Molecular biology: Early convolutions
Three-dimensional structure of a nucleolar ribosomal complex. Different biogenesis factors are colored, RNA and ribosomal proteins are shown in light gray or dark gray. Image: L. Kater, LMU
As cells grow, the cellular protein factories - the ribosomes - produce large amounts of proteins. For this, the cell must first form the ribosomes themselves on a large scale. A yeast cell produces about 100,000 ribosomes per hour, with each ribosome producing about 80 ribosomal proteins and four ribosomal RNAs. In order for these functional components to make a functioning protein factory, the yeast needs about 200 so-called biogenesis factors. These proteins regulate the assembly of the ribosome and, among other things, ensure that the ribosomal RNA is correctly folded into its complex spatial structure. LMU scientists led by Professor Roland Beckmann, in cooperation with researchers led by Professor Ed Hurt (University of Heidelberg), are now showing for the first time in a three-dimensional representation how the early steps of this folding are taking place. They report on their results in the renowned journal Cell.
Ribosomes always consist of two subunits, which in turn consist of several ribosomal RNA molecules and proteins. The structure of the ribosome begins already in the so-called nucleolus of the nucleus. From there, the maturing subunits are first transported into the nucleoplasm and then for further completion from the cell nucleus into the cytoplasm. "This multi-stage process shows that it is an extremely complex process," says Lukas Kater, first author of the study. To investigate how ribosomal RNA folding begins in the nucleolus, scientists used cryo-electron microscopy to elucidate the structure of five ribosomal complexes associated with early biogenesis factors. "In this way, we were able to identify the function of several early biogenesis factors and, for the first time, elucidate the basic sequence of folding and assembly of a nucleolar - long-unfinished - ribosomal subunit," Kater says.
It showed that the resulting ribosomal RNA folds from both ends and not strictly in the order in which their building blocks are lined up. "This creates already during the first phase of assembly a kind of exoskeleton for the later forming catalytic center and the so-called exit tunnel, are left by the newly produced proteins, the ribosome," Beckmann says. Next, the scientists want to solve further intermediate steps in the construction of ribosomes in order to obtain a more detailed picture of the processes in the nucleus.