Schmeing, M. | bioRxiv Science

bioRxiv2023

Repression of mRNA translation initiation by GIGYF1 via blocking the eIF3-eIF4G1 interaction

Viruses commonly interfere with the function of the eukaryotic translation initiation factor 4G1 (eIF4G1), a pivotal factor in the recruitment of the eIF3 complex and ribosome to the mRNA. This results in the inhibition of general host protein synthesis and redirecting ribosomes toward viral mRNAs. Certain viruses also selectively repress the translation of mRNAs involved in the host antiviral response. GIGYF2 and its interacting cap-binding protein 4EHP enable the transcript-specific repression of mRNA translation mediated by microRNAs and RNA-binding proteins (RBPs). RNA viruses, such as SARS-CoV-2, exploit the GIGYF2/4EHP complex to selectively repress the translation of transcripts such as Ifnb1 mRNA, which encodes the antiviral cytokine Interferon {beta} (IFN-{beta}). Herein, we reveal that GIGYF1, a paralogue of GIGYF2, robustly represses cellular mRNA translation through a distinct mechanism independent of 4EHP. Upon recruitment to a target mRNA by RBPs, the C-terminal region of GIGYF1 binds to subunits of eIF3 at the interaction interface of eIF3-eIF4G1. This disrupts binding of eIF3 to eIF4G1, resulting in mRNA-specific translational repression. This mechanism exerts profound influences on the host cells response to viral infection. Depletion of GIGYF1 induces a robust immune response by derepressing Ifnb1 mRNA translation. Overall, our study highlights a unique mechanism of translational regulation by GIGYF1 that involves sequestering eIF3 and abrogating its binding to eIF4G1. This mechanism can be utilized by RBPs that interact with GIGYF1 to specifically repress the translation of their target mRNAs, significantly affecting critical biological processes, including host-pathogen interactions.

cell biology↗

bioRxiv2023

Structure and function of a hexameric cyanophycin synthetase 2

Cyanophycin is a natural polymer composed of a poly-aspartate backbone with arginine attached to each of the aspartate sidechains. Produced by a wide range of bacteria, which mainly use it as a store of fixed nitrogen, it has many promising industrial applications. Cyanophycin can be synthesized from the amino acids Asp and Arg by the widespread cyanophycin synthetase 1 (CphA1), or from the dipeptide {beta}-Asp-Arg by the cyanobacterial enzyme cyanophycin synthetase 2 (CphA2). CphA2 enzymes display a range of oligomeric states, from dimers to dodecamers. Recently, the crystal structure of a CphA2 dimer was solved but could not be obtained in complex with substrate. Here, we report cryo-EM structures of the hexameric CphA2 from Stanieria sp. at ~2.8 [A] resolution, both with and without ATP and cyanophycin. The structures show a trimer-of-dimers hexameric architecture, and substrate-binding interactions that are similar to those of CphA1. Mutagenesis experiments demonstrate the importance of several conserved substrate-binding residues. We also find that a Q416A/R528G double mutation prevents hexamer formation and use this double mutant to show that hexamerization augments the rate of cyanophycin synthesis. Together, these results increase our mechanistic understanding of how an interesting green polymer is biosynthesized.

biochemistry↗