NTU Scientists Reveal First High Resolution Cryo-EM Structure of Chloroplast Ribosome

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Are we a step closer to understanding molecular evolution?

Ribosome! Protein synthesizers in the cell!

This tiny but indispensable organelle has been gaining popularity for quite some time now. Ever since the 2009 Noble Prize was awarded to Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath “for studies of the structure and function of the ribosome”, studies involving them have made it to the front page of most reputed science journals, not that their importance was not known earlier.

With the advent of Cryo-electron microscopy, they presented themselves as best samples, thanks to their molecular weight which is in the range of multi-megadalton. Owing to this, ribosome structure has been studied in great detail and are very well-characterized.

So what is significance of this latest study by Ahmed et. al. published recently in Nature’s Scientific Reports.

The team headed by Dr. Shashi Bhushan, Asst. Professor at School of Biological Sciences, Nanyang Technological University, Singapore has elucidated the cryo-EM structure of the large subunit of the chloro-ribosome from spinach (Spinacia oleracea) at an average resolution of 3.5 Å.

Cryo-EM reconstructed density of the spinach chloro-ribosome LSU, showing solvent-facing and subunit interface side. All proteins except two PSRPs (red) are coloured yellow. Chloroplast-specific extensions are coloured green. rRNAs are coloured as: 5S (goldenrod), 4.8S (sienna) and 23S (grey).
Cryo-EM reconstructed density of the spinach chloro-ribosome LSU, showing solvent-facing and subunit interface side. All proteins except two PSRPs (red) are coloured yellow. Chloroplast-specific extensions are coloured green. rRNAs are coloured as: 5S (goldenrod), 4.8S (sienna) and 23S (grey). Credit: Nature Scientific reports

Significance:

To understand the significance of this study, we need to understand the fact that, while the ribosome structure has been solved at very high-resolution, these were predominantly bacterial and mitochondrial ribosomes. A high-resolution structure of chloroplast ribosome was missing until this study was published.

This structure of the chloroplast ribosome could very well pave way to deciphering the course of molecular evolution.

According to the endosymbiotic theory, both chloroplast and mitochondria originated from prokaryotic cells. Therefore, from an evolutionary perspective, studying ribosomes from these organelles may shed new light on the evolution of life, in general.

In the course of evolution, most of the chloroplastic genes have been transferred to the nucleus. But still, chloroplast retains a small genome which synthesizes around 100 proteins. The exact reason is a matter of debate and the detailed analysis of this high-resolution structure at 3.5 angstrom could be key to unravelling this mystery.

Getting into some detail, one cannot help but notice that mitochondrial ribosomes have evolved a bit differently compared to their bacterial ancestors while chloroplastic ribosomes are similar to their bacterial counterparts.

Chloroplastic proteins which are known as PRPs (Plastid Ribosomal proteins) are usually larger in length because they contain plastid specific extensions in their N and C termini. Moreover, chloroplast ribosome contains six plastid specific ribosomal proteins (2 in the large subunit and 4 in the small subunit) which are known to help in light-dependent regulation of translation in chloroplast.

While the above reasons are compelling enough to investigate the chloroplastic ribosome, the work carried out by the group of Dr. Shashi Bhushan, Asst. Professor at School of Biological Sciences, Nanyang Technological University, Singapore also hints towards understanding protein folding, the holy grail of molecular biology, in the chloroplast.

The full text of article can be accessed from this link: http://www.nature.com/articles/srep35793