Translation of MMTV Gag requires nuclear events involving splicing motifs in addition to the viral Rem protein and RmRE

• Published in: Retrovirology Vol. 9; no. 1; p. 8
• Main Authors: Boeras, Ioana, Sakalian, Michael, West, John T
• Format: Journal Article
• Language: English
• Published: London BioMed Central 25.01.2012; BioMed Central Ltd; BMC
• Subjects: Animals; Antibodies; Betaretrovirus; Biomedical and Life Sciences; Biomedicine; Breast cancer; Cancer Research; Cell Line; Colleges & universities; Cytoplasm; Deoxyribonucleic acid; DNA; gag; Gene Expression; Gene Expression Regulation, Viral; Gene Products, gag - biosynthesis; Genes; Genetic aspects; Genetic regulation; Genomes; Health sciences; Humans; Infectious Diseases; Mammary Tumor Virus, Mouse - genetics; Mammary Tumor Virus, Mouse - physiology; Messenger RNA; Open reading frames; Physiological aspects; post-transcriptional regulation; Protein Biosynthesis; Protein Structure; Proteins; Rem; RNA Splicing; Vaccine; Viral proteins; Virology; United States; Betaretrovirus; post-transcriptional regulation; Rem
• ISSN: 1742-4690; 1742-4690
• DOI: 10.1186/1742-4690-9-8

Background Retroviral Gag proteins are encoded in introns and, because of this localization, they are subject to the default pathways of pre-mRNA splicing. Retroviruses regulate splicing and translation through a variety of intertwined mechanisms, including 5'- post-transcriptional control elements, 3'- constitutive transport elements, and viral protein RNA interactions that couple unspliced and singly spliced mRNAs to transport machinery. Sequences within the gag gene termed inhibitory or instability sequences also appear to affect viral mRNA stability and translation, and the action of these sequences can be countered by silent mutation or the presence of RNA interaction proteins like HIV-1 Rev. Here, we explored the requirements for mouse mammary tumor virus (MMTV) Gag expression using a combination of in vivo and in vitro expression systems. Results We show that MMTV gag alleles are inhibited for translation despite possessing a functional open reading frame (ORF). The block to expression was post-transcriptional and targeted the mRNA but was not a function of mRNA transport or stability. Using bicistronic reporters, we show that inhibition of gag expression imparted a block to both cap-dependent and cap-independent translation onto the mRNA. Direct introduction of in vitro synthesized gag mRNA resulted in translation, implying a nuclear role in inhibition of expression. The inhibition of expression was overcome by intact proviral expression or by flanking gag with splice sites combined with a functional Rem-Rem response element (RmRE) interaction. Conclusions Expression of MMTV Gag requires nuclear interactions involving the viral Rem protein, its cognate binding target the RmRE, and surprisingly, both a splice donor and acceptor sequence to achieve appropriate signals for translation of the mRNA in the cytoplasm.