Poxvirus-encoded decapping enzymes promote selective translation of viral mRNAs.

Fernando Cantu,Shuai Cao,Candy Hernandez,Zhilong Yang,Joshua Spradlin,Pragyesh Dhungel,Stefan Rothenburg

doi:10.1371/journal.ppat.1008926

Abstract

Cellular decapping enzymes negatively regulate gene expression by removing the methylguanosine cap at the 5’ end of eukaryotic mRNA, rendering mRNA susceptible to degradation and repressing mRNA translation. Vaccinia virus (VACV), the prototype poxvirus, encodes two decapping enzymes, D9 and D10, that induce the degradation of both cellular and viral mRNAs. Using a genome-wide survey of translation efficiency, we analyzed vaccinia virus mRNAs in cells infected with wild type VACV and mutant VACVs with inactivated decapping enzymes. We found that VACV decapping enzymes are required for selective translation of viral post-replicative mRNAs (transcribed after viral DNA replication) independent of PKR- and RNase L-mediated translation repression. Further molecular characterization demonstrated that VACV decapping enzymes are necessary for efficient translation of mRNA with a 5'-poly(A) leader, which are present in all viral post-replicative mRNAs. Inactivation of D10 alone in VACV significantly impairs poly(A)-leader-mediated translation. Remarkably, D10 stimulates mRNA translation in the absence of VACV infection with a preference for RNA containing a 5’-poly(A) leader. We further revealed that VACV decapping enzymes are needed for 5’-poly(A) leader-mediated cap-independent translation enhancement during infection. Our findings identified a mechanism by which VACV mRNAs are selectively translated through subverting viral decapping enzymes to stimulate 5’-poly(A) leader-mediated translation.

Highlights

The 50-methylguanosine (m7G) cap at the 5’ end of eukaryotic mRNA protects RNA from exonuclease digestion and facilitates recruiting translation machinery [1]
Previous studies indicated that decapping enzymes are negative gene expression regulators by accelerating mRNA degradation and repressing translation
In this study we found that vaccinia virus (VACV) encoded-decapping enzymes, D9 and D10, are required to promote selective synthesis of viral proteins, they are known to promote both cellular and viral mRNA degradation

Summary

Introduction

The 50-methylguanosine (m7G) cap at the 5’ end of eukaryotic mRNA protects RNA from exonuclease digestion and facilitates recruiting translation machinery [1]. Decapping enzymes belong to the Nudix hydrolases superfamily of proteins containing a conserved Nudix motif that is critical for removing the mRNA 5’-cap by hydrolysis of the phosphodiester bond, resulting in a 50-monophosphate RNA and a 7-methyl guanosine diphosphate [2]. Decapping enzymes can reduce protein production by inducing mRNA degradation and interfering with mRNA translation processes. In addition to eliminating mRNA, the translation template, decapping enzymes directly impede the translation process. Processing bodies (PBs) are distinct cytoplasmic foci of decapping machinery containing Dcp, mRNAs, and other enzymes, in which mRNA translation is suppressed [9,10,11,12,13]. One likely mechanism is to slow the ribosome movement [17,18,19]

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Conclusion