Ribosomes collect all proteins in cells. Studies primarily in yeast have revealed a lot about how ribosomes come together, but the Ludwig-Maximilians-Universitaet (LMU) team in Munich now reports that assembling ribosomes in human cells requires factors that have no analogues in simpler, typical organisms.
In each cell, hundreds of thousands of complex molecular machines called ribosomes synthesize new proteins, and each growth chain is stretched at a rate of few amino acids per second. It is therefore not surprising that the construction of these vital protein plants in and of itself is a very complex process, with more than 200 assembly workers participating in passage. Mature ribosomes consist of about 80 proteins and four RNAs ribosomes. But how these components are assembled in the correct order to produce a functional ribosome is still not fully understood. Moreover, most of our knowledge of the process comes from studies of model organisms such as bacteria and yeast, and it may not necessarily be applicable to cells of higher organisms. Researchers led by Professor Roland Beckmann (Gene Center, LMU in Munich) have now revealed new details of critical steps in the maturation of ribosomes in human cells.
Active ribosomes consist of two separately assembled particles, differing in size and interacting with each other only after the first steps in protein synthesis occur on the smallest of them (in human cells, “40S subunit”). Beckmann’s team used cryo-electron microscopy to identify the structures of several 40S subunit precursors isolated from human cells and track their maturation pathway. “This study comes from a previous project in which we got initial insights into the process,” says Michael Ameismeier. He is a PhD student on the Beekman team and the lead author of the new report, which is concerned with the final steps in putting together the small subunit.
At this late stage in the process, one end of the RNA attached to the small particle protrudes from the body of the immature subunit. The final step in maturing the 18S subunit is to remove this excess now. To ensure this reaction does not occur prematurely, the responsible enzyme – NOB1 – is kept in an inactive state until required. The new study shows that NOB1 activation is preceded by a combative change that results in the separation of a binding partner from the enzyme. This in turn leads to a structural rearrangement in NOB1 itself, enabling the enzyme to shear the prominent rRNA fragment. Ameismeier explains: “NOB1 activation is coordinated by another enzyme.” Together with a protein we discovered – not found in yeast – the latter enzyme inserts like a wedge into the mature 40S subunit, and this facilitates the critical harmonic change in NOB1.
The authors also showed that another protein not present in yeast plays (yet) an ambiguous role in the maturation of the 40S subunit. “This demonstrates the importance of looking at the human system separately from other experimental paradigms,” says Beekman. Using the evolutionarily simpler yeast system is sufficient for a basic understanding of the process. But some disease syndromes have been linked to errors in the biogenesis of ribosomes in humans, providing a clear rationale for studying ribosomal assembly in human cell systems.
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