Site-directed mutagenesis of hepatitis A virus protein 3A: effects on membrane interaction

Evolutionary Origins of Enteric Hepatitis Viruses

The enterically transmitted hepatitis A (HAV) and hepatitis E viruses (HEV) are the leading causes of acute viral hepatitis in humans. Despite the discovery of HAV and HEV 40-50 years ago, their evolutionary origins remain unclear. Recent discoveries of numerous nonprimate hepatoviruses and hepeviruses allow revisiting the evolutionary history of these viruses. In this review, we provide detailed phylogenomic analyses of primate and nonprimate hepatoviruses and hepeviruses. We identify conserved and divergent genomic properties and corroborate historical interspecies transmissions by phylogenetic comparisons and recombination analyses. We discuss the likely non-recent origins of human HAV and HEV precursors carried by mammals other than primates, and detail current zoonotic HEV infections. The novel nonprimate hepatoviruses and hepeviruses offer exciting new possibilities for future research focusing on host range and the unique biological properties of HAV and HEV.

A Novel Marsupial Hepatitis A Virus Corroborates Complex Evolutionary Patterns Shaping the Genus Hepatovirus

The discovery of highly diverse nonprimate hepatoviruses illuminated the evolutionary origins of hepatitis A virus (HAV) ancestors in mammals other than primates. Marsupials are ancient mammals that diverged from other Eutheria during the Jurassic. Viruses from marsupials may thus provide important insight into virus evolution. To investigate Hepatovirus macroevolutionary patterns, we sampled 112 opossums in northeastern Brazil. A novel marsupial HAV (MHAV) in the Brazilian common opossum (Didelphis aurita) was detected by nested reverse transcription-PCR (RT-PCR). MHAV concentration in the liver was high, at 2.5 × 10⁹ RNA copies/g, and at least 300-fold higher than those in other solid organs, suggesting hepatotropism. Hepatovirus seroprevalence in D. aurita was 26.6% as determined using an enzyme-linked immunosorbent assay (ELISA). Endpoint titers in confirmatory immunofluorescence assays were high, and marsupial antibodies colocalized with anti-HAV control sera, suggesting specificity of serological detection and considerable antigenic relatedness between HAV and MHAV. MHAV showed all genomic hallmarks defining hepatoviruses, including late-domain motifs likely involved in quasi-envelope acquisition, a predicted C-terminal pX extension of VP1, strong avoidance of CpG dinucleotides, and a type 3 internal ribosomal entry site. Translated polyprotein gene sequence distances of at least 23.7% from other hepatoviruses suggested that MHAV represents a novel Hepatovirus species. Conserved predicted cleavage sites suggested similarities in polyprotein processing between HAV and MHAV. MHAV was nested within rodent hepatoviruses in phylogenetic reconstructions, suggesting an ancestral hepatovirus host switch from rodents into marsupials. Cophylogenetic reconciliations of host and hepatovirus phylogenies confirmed that host-independent macroevolutionary patterns shaped the phylogenetic relationships of extant hepatoviruses. Although marsupials are synanthropic and consumed as wild game in Brazil, HAV community protective immunity may limit the zoonotic potential of MHAV.

IMPORTANCE Hepatitis A virus (HAV) is a ubiquitous cause of acute hepatitis in humans. Recent findings revealed the evolutionary origins of HAV and the genus Hepatovirus defined by HAV in mammals other than primates in general and in small mammals in particular. The factors shaping the genealogy of extant hepatoviruses are unclear. We sampled marsupials, one of the most ancient mammalian lineages, and identified a novel marsupial HAV (MHAV). The novel MHAV shared specific features with HAV, including hepatotropism, antigenicity, genome structure, and a common ancestor in phylogenetic reconstructions. Coevolutionary analyses revealed that host-independent evolutionary patterns contributed most to the current phylogeny of hepatoviruses and that MHAV was the most drastic example of a cross-order host switch of any hepatovirus observed so far. The divergence of marsupials from other mammals offers unique opportunities to investigate HAV species barriers and whether mechanisms of HAV immune control are evolutionarily conserved.

Amino acid changes in proteins 2B and 3A mediate rhinovirus type 39 growth in mouse cells

Many steps of viral replication are dependent on the interaction of viral proteins with host cell components. To identify rhinovirus proteins involved in such interactions, human rhinovirus 39 (HRV39), a virus unable to replicate in mouse cells, was adapted to efficient growth in mouse cells producing the viral receptor ICAM-1 (ICAM-L cells). Amino acid changes were identified in the 2B and 3A proteins of the adapted virus, RV39/L. Changes in 2B were sufficient to permit viral growth in mouse cells; however, changes in both 2B and 3A were required for maximal viral RNA synthesis in mouse cells. Examination of infected HeLa cells by electron microscopy demonstrated that human rhinoviruses induced the formation of cytoplasmic membranous vesicles, similar to those observed in cells infected with other picornaviruses. Vesicles were also observed in the cytoplasm of HRV39-infected mouse cells despite the absence of viral RNA replication. Synthesis of picornaviral nonstructural proteins 2C, 2BC, and 3A is known to be required for formation of membranous vesicles. We suggest that productive HRV39 infection is blocked in ICAM-L cells at a step posttranslation and prior to the formation of a functional replication complex. The observation that changes in HRV39 2B and 3A proteins lead to viral growth in mouse cells suggests that one or both of these proteins interact with host cell proteins to promote viral replication.

Membrane-association properties of avian encephalomyelitis virus protein 3A

Avian encephalomyelitis virus (AEV) protein 3A is a membrane-interacting protein containing a stretch of 21 hydrophobic amino acid residues. Membrane-association property was assayed using chick embryo brain (CEB) cells transfected with the fusion GFP-3A and its various deletion mutants demonstrate that 3A is integrally interacted with membranes by its hydrophobic domain and further defines that the motif of amino acid residues 45-51, the most C-terminal hydrophobic domain essential for this feature. Expression of 3A in transfected CEB cells results in membrane permeability modifications through association of the third motif with membranes, which can be demonstrated by release of lactate dehydrogenase (LDH) into the medium. Furthermore, the localization of the protein 3A in transfected CEB and Cos-7 cells exhibited an overlapping staining pattern with an endoplasmic reticulum (ER) and involved in the disassembly of the Golgi apparatus under double-staining and confocal microscopic observations, whereas the 3A mutants lacking amino acids 45-51 could not localize to the ER and display an intact Golgi morphology as seen in the mutant devoid of the complete hydrophobic domain after transfection. Taken together, our results demonstrate that the motif (aa 45-51) of the transmembrane domain might be fundamental for the stable interaction of the protein 3A with the ER membrane regardless of the cell types. Although this motif was deleted, the resultant protein did not localize to the ER, which directly results in the loss of the ability to block the ER-to-Golgi transport by 3A protein and hence makes the morphology of the Golgi apparatus return to normal.

Microbial quality of wastewater: detection of hepatitis A virus by reverse transcriptase-polymerase chain reaction

AIMS

The persistent circulation of hepatitis A virus (HAV) in the Mediterranean area suggests the need for monitoring its presence in the environment. A reverse transcriptase-polymerase chain reaction (RT-PCR) was used to detect the presence of HAV in several consecutive raw sewage and final effluent samples, collected over an 8-month period from an activated sludge treatment plant in southern Italy.

METHODS AND RESULTS

Two distinct purification protocols, either based on antigen-capture with monoclonal antibody (AC) or RNA extraction, were compared. The possible influence of the antibody used in the AC phase was evaluated in preliminary experiments on HAV-spiked samples, using two different monoclonal antibodies. Hepatitis A virus RNA was detected in all but one sewage environmental sample examined. The contemporary presence of enteroviruses, reoviruses and phages was observed, while HAV growth in cell culture was hampered.

CONCLUSIONS

The RT-PCR technique was confirmed to be a valuable tool for the rapid monitoring of HAV in sewage samples. In addition, this study demonstrated that application of different sample purification methods can result in different levels of sensitivity of the assay and that, in the antigen-capture method, the choice of antibody can have a crucial role.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work underlines the need for technical uniformity in environmental studies from different laboratories for a correct and useful comparison of the results.

Improving proteolytic cleavage at the 3A/3B site of the hepatitis A virus polyprotein impairs processing and particle formation, and the impairment can be complemented in trans by 3AB and 3ABC

The orchestrated liberation of viral proteins by 3C(pro)-mediated proteolysis is pivotal for gene expression by picornaviruses. Proteolytic processing is regulated either by the amino acid sequence at the cleavage site of the substrate or by cofactors covalently or noncovalently linked to the viral proteinase. To determine the role of the amino acid sequence at cleavage sites 3A/3B and 3B/3C that are essential for the liberation of 3C(pro) from its precursors and to assess the function of the stable processing intermediates 3AB and 3ABC, we studied the effect of cleavage site mutations on hepatitis A virus (HAV) polyprotein processing, particle formation, and replication. Using the recombinant vaccinia virus system, we showed that the normally retarded cleavage at the 3A/3B junction can be improved by altering the amino acid sequence at the scissile bond such that it matches the preferred HAV 3C cleavage sites. In contrast to the processing products of the wild-type polyprotein, 3ABC was no longer detectable in the mutant. VP0 and VP3 were generated less efficiently, implying that processing of the structural protein precursor P1-2A depends on the presence of stable 3ABC and/or 3AB. In addition, cleavage of 2BC was impaired in 3AB/3ABC-deficient mutants. Formation of HAV particles was not affected in mutants with blocked 3A/3B and/or 3B/3C cleavage sites. However, 3ABC-deficient mutants produced small numbers of HAV particles, which could be augmented by coexpressing 3AB or 3ABC. The hydrophobic domain of 3A that has been proposed to mediate membrane anchorage of the replication complex was crucial for restoration of defective particle formation. In vitro transcripts of the various cleavage site mutants were unable to initiate an infectious cycle, and no progeny viruses were obtained even after blind passages. Taken together, the data suggest that accumulation of uncleaved HAV 3AB and/or 3ABC is pivotal for both viral replication and efficient particle formation.

Mapping of protein domains of hepatitis A virus 3AB essential for interaction with 3CD and viral RNA

The small hydrophobic protein 3AB of the picornaviruses, encompassing the replication primer 3B, has been suggested to anchor the viral replication complex to membranes. For hepatitis A virus (HAV) 3AB, we have previously demonstrated its ability to form stable homodimers, to bind to membranes, and to interact specifically with RNA, implicating its multiple involvement in viral replication. In the present report, we show that HAV 3AB additionally interacts with HAV protein 3CD, a feature also described for the corresponding polypeptide of poliovirus. By assessing the interactions of three deletion mutants, distinct domains of HAV 3AB were mapped. The hydrophobic domain and the 3B moiety were found to be essential for the 3AB interaction with 3CD. Both electrostatic and hydrophobic forces are involved in this interaction. The cluster of charged amino acid residues at the C terminus of 3A seems to determine the specificity of 3AB interaction with RNA structures formed at either terminus of the HAV genome. Furthermore, our data implicate that 3A can interact with HAV RNA. Compared with poliovirus 3AB, which by itself is a nonspecific RNA-binding protein, HAV 3AB specifically recognizes HAV RNA structures that might be of relevance for initiation of viral RNA replication.

Polypeptide 3AB of hepatitis A virus is a transmembrane protein

Hepatitis A virus (HAV) protein 3AB is a membrane-interacting protein containing a stretch of 21 hydrophobic amino acid residues. The nature of its membrane association was studied in detail by analysing various deletion mutants. In vivo and in vitro expression of the wild-type protein and its mutants allowed to demonstrate that the hydrophobic domain interacts with membranes and to define the portions essential for this feature. Furthermore, the results suggest that 3AB behaves as an integral membrane protein. Expression in Escherichia coli showed that 3AB can be isolated, in association with membranes, both in monomeric and in dimeric form. This finding was confirmed in vitro after post-translational incubation of the protein with microsomal membranes. Analysis of deletion mutants demonstrated that the dimerization region colocalises with the hydrophobic transmembrane domain, implicating that HAV 3AB could form oligomers mediated by the interaction of transmembrane alpha-helices.