Poster abstracts

Poster number 155 submitted by Suada Leskaj

Visualizing translation initiation on a very short 5′ UTR

Suada Leskaj (Department of Biological Chemistry, University of Michigan), Ritam Neupane (Department of Biological Chemistry, University of Michigan), Andrew Shurer (Department of Biological Chemistry), Rachel O. Niederer (Department of Biological Chemistry, University of Michigan), Jailson Brito Querido (Department of Biological Chemistry, Life Sciences Institute, University of Michigan)

Abstract:
Human Papillomaviruses (HPVs) are causative agents of nearly all cervical cancers and are implicated in various other human malignancies. It was shown that a high-risk strain of HPV-18 uses an mRNA with an extremely short, 3-nucleotide ACC 5′-UTR to produce the oncogenic protein E6. Such a short UTR challenges the canonical eIF4F-dependent model of translation initiation, which predicts a minimal distance between the cap and the start site. The precise mechanism by which ribosomes are recruited and initiate translation on such minimal 5′-UTRs remains unknown. To investigate translation initiation on a 3-nucleotide 5′-UTR, we purified translation initiation complexes from HeLa cell extracts supplemented with an in vitro transcribed HPV-18 E6 mRNA and determined the structure by cryo-electron microscopy at 2.7 Å resolution. We captured a late-stage initiation complex in which the initiator tRNA is directly base-paired with the AUG start codon. We observe density 5′ of the start codon that could accommodate the ACC 5′-UTR as well as possible density for the 5′ m7G cap structure. In our structure, the 5′ end of the mRNA interacts with eIF2α in a manner that resembles the interaction between eIF2α and the −3 nucleotide of the Kozak sequence observed during canonical start codon recognition. This interaction provides a potential mechanistic explanation for how the start site is selected on the HPV-18 E6 mRNA. To further characterize the mechanism of initiation on HPV-18 E6 unusual short UTR, we are also extending these studies to other UTR isoforms. These structural insights will advance our understanding of non-canonical translation initiation mechanisms and how they are exploited by viruses.

References:
García, A. et al. High-risk human papillomavirus-18 uses an mrna sequence to synthesize oncoprotein E6 in tumors. Proceedings of the National Academy of Sciences 118, (2021).

Keywords: translation initiation , viral RNA, cryo-EM