The 5′ Untranslated Region of the Human T-Cell Lymphotropic Virus Type 1 mRNA Enables Cap-Independent Translation Initiation

Abstract

The human T-cell leukemia virus type 1 (HTLV-1) is a complex human retrovirus that causes adult T cell leukemia and of HTLVassociated myelopathy/tropical spastic paraparesis. The mRNA of some complex retroviruses, including the human and simian immunodeficiency viruses (HIV and SIV), can initiate translation using a canonical cap-dependent mechanism or through an internal ribosome entry site (IRES). In this study, we present strong evidence showing that like HIV-1 and SIV, the 5=-untranslated region (5=UTR) of the HTLV-1 full-length mRNA harbors an IRES. Cap-independent translational activity was evaluated and demonstrated using dual luciferase bicistronic mRNAs in rabbit reticulocyte lysate, in mammalian cell culture, and in Xenopus laevis oocytes. Characterization of the HTLV-1 IRES shows that its activity is dependent on the ribosomal protein S25 (RPS25) and that its function is highly sensitive to the drug edeine. Together, these findings suggest that the 5=UTR of the HTLV-1 full-length mRNA enables internal recruitment of the eukaryotic translation initiation complex. However, the recognition of the initiation codon requires ribosome scanning. These results suggest that, after internal recruitment by the HTLV-1 IRES, a scanning step takes place for the 40S ribosomal subunit to be positioned at the translation initiation codon. IMPORTANCEThe mechanism by which retroviral mRNAs recruit the 40S ribosomal subunit internally is not understood. This study provides new insights into the mechanism of translation initiation used by the human T-cell lymphotropic virus type 1 (HTLV-1). The results show that the HTLV-1 mRNA can initiate translation via a noncanonical mechanism mediated by an internal ribosome entry site (IRES). This study also provides evidence showing the involvement of cellular proteins in HTLV-1 IRES-mediated translation initiation. Together, the data presented in this report significantly contribute to the understanding of HTLV-1 gene expression.

Structural domains within the HIV‐1 mRNA and the ribosomal protein S25 influence cap‐independent translation initiation

“…In agreement with our previous observations (Fig. A–C and ), the presence of edeine negatively impacted cap‐dependent (RLuc) translation (Fig. D, left panel).…”

“…These results are consistent with the ability of the HCV IRES to directly recruit the 40S ribosomal subunit to the initiation codon without scanning . Inhibition of the HCV IRES activity at 0.5 μ m of edeine has been previously described , and likely reflects inhibition of elongation, which is observed at high concentrations of the drug .…”

“…A), containing the defective EMCV IRES inserted upstream of the FLuc reporter, was used as negative control for the IRES activity . The positive controls for IRES activity were the dl hepatitis C virus (HCV) IRES and dl human T‐cell lymphotropic virus type 1 (HTLV‐1) IRES reporters, which harbored the HCV (genotype 1b) and HTLV‐1 IRESs, respectively (Fig. A).…”

“…The bicistronic reporters are expressed from the SV 40 promoter and receive a poly(A) tail by the SV 40 poly(A) signal. The dl ∆ EMCV , dl HIV ‐1 IRES , dl VAR 2 IRES , dl HTLV ‐1 IRES , or the dl HCV IRES (200 ng) vectors were cotransfected into HeLa (B) or COS ‐7 (C) cells with the pc DNA 3.1 lacZ plasmid (50 ng) that expresses β‐Galactosidase from a cap‐dependent transcript . Co‐transfection of β‐Galactosidase serves as an internal control for transfection efficiency.…”

RPLP1 and RPLP2 Are Essential Flavivirus Host Factors That Promote Early Viral Protein Accumulation

“…DENV-2 infection without affecting global cellular translation or cell viability (52). RPS25 appears to be broadly required for IRES-mediated translation initiation of HCV, human T-cell leukemia virus 1, Cricket paralysis virus, and poliovirus, as well as ribosome shunting in adenovirus infection (21,53,54). For HCV, RPs of the small subunit were found to be disproportionately required for translation and virus replication compared to RPs of the large subunit (55), suggesting that the rate-limiting step of HCV translation is at the stage of 40S subunit recruitment.…”