The ribosome is a large RNA-protein molecular machine that synthesizes protein in all organisms on earth. In eukaryotes, assembly of ribosome is a highly complicated process. During its maturation, the ribosome transiently associates with numerous assembly factors, forming a series of pre-ribosomal particles. The small ribosomal subunit forms two major assembly intermediates: the early 90S and the late pre-40S pre-ribosomes. The 90S pre-ribosome is a gigantic complex that contains ribosomal components and additionally ~50 non-ribosomal proteins and U3 snoRNA and has a molecular mass of 5 megadaltons.
The YE Keqiong group at the Institute of Biophysics, Chinese Academy of Sciences (CAS), has determined three electron density maps of Saccharomyces cerevisiae 90S pre-ribosome by cryo-electron microscopy (cryo-EM) and single-particle reconstruction approach. The best map has an overall resolution of 4.5 Å. The researchers have constructed a nearly complete model of 90S based on available crystal structures of component RNAs and proteins, de novo model building and chemical crosslinking and mass spectrometry data.
The structure of 90S shows that the nascent ribosome is assembled into isolated native-like substructures that are stabilized by abundant assembly factors. The 5' external transcribed spacer of pre-rRNA, U3 snoRNA and many assembly factors form a large base that scaffolds the nascent ribosome. The structure also reveals a novel interaction between U3 snoRNA and pre-rRNA. The 90S model provides significant insight into the principle of small subunit assembly and the function of assembly factors.
The paper entitled "Molecular architecture of the 90S small subunit pre-ribosome" was published online in eLife on February 28, 2017 (http://dx.doi.org/10.7554/eLife.22086). Prof. YE Keqiong is the corresponding author and SUN Qi, Dr. ZHU Xing, QI Jia and AN Weidong are the co-first authors. The collaborators Drs. DONG Meng-Qiu and TAN Dan and other researchers from the YE group also made important contributions to the work. The Center for Biological Imaging of CAS provided key facilities and supports for the cryo-EM study. The research was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of CAS, the Ministry of Science and Technology of China and the Beijing Municipal Government.
Cryo-EM structure of 90S pre-ribosome (Image by IBP)
Institute of Biophysics, Chinese Academy of Sciences