Resumen de Requerimiento de factores de iniciación para la traducción de mRNAs de picornavirus
Viruses are intracellular parasites that rely on the components of the host cell for gene expression and replication. Soon after infection, the host cell often tends to limit viral production and replication by shutting-off global translation. Many viral genomes have evolved mechanisms to bypass this general inhibition of translation by developing strategies of initiation independent of the classical recognition of an m7G cap structure at the 5¿ end of the mRNA. These mechanisms imply the utilization of internal ribosome entry sites (IRES) which can promote 5¿ end independent initiation. The IRES were first recognized within the RNA genomes of picornaviruses 20 years ago. Since the initial characterization of picornavirus IRES, other RNA virus have shown to initiate translation internally. Each class of IRES varies in size, structure and requirements for cellular protein to allow them to function.
Poliovirus IRES translation is not blocked when eIF4G is cleaved either when eIF2 is inactivated by phosphorylation at late times of infection. However, RNA is blocked when eIF2 is inactivated at earlier times. Thus, poliovirus RNA translation exhibits a dual mechanism for the initiation of protein synthesis as regards to the requirement for eIF2. Analysis of individual poliovirus non-structural proteins indicates that the presence of 2Apro alone is sufficient to provide eIF2 independence for IRES-driven translation. This effect is not observed with a 2Apro variant unable to cleave eIF4G. The level of 2Apro synthesized in culture cells is crucial for obtaining eIF2 independence. Expression of the N-or C-terminus fragments of eIF4G did not stimulate IRES-driven translation, nor provide eIF2 independence, consistent with the idea that the presence of 2Apro at high concentrations is necessary. The finding that 2Apro provides eIF2-independent translation opens a new and unsuspected area of research in the field of picornavirus protein synthesis.
The hepatitis A virus (HAV) IRES is thought that requires intact eIF4F complex for translation. In line with previous results we report that poliovirus (PV) 2Apro strongly blocks protein synthesis directed by HAV IRES. However, in contrast to previous findings we now demonstrate that eIF4G cleavage by foot-and-mouth disease virus (FMDV) Lpro strongly stimulates HAV IRES-driven translation. Thus, this is the first observation that 2Apro and Lpro exhibit opposite effects to what was previously thought to be the case in HAV IRES. Notably, in presence of this FMDV protease, translation directed by HAV IRES takes place when eIF2¿ has been inactivated by phosphorylation. Our present findings clearly demonstrate that protein synthesis directed by HAV IRES can occur when eIF4G has been cleaved and after inactivation of eIF2. Therefore, factorless translation directed by HAV IRES is similar to that observed with other picornavirus IRESs.
