in a new paradigm in treating infections caused by these viruses, as host kinases that regulate paramyxovirus replication are investigated as potential targets of therapeutic intervention.

Evidence for a new viral late-domain core sequence, FPIV, necessary for budding of a paramyxovirus

Enveloped virus budding has been linked to both the ubiquitin-proteasome pathway and the vacuolar protein-sorting pathway of cells. We show here for the paramyxovirus SV5 that proteasome inhibitors and expression of dominant-negative VPS4(E228Q) ATPase blocks budding. The SV5 matrix (M) protein lacks previously defined late domains (e.g., P[T/S]AP, PPxY, YPDL) that recruit cellular factors. We identified a new motif for budding (core sequence FPIV) that can compensate functionally for lack of a PTAP late domain in budding human immunodeficiency virus type 1 virus-like particles (VLPs). Mutagenesis experiments suggest the more general sequence O-P-x-V. The proline residue was found to be critically important for function of this sequence, as substitution of this proline in the SV5 M protein resulted in poor budding of SV5 VLPs and failure of recombinant SV5 virus to replicate normally. Adaptation of mutant virus occurred rapidly, resulting in new proline residues elsewhere in the M protein. We hypothesize that these proline residues act to partially restore virus budding by generation of new motifs that act as suboptimal late domains.

Escaping from the cell: assembly and budding of negative-strand RNA viruses

Negative-strand RNA virus particles are formed by a process that includes the assembly of viral components at the plasma membranes of infected cells and the subsequent release of particles by budding. Here, we review recent progress that has been made in understanding the mechanisms of negative-strand RNA virus assembly and bud- ding. Important topics for discussion include the key role played by the viral matrix proteins in assembly of viruses and viruslike particles, as well as roles played by additional viral components such as the viral glycoproteins. Various interactions that contribute to virus assembly are discussed, including interactions between matrix proteins and membranes, interactions between matrix proteins and glycoproteins, interactions between matrix proteins and nucleocapsids, and interactions that lead to matrix protein self-assembly. Selection of specific sites on plasma membranes to be used for virus assembly and budding is described, including the asymmetric budding of some viruses in polarized epithelial cells and assembly of viral components in lipid raft microdomains. Evidence for the involvement of cellular proteins in the late stages of rhabdovirus and filovirus budding is discussed as well as the possible involvement of similar host factors in the late stages of budding of other negative-strand RNA viruses.

Roles for the cytoplasmic tails of the fusion and hemagglutinin-neuraminidase proteins in budding of the paramyxovirus simian virus 5

The efficient release of many enveloped viruses from cells involves the coalescence of viral components at sites of budding on the plasma membrane of infected cells. This coalescence is believed to require interactions between the cytoplasmic tails of surface glycoproteins and the matrix (M) protein. For the paramyxovirus simian virus 5 (SV5), the cytoplasmic tail of the hemagglutinin-neuraminidase (HN) protein has been shown previously to be important for normal virus budding. To investigate a role for the cytoplasmic tail of the fusion (F) protein in virus assembly and budding, we generated a series of F cytoplasmic tail-truncated recombinant viruses. Analysis of these viruses in tissue culture indicated that the cytoplasmic tail of the F protein was dispensable for normal virus replication and budding. To investigate further the requirements for assembly and budding of SV5, we generated two double-mutant recombinant viruses that lack 8 amino acids of the predicted 17-amino-acid HN protein cytoplasmic tail in combination with truncation of either 10 or 18 amino acids from the predicted 20-amino-acid F protein cytoplasmic tail. Both of the double mutant recombinant viruses displayed a replication defect in tissue culture and a budding defect, the extent of which was dependent on the length of the remaining F cytoplasmic tail. Taken together, this work and our earlier data on virus-like particle formation (A. P. Schmitt, G. P. Leser, D. L. Waning, and R. A. Lamb, J. Virol. 76:3953-3964, 2002) suggest a redundant role for the cytoplasmic tails of the HN and F proteins in virus assembly and budding.

Requirements for budding of paramyxovirus simian virus 5 virus-like particles

Enveloped viruses are released from infected cells after coalescence of viral components at cellular membranes and budding of membranes to release particles. For some negative-strand RNA viruses (e.g., vesicular stomatitis virus and Ebola virus), the viral matrix (M) protein contains all of the information needed for budding, since virus-like particles (VLPs) are efficiently released from cells when the M protein is expressed from cDNA. To investigate the requirements for budding of the paramyxovirus simian virus 5 (SV5), its M protein was expressed in mammalian cells, and it was found that SV5 M protein alone could not induce vesicle budding and was not secreted from cells. Coexpression of M protein with the viral hemagglutinin-neuraminidase (HN) or fusion (F) glycoproteins also failed to result in significant VLP release. It was found that M protein in the form of VLPs was only secreted from cells, with an efficiency comparable to authentic virus budding, when M protein was coexpressed with one of the two glycoproteins, HN or F, together with the nucleocapsid (NP) protein. The VLPs appeared similar morphologically to authentic virions by electron microscopy. CsCl density gradient centrifugation indicated that almost all of the NP protein in the cells had assembled into nucleocapsid-like structures. Deletion of the F and HN cytoplasmic tails indicated an important role of these cytoplasmic tails in VLP budding. Furthermore, truncation of the HN cytoplasmic tail was found to be inhibitory toward budding, since it prevented coexpressed wild-type (wt) F protein from directing VLP budding. Conversely, truncation of the F protein cytoplasmic tail was not inhibitory and did not affect the ability of coexpressed wt HN protein to direct the budding of particles. Taken together, these data suggest that multiple viral components, including assembled nucleocapsids, have important roles in the paramyxovirus budding process.

Involvement of the cytoplasmic domain of the hemagglutinin-neuraminidase protein in assembly of the paramyxovirus simian virus 5

Efficient assembly of enveloped viruses at the plasma membranes of virus-infected cells requires coordination between cytosolic viral components and viral integral membrane glycoproteins. As viral glycoprotein cytoplasmic domains may play a role in this coordination, we have investigated the importance of the hemagglutinin-neuraminidase (HN) protein cytoplasmic domain in the assembly of the nonsegmented negative-strand RNA paramyxovirus simian virus 5 (SV5). By using reverse genetics, recombinant viruses which contain HN with truncated cytoplasmic tails were generated. These viruses were shown to be replication impaired, as judged by small plaque size, reduced replication rate, and low maximum titers when compared to those features of wild-type (wt) SV5. Release of progeny virus particles from cells infected with HN cytoplasmic-tail-truncated viruses was inefficient compared to that of wt virus, but syncytium formation was enhanced. Furthermore, accumulation of viral proteins at presumptive budding sites on the plasma membranes of infected cells was prevented by HN cytoplasmic tail truncations. We interpret these data to indicate that formation of budding complexes, from which efficient release of SV5 particles can occur, depends on the presence of an HN cytoplasmic tail.

Transcriptional factor mutations reveal regulatory complexities of heat shock and newly identified stress genes in Saccharomyces cerevisiae

A computer-aided pattern search of the entire yeast genome was designed and used to identify 186 putative stress response element-regulated genes in Saccharomyces cerevisiae. Transcript levels of eight of these candidate genes were examined, and three (37%) were shown to be heat shock- and DNA damage-inducible and to require the Msn2p and Msn4p transcriptional activators for stress regulation. Significantly, several heat shock protein (HSP) genes were identified in this computer search. Using a series of single and multiple regulatory mutants, we demonstrate unexpected regulatory complexities among the HSP genes from S. cerevisiae following heat shock.

Msn2p, a zinc finger DNA-binding protein, is the transcriptional activator of the multistress response in Saccharomyces cerevisiae

The stress response promoter element (STRE) confers increased transcription to a set of genes following environmental or metabolic stress in Saccharomyces cerevisiae. A lambda gt11 library was screened to isolate clones encoding STRE-binding proteins, and one such gene was identified as MSN2, which encoded a zinc-finger transcriptional activator. Disruption of the MSN2 gene abolished an STRE-binding activity in crude extracts as judged by both gel mobility-shift and Southwestern blot experiments, and overexpression of MSN2 intensified this binding activity. Northern blot analysis demonstrated that for the known or suspected STRE-regulated genes DDR2, CTT1, HSP12, and TPS2, transcript induction was impaired following heat shock or DNA damage treatment in the msn2-disrupted strain and was constitutively activated in a strain overexpressing MSN2. Furthermore, heat shock induction of a STRE-driven reporter gene was reduced more than 6-fold in the msn2 strain relative to wild-type cells. Taken together, these data indicate that Msn2p is the transcription factor that activates STRE-regulated genes in response to stress. Whereas nearly 85% of STRE-mediated heat shock induction was MSN2 dependent, there was significant MSN2-independent expression. We present evidence that the MSN2 homolog, MSN4, can partially replace MSN2 for transcriptional activation following stress. Moreover, our data provides evidence for the involvement of additional transcription factors in the yeast multistress response.