Dual Mechanisms of Translation Initiation of the Full-Length HIV-1 mRNA Contribute to Gag Synthesis

Abstract

The precursor group-specific antigen (pr55Gag) is central to HIV-1 assembly. Its expression alone is sufficient to assemble into virus-like particles. It also selects the genomic RNA for encapsidation and is involved in several important virus-host interactions for viral assembly and restriction, making its synthesis essential for aspects of viral replication. Here, we show that the initiation of translation of the HIV-1 genomic RNA is mediated through both a cap-dependent and an internal ribosome entry site (IRES)-mediated mechanisms. In support of this notion, pr55Gag synthesis was maintained at 70% when cap-dependent translation initiation was blocked by the expression of eIF4G- and PABP targeting viral proteases in two in vitro systems and in HIV-1-expressing cells directly infected with poliovirus. While our data reveal that IRES-dependent translation of the viral genomic RNA ensures pr55Gag expression, the synthesis of other HIV-1 proteins, including that of pr160Gag/Pol, Vpr and Tat is suppressed early during progressive poliovirus infection. The data presented herein implies that the unspliced HIV-1 genomic RNA utilizes both cap-dependent and IRES-dependent translation initiation to supply pr55Gag for virus assembly and production.

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

“…Therefore, our data do not support the conclusion of others suggesting that translation initiation from the full-length HTLV-1 mRNA is exclusively cap dependent (22), nor do they indicate that translation initiation from the full-length HTLV-1 mRNA is exclusively IRES dependent. Several studies have shown that primate lentiviruses exhibit a redundancy in the mechanism of initiation of protein synthesis (reviewed in reference 8), suggesting that these mRNAs can initiate translation using both a cap-dependent and an IRES-dependent mechanism (18,19). Based on the evidence and on the apparent conservation of functions among the Retroviridae (8), we expect the HTLV-1 full-length mRNA to also use dual cap-and IRES-mediated mechanisms of translation initiation.…”

“…The full-length mRNA of some retroviruses can initiate protein synthesis through both a cap-dependent and a cap-independent mechanism (8,18,19). At present, an IRES element is defined solely by performing a functional assay and cannot be predicted by the presence of characteristic RNA sequences or structural motifs (7,38).…”

“…More recently, the presence of an IRES within the 5=UTR of the HIV-1 mRNA has been reconfirmed (14)(15)(16)(17). Taken together, these reports suggest that both mechanisms (cap-and IRESdriven) are used by the HIV-1 mRNA to drive the synthesis of the viral Gag protein (8,9,18,19).…”

Translation initiation of the HIV-1 mRNA

“…30,31 In sharp contrast to picornavirus mRNAs, retroviral mRNAs possess a 5 0 cap structure and can initiate translation either by a canonical cap-dependent mechanism or via an IRES. 32,33 RNA Structures Modulate Protein Synthesis of the Full-Length HIV-1 mRNA…”

“…132,133 In the case of the HIV-1, the 5 0 UTR has been shown to drive cap-dependent translation initiation under normal physiological conditions 33,51 and to mediate IRES-dependent protein synthesis during stress conditions known to hinder cap-dependent translation initiation. 32,86,88,91,118 Interestingly, in the course of viral replication, HIV-1 has been shown to change cellular homeostasis altering several components of the host translational apparatus which results in the inhibition of cap-dependent translation. For instance, HIV-1 induces an endogenous cellular increase of reactive oxygen intermediates leading to an oxidative stress that has been demonstrated to facilitate NF-kappa B-dependent activation of HIV transcription and to stimulate the HIV-1 5 0 UTR IRES activity.…”