The labs of Alexey Amunts and Erik Lindahl (both Stockholm University/SciLifeLab) have successfully determined the structure of chlororibosomes using Cryo-EM, providing novel insights into plant protein synthesis and a new perspective on the evolution of translation. This new study is published in Nature Plants.
The chlororibosomes are ribosomes located in the chloroplast organelles of plants and algae, where the photosynthetic reaction takes place. They synthesize the protein components responsible for coordinating the chlorophyll pigments and driving the energy-making processes in green plants. Using cryo-electron microscopy (Cryo-EM), the present study investigated chlororibosomes purified from spinach leaves. From the resulting density map, calculated to 2.91 Å and 3.07 Å for the large and small chlororibosomal subunit, respectively, the researchers were able to build a detailed model of the chlororibosome. This has led to the discovery of several previously unconfirmed features of the molecular structure. Remarkably, the quality of the density map allowed identification of errors in sequencing and mis-annotation in the UniProt database.
The model of the spinach chlororibosome reveals several molecular specialties of the large subunit. The most intriguing is a prominent channel, at least 9 Å wide x 30 Å long, extending from the exit tunnel to the chlororibosome exterior. Protein deletion resulted in more than twofold widening of the tunnel that is likely to affect an initial protein folding. In addition, two native translation factors were identified in complex with the chlororibosome: the recycling factor and long hibernation-promoting factor (PSRP1). Both were resolved better in the cryo-EM map than in the corresponding crystal structures of reconstituted ribosomal complexes from E. coli and T. thermophilus bacteria.
From the evolutionary perspective, the work revealed that a particular protein-protein bridge connecting the large and small subunits of chlororibosomes in a similar fashion to a ribosome from human mitochondria. This prompted the researchers to assess if there might be structural adaptations that have taken parallel evolutionary paths in ribosomes of chloroplasts and mitochondria. The results show corresponding patterns in chloro- and mitoribosomes on the level of sequence variability of ribosomal RNA, as well as on the protein level. This provides a new outlook for the further exploration of the evolution of translation.
Read the full paper in Nature Plants