Chapter 5 : Translation and Host Cell Shutoff

Diagram of computer-predicted secondary structure of the 5′ NCR of poliovirus RNA. This structure has been confirmed, in part, by RNase and chemical probing experiments. Numbers refer to the poliovirus type 1 RNA genome. The seven stem-loop structures are labeled A through G.An alternative nomenclature employing roman numerals for the stem-loop structures is indicated in parentheses ( 36 ). In addition, Andino et al. ( 2 ) suggested that stem-loops A and B form a cloverleaf structure. This structure would compose the I stem-loop region shown in the figure. The asterisks indicate AUGs conserved among the enteroviruses and rhinoviruses. The conserved polypyrimidine tract (UUUCCUU) is located at nt 558 to 577; the conserved AUG at position 586 is indicated. The AUG at nt 743 represent the authentic translation start codon. The figure is modified from that of Dildine and Semler ( 18 ).

Diagram of bicistronic mRNAs directing cap-independent translation initiation in uninfected and poliovirus-infected cells. The bicistronic RNA consists of a capped mRNA encoding TK that is fused to the poliovirus 5′ NCR linked to the second cistron encoding CAT. In uninfected cells, the capped mRNA encoding TK is translated in a cap-dependent manner. Translation of the second cistron encoding CAT could occur in two ways: by ribosomal readthrough and reinitiation of ribosomes that bound at the 5′ end of the bicistronic mRNA or by internal ribosome entry in the 5′ NCR of poliovirus. In poliovirus-infected cells, cap-dependent translation initiation is inhibited by the proteolytic processing of translation initiation factor eIF-4F. Thus, no TK protein is observed. However, the CAT mRNA is still translated, presumably by ribosomes binding internally at the IRES element within the 5′ NCR of poliovirus RNA. Reprinted with permission from Nature ( 85 ). © 1988 Macmillan Magazines Limited.

Diagram of conserved sequence elements in the IRES of enteroviral RNAs. The conserved sequence elements, i.e., the polypyrimidine tract and the upstream AUG codon (at nt 586 in poliovirus type 1), are boxed. The AUG codon is present in stem-loop G of the 5′ NCR of enterovirus RNA. The nucleotide sequences of the AUG codon are predicted to be base paired in a stem structure. A distance of 20 nt between the pyrimidine-rich region and the AUG at nt 586 is required for efficient translation initiation by internal ribosome entry on viral RNAs. Stem-loop G sequences are followed by a region of viral RNA that is not well conserved among the enteroviruses (the variable region). The authentic translation start codon is located at nt 743 and is thus ∼150 nt downstream of the polypyrimidine tract. This AUG codon may be located by ribosome scanning of the viral RNA.

Kinetics of protein synthesis in poliovirus-infected cells compared to the rate of protein synthesis in uninfected cells. By approximately 2 h postinfection, cap-dependent cellular protein synthesis has ceased in infected cells. By 3.5 to 4 h postinfection, cap-independent virus-specific protein synthesis occurs at high levels. In uninfected cells, continuous cellular protein synthesis is observed. (Adapted from reference 22 with permission.)

Models of shutoff of host cell protein synthesis for poliovirus and coxsackievirus. In poliovirus-infected cells, a latent cellular protease is activated by the viral 2Apro, perhaps by proteolysis. The cellular protease carries out the cleavage of p220, a component of eIF-4F, in the presence of eIF-3, resulting in inactive eIF-4F and an inhibition of cap-dependent translation initiation. In contrast, the coxsackievirus 2Apro can carry out p220 cleavage itself without a requirement for eIF-3.