NAR: Huang Chuan's team at Chongqing University revealed key molecular mechanisms of circular RNA translation inhibition

Circular RNA (circRNA) is a class of single-stranded RNA molecules
with a covalently closed circular structure.
Except for a small number of circular RNAs derived from introns, non-coding genes, or non-coding regions between protein-coding genes, the vast majority of circular RNAs are back-splicing products
of exons of protein-coding genes.
Although circular RNA is mainly localized to the cytoplasm, it lacks a classical translation initiation signal, so circular RNA has long been considered a class of non-coding RNA
.
Until recent years, some studies and multi-omics evidence have shown that at least a portion of endogenous circular RNA in fruit flies and mammalian cells binds directly to polyribosomes and can serve as translation templates for proteins, and exert important regulatory functions
in cells through expressed proteins.
Due to the structural specificity of circular RNA, its translation process may be regulated differently from linear mRNA, and cells may have some regulatory method that helps general-purpose translation machines distinguish between linear RNA and circular RNA translation templates
。 However, most of the current research in the field of circular RNA translation focuses on the physiological and pathological functions of circular RNA-coding proteins, but the regulatory mechanism of circular RNA translation process is not deeply explored, especially the types of eukaryotic translation initiation factors (eIFs) involved in regulating circular RNA translation and their specific molecular mechanisms are not clear
。 Recently, the Huang Chuan team of the School of Life Sciences of Chongqing University published a research paper
entitled: eIF3j inhibits translation of a subset of circular RNAs in eukaryotic cells in the journal Nucleic Acids Research.
This study revealed the molecular mechanism and physiological significance
of eIF3j protein-specific inhibition of circular RNA translation initiation.
In this study, a circular RNA circSfl-derived protein reporter system was established, and then all 43 flies translation initiation factor eIFs were screened by RNAi interference technology, and the functions of
various eIFs in the process of circular RNA translation initiation were systematically explored 。 The study found that different members of the eIF3 complex have opposite regulatory functions for circular RNA translation: eIF3 core complexes (such as eIF3a, 3b, 3c, 3d1, etc.
) are required for circular RNA translation, while eIF3j and others are negative regulators of circular RNA translation
.
In addition, the eIF3j protein did not significantly inhibit the translation ability of linear Sfl mRNA, which proved the specificity
of eIF3j protein on the regulation of circular RNA translation.
A series of molecular mechanism experiments showed that the eIF3j protein directly bound to the protein-coding circular RNA through its C-terminal domain, which hindered the recruitment of the eIF3 core complex to the UTR region of the circular RNA, preventing the assembly of the translation initiation complex PIC and the correct translation initiation
of circular RNA 。 Further, the research team used circSfl stable transfection cell lines to simulate circSfl high-expression cells (Drosophila nerve tissue), and found that eIF3j protein inhibits the expression of circular RNA translator CdSfl (circSfl-derived Sfl) in a heat-stressed environment, allowing heat-excited cells to enter the death process
normally.
This regulatory behavior may be a response mechanism to clear damaged cells and ensure the vital activity
of normal cells.
To continue exploring the broad spectrum of eIF3j protein inhibition of circular RNA translation initiation, the authors analyzed all endogenous ribo-circRNAs identified in S2 cells by ribosome profiling, and found that the eIF3j protein can hinder the binding of eIF3 core complexes to a subset of ribo-circRNAs (such as circPde8), thereby inhibiting their translational activity
.
This proves that the eIF3j protein-mediated circular RNA translation inhibition mechanism is an important and extensive molecular mechanism
for protein expression regulation.
This study1) systematically evaluated the regulatory effects of all 43 eIFs on circular RNA translation initiation; 2) elucidating the specific molecular mechanism by which eIF3j protein inhibits circular RNA translation initiation by preventing eIF3 core complex binding proteins from encoding circular RNA; 3) The cellular physiological significance
of the eIF3j-mediated circular RNA translation inhibitory pathway was revealed.
Overall, this study contributes to a deeper understanding of the diversity of ways protein translation is regulated and the way general transcription machines distinguish between linear and circular RNA translation templates, while also providing updated insights
into circular RNA biology.
The study was completed by the team of Professor Huang Chuan from the School of Life Sciences of Chongqing University and strongly supported
by Professor Shan Ge of the University of Science and Technology of China.
The co-first authors of the paper are Song Zhenxing and Lin Jiamei, doctoral students from the School of Life Sciences, Chongqing University, and the corresponding authors of the paper are Professor
Huang Chuan.