Prof. Ning Gao’ group, from the School of Life Science, Tsinghua University, in collaboration with Prof. Hyouta Himeno’ group from Hirosaki University, Japan, have now revealed how bacteria make use of a small protein called ArfA to rescue ribosomes stalled on defective messager RNAs. Their paper, “Mechanistic insights into the alternative translation termination by ArfA and RF2”, was published online on Nature on December 1st, 2016.
The ribosome is a gigantic molecular machine responsible for protein synthesis according to the genetic information carried on messager RNAs (mRNA). For each mRNA, there is a tri-nucleotide code in its 3’-end called “stop-codon”, which signals the ribosome to terminate the translation. When a truncated mRNA lacking a stop codon (non-stop mRNA) is translated, the ribosome would be stalled at the 3’end of mRNA and translation termination could not occur. Accumulation of stalled ribosomes in bacteria is a matter of life or death. As a response, bacteria have evolved multiple systems to rescue stalled ribosomes. ArfA is a newly identified ribosome rescuing factor.
With the state-of-art cryo-EM technique, Gao’s group has elucidated a high-resolution structure (3Å) of the 70S ribosome in complex with ArfA, non-stop mRNA and canonical translation termination factor RF2. Their structure reveals two essential roles for ArfA in this rescuing process: (1) The C-terminal loop of ArfA senses the length of mRNA, to ensure that it only works on stalled ribosomes with 3’-end truncated mRNAs; (2) The N-terminus of ArfA interacts with RF2 to compensate the lack of stop-codon and stimulate RF2’s function in translation termination.
Figure. Binding site of ArfA on the 70S ribosome.
First-authors of the paper are Ph.D. student Chengying Ma from Gao’ lab and Dr. Daisuke Kurita from Himeno’ lab. Profs. Gao and Himeno are the corresponding authors. Data collection and computation were supported by the National Facility for Protein Sciences (Beijing). Part of the computation was performed on the Computing Platform of the Center for Life Science, Peking University. This work was supported research funds from the Beijing Advanced Innovation Center for Structural Biology, the Ministry of Science and Technology of China, the National Natural Science Foundation of China, and Japan Society for the Promotion of Science.