The mouse mammary tumor virus (MMTV) Pr77Gag polypeptide is an essential retroviral structural protein without which infectious viral particles cannot be formed. This process requires specific recognition and packaging of dimerized genomic RNA (gRNA) by Gag during virus assembly. Most of the previous work on retroviral assembly has used either the nucleocapsid portion of Gag, or other truncated Gag derivatives—not the natural substrate for virus assembly. In order to understand the molecular mechanism of MMTV gRNA packaging process, we expressed and purified full-length recombinant Pr77Gag-His6-tag fusion protein from soluble fractions of bacterial cultures. We show that the purified Pr77Gag-His6-tag protein retained the ability to assemble virus-like particles (VLPs) in vitro with morphologically similar immature intracellular particles. The recombinant proteins (with and without His6-tag) could both be expressed in prokaryotic and eukaryotic cells and had the ability to form VLPs in vivo. Most importantly, the recombinant Pr77Gag-His6-tag fusion proteins capable of making VLPs in eukaryotic cells were competent for packaging sub-genomic MMTV RNAs. The successful expression and purification of a biologically active, full-length MMTV Pr77Gag should lay down the foundation towards performing RNA–protein interaction(s), especially for structure-function studies and towards understanding molecular intricacies during MMTV gRNA packaging and assembly processes.

MPMV precursor polypeptide Pr78Gag orchestrates assembly and packaging of genomic RNA (gRNA) into virus particles. Therefore, we have expressed recombinant full-length Pr78Gag either with or without His6-tag in bacterial as well as eukaryotic cultures and purified the recombinant protein from soluble fractions of the bacterial cultures. The recombinant Pr78Gag protein has the intrinsic ability to assemble in vitro to form virus like particles (VLPs). Consistent with this observation, the recombinant protein could form VLPs in both prokaryotes and eukaryotes. VLPs formed in eukaryotic cells by recombinant Pr78Gag with or without His6-tag can encapsidate MPMV transfer vector RNA, suggesting that the inclusion of the His6-tag to the full-length Pr78Gag did not interfere with its expression or biological function. This study demonstrates the expression and purification of a biologically active, recombinant Pr78Gag, which should pave the way to study RNA-protein interactions involved in the MPMV gRNA packaging process.

Retroviral RNA genome (gRNA) harbors cis-acting sequences that facilitate its specific packaging from a pool of other viral and cellular RNAs by binding with high-affinity to the viral Gag protein during virus assembly. However, the molecular intricacies involved during selective gRNA packaging are poorly understood. Binding and footprinting assays on mouse mammary tumor virus (MMTV) gRNA with purified Pr77Gag along with in cell gRNA packaging study identified two Pr77Gag binding sites constituting critical, non-redundant packaging signals. These included: a purine loop in a bifurcated stem-loop containing the gRNA dimerization initiation site, and the primer binding site (PBS). Despite these sites being present on both unspliced and spliced RNAs, Pr77Gag specifically bound to unspliced RNA, since only that could adopt the native bifurcated stem–loop structure containing looped purines. These results map minimum structural elements required to initiate MMTV gRNA packaging, distinguishing features that are conserved amongst divergent retroviruses from those perhaps unique to MMTV. Unlike purine-rich motifs frequently associated with packaging signals, direct involvement of PBS in gRNA packaging has not been documented in retroviruses. These results enhance our understanding of retroviral gRNA packaging/assembly, making it not only a target for novel therapeutic interventions, but also development of safer gene therapy vectors.

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ObjectivesThe prevalence of hMPV infections in Sri Lanka has not been reported and here we report a case series of hMPV infection in children less than 5 years. Patients with ARTI were included from Teaching Hospital, Anuradhapura from March 2013 to August 2014. Indirect fluorescence assay was performed on nasopharyngeal aspirates for the identification of respiratory viruses [respiratory syncytial virus (RSV), parainfluenza virus 1, 2 and 3, influenza A and B and hMPV]. Moreover, reverse transcriptase-polymerase chain reaction was done to further confirm the hMPV infection.ResultsIn this case series, hMPV infection showed a range of respiratory symptoms from common cold to life threatening lower respiratory tract infections with varying severity. In some cases, the clinical presentation of hMPV infection was similar to the ARTI caused by RSV. hMPV co-infections with of RSV have also been seen in some cases of ARTI. A child delivered through cesarean section and birth order > 3 has an Odds ratio of 3.5 and 4.3 (95% CI) for developing co-infection with RSV compared to hMPV mono-infections. Lack of diagnostic facilities to identify the viral aetiology has contributed to the use of antibiotics indicating the need for establishing viral diagnostic facilities in the country.Electronic supplementary materialThe online version of this article (10.1186/s13104-018-3239-3) contains supplementary material, which is available to authorized users.