Translation-targeted therapeutics for viral diseases

Abstract

Viruses utilize the protein synthetic machinery of their host. Nonetheless, certain features of the synthesis of viral proteins are distinct from those of host-cell translation. Examples include internal ribosome entry sites in some viral mRNAs and ribosomal frameshifting during production of retroviral proteins. Viruses often inhibit host translation and/or possess mechanisms leading to preferential synthesis of viral proteins. In addition, a participant in the cellular antiviral response is the enzyme PKR (protein kinase, RNA activated), which is involved in the control of cellular translation. Thus, viruses and host cells wage war on the battlefield of translation. The distinctive features of protein synthesis in virally infected cells provide potential targets for therapeutic intervention. Translation-targeted therapeutics have precedence in antibiotics like tetracycline and erythromycin. Means for discovery of translation-targeted therapeutics for viral disease are discussed.

FIG. 2

The translation of an mRNA. The “anatomy” of a typical mRNA molecule in higher eukaryotes is shown in the upper portion of the figure. The mature mRNA molecule can be envisioned as consisting of three divisions. Central to the structure and function of the molecule is the open reading frame that codes for protein. The open reading frame is defined by a start codon and an in-frame stop codon. On either side of the protein coding region are the untranslated regions (UTRs). The 5′UTR is modified at its 5′ end by the post- or cotranscriptional addition of a 7-methylguanosine cap that is linked to the remainder of the transcript in 5′-5′ triphosphate linkage. The 3 ′UTR is modified at its 3′ end by the posttranscriptional addition of a poly(A) tail. Translation is divided into the steps of Initiation, Elongation, and Termination as indicated. Initiation involves the binding of 40S ribosomal subunit to the cap, scanning to the start codon, and 60S ribosomal subunit joining. Elongation is a cyclical process by which the triplet nucleotide codons of the mRNA are decoded as aminoacyl tRNAs deliver the appropriate amino acid for the growth of the nascent polypeptide chain. Termination occurs when the ribosome reaches the stop codon where the completed protein and the components of the translation machinery are released. Soluble factors that have been identified as participants in each of the steps of translation factors listed are themselves comprised of multiple polypeptide subunits.

FIG. 3

The scanning model for translation initiation. The complex consisting of a 40S ribosome, met-tRNAi, and initiation factors interacts with the mRNA cap (step 1). The complex then scans in a 5′ to 3′ direction (step 2) until the start codon is encountered (step 3). The 60S ribosome joins at the AUG (step 4) and elongation proceeds (step 5). The nascent polypeptide is shown after the nth codon has directed the addition of its amino acid (aa_n).

FIG. 4

The initiation of translation via a viral IRES element. Certain picornaviruses (e.g., poliovirus, rhinovirus) encode a protease that inactivates the cap binding translation factor eIF-4F resulting in an inhibition of cellular, cap-dependent translation initiation. The 5′UTR of the mRNAs of these viruses contain an Internal Ribosome Entry Site (IRES) that allows translation initiation in the absence of functional eIF-4F.

FIG. 5

The PKR pathway. Double-stranded RNA (dsRNA) resulting from a viral infection triggers interferon production. Interferon is a transciptional inducer of a number of genes including the protein kinase, RNA activated (PKR). The viral dsRNA also activates PKR through its two RNA binding motifs. Active PKR catalyzes phosphorylation of eIF-2 at serine 51 of its α subunit. The phosphorylated eIF-2 forms a complex with eIF-2B, the guanine nucleotide exchange factor, and the trapped eIF-2B is inactive. Failure to exchange bound GDP for GTP prevents eIF-2 from participating in translation initiation, resulting in a global inhibition of protein synthesis. If this pathway is effective, the infected host cell would perish, but viral replication and spread to other host cells would be aborted. Various viruses possess countermeasures to prevent either PKR activation or the phosphorylation of eIF-2.

FIG. 6

The frameshifting sequence of HIV-1. The gag and pol gene products of HIV-1 are encoded by a single mRNA. The open reading frames (ORFs) of the gag protein and the pol proteins overlap by approximately 200 nucleotides. Within the region of overlap is contained a frame-shifting sequence (HIV FS) consisting of “slippery sequence” (six consecutive U residues) and a moderately stable stem-loop structure. Frameshifting from the gag ORF to the pol ORF (a −1 frameshift) is thought to occur when the elongating ribosome is paused over the slippery sequence as a consequence of being impeded by the RNA secondary structure. Frameshifting in HIV-1 occurs at a frequency of 5–10% and is the only means for producing the pol proteins required for viral replication.