"Our findings are the first to link the structure of a ribosomal protein to a critical step in the pathway to assembling a fully functional ribosome," explained John Woolford, professor of biological sciences at the Mellon College of Science at Carnegie Mellon. "Understanding the molecular basis of ribosome assembly offers a rational scheme for designing drugs to interfere with that process."
A complex of protein and ribonucleic acid (RNA), ribosomes are present in vast quantities inside every cell. There, they translate genetic information into proteins that control many activities, including cell movement, metabolism, division and response to the environment. Because ribosomes are essential for protein production, problems with their assembly inevitably spell cell death.
Woolford found that changing the tail of a ribosomal protein called S14 prevented it from processing a chunk of RNA destined to become part of a mature ribosome. Drugs that target the tail of S14 would likely interfere with ribosome assembly, according to Woolford, who added that such agents would destroy an infectious fungus while leaving animal or plant cells unharmed.
Using processes known as transformation and gene disruption, Woolford's group engineered the yeast Saccharomyces cerevisiae, (common baker's yeast) to contain two genes for S14. One normal, or wild-type, gene instructed production of a fully functional S14 protein, while a mutant gene coded for the production of an S14 with an altered tail. After growing the yeast under normal conditions, Woolford turned off the wild-type gene and observed the consequences when only the mutant gene worked. He found that a slightly altered tail structure prevented the S14 protein from "cutting" its target RNA molecule, thus halting ribosome assembly. Because it wasn't processed, this typically short-lived RNA molecular intermediate accumulated within yeast cells, making it easy to isolate and study. Yeast engineered with mutations in genes for other proteins that direct ribosome assembly should yield even more intermediates for study, according to Woolford, whose research was supported by the National Institutes of Health and reported in the May 7 issue of Molecular Cell.
In collaboration with Martin Farach-Colton at Rutgers University, Woolford is currently developing computer models to outline the many proteins involved in ribosome assembly and the step-by-step process by which various parts come together to make a new ribosome. In addition, Woolford is carrying out genetic experiments to test their idea that certain non-ribosomal proteins that regulate ribosome assembly (called ribosomal assembly factors) also regulate cell proliferation.
"We think that specific ribosome assembly factors we discovered might have a second 'moonlighting' job," said Woolford. "Thus, if such a protein functions in both ribosome assembly and growth regulation, cells could coordinate these two processes by 'talking' to the same molecule in two places."
In this scenario, if a cell told a ribosome assembly factor to stop working, it would effectively shut down ribosome production and at the same time trigger cells to stop dividing. But if that factor failed to hear what the cell dictated, it would continue to build ribosomes and spur cell division that could lead to cancer, according to Woolford.
Molecular Cell