A division of labor in cells' protein factories.


1218 23 JUNE 2017 • VOL 356 ISSUE 6344 sciencemag.org SCIENCE IL L U S T R A T IO N : V . A L T O U N IA N / S C IE N C E T he plant that built your computer isn’t churning out cars and toys as well. But many researchers think cells’ crucial protein factories, organelles known as ribosomes, are interchangeable, each one able to make any of the body’s proteins. Now, a provocative study suggests that some ribosomes, like modern factories, specialize to manufacture only certain products. Such tailored ribosomes could provide a cell with another way to control which proteins it generates. They could also help explain the puzzling symptoms of certain diseases, which might arise when particular ribosomes are defective. Biologists have long debated whether ribosomes specialize, and some remain unconvinced by the new work. But other researchers say they are sold on the finding, which relied on sophisticated analytical techniques. “This is really an important step in redefining how we think about this central player in molecular biology,” says Jonathan Dinman, a molecular biologist at the University of Maryland in College Park. A mammalian cell may harbor as many as 10 million ribosomes, and it can devote up to 60% of its energy to constructing them from RNA and 80 different types of proteins. Although ribosomes are costly, they are essential for translating the genetic code, carried in messenger RNA (mRNA) molecules, into all the proteins the cell needs. “Life evolved around the ribosome,” Dinman says. The standard view has been that a ribosome doesn’t play favorites with mRNAs— and therefore can synthesize every protein variety. But for decades, some researchers have reported hints of customized ribosomes. For example, molecular and developmental biologist Maria Barna of Stanford University in Palo Alto, California, and colleagues reported in 2011 that mice with too little of one ribosome protein have short tails, sprout extra ribs, and display other anatomical defects. That pattern of abnormalities suggested that the protein shortage had crippled ribosomes specialized for manufacturing proteins key to embryonic development. Definitive evidence for such differences has been elusive, however. “It’s been a really hard field to make progress in,” says structural and systems biologist Jamie Cate of the University of California (UC), Berkeley. For one thing, he says, measuring the concentrations of proteins in naturally occurring ribosomes has been difficult. In their latest study, published online last week in Molecular Cell, Barna and her team Ribosomes, which build a protein (black) from an RNA strand (blue), may specialize in making particular sets of proteins. nually add about 13 tons of phosphorus, 25 tons of nitrogen, and 107 tons of carbon to the ecosystem in half a dozen pulses that each last about a month. During those weeks some nutrient levels in the river can quadruple temporarily. The bones, which make up half the biomass, are the last to decay, taking 7 years. Along the way they support a film of microbes that in turn become food for fish and other river-dwellers. “I am stunned by the extent of the annual mass wildebeest drownings and their large contribution of [carbon, nitrogen, and phosphorus] to the energy budget of the Mara River,” says Gary Lamberti, an aquatic ecologist at the University of Notre Dame in South Bend, Indiana. The boon likely extends beyond the river, as vultures and storks move wildebeest-derived nutrients tens of kilometers inland. The study, which required spending time in the crocodile-filled river, adds to a growing body of evidence that mass mortality can have ecosystem impacts. Researchers like Sumida have found, for example, that dead whales provide a pulse of food to nutrient-starved ocean floors, enabling a specialized ecosystem to flourish on the decaying carcasses. Others have tracked how salmon that die after they finish their final upstream journey to spawn add nutrients to river ecosystems. The impact of the wildebeest appears to be larger, however; they contribute four times more biomass to the Mara than dying salmon add to British Columbia’s rivers, Subalusky notes. “These phenomena highlight the multiple pathways—nutrients, direct consumption, food web transfers—by which animal tissue can influence food webs,” Lamberti says. On a broader scale, “the [wildebeest] findings have implications for understanding the ecological role of past and present animal migrations,” says David Janetski, an aquatic ecologist at Indiana University of Pennsylvania. The bison in North America, the saiga antelope in central Asia, and many caribou in the Arctic once migrated by the millions, sustaining ecosystems in the rivers they crossed. When the migrations dwindled, the organisms that relied on the carcasses of animals that came to grief may have declined or vanished, he says. On the positive side, the wildebeest drownings kill only about 0.7% of the total herd each year. Illegal harvesting, starvation, and predation kill many more. “Although drowning events are horrendous and graphic, they should not be our primary concern for the long-term sustainability of this population,” Hopcraft says. “If anything,” he says, “the Serengeti shows us what an ecosystem should look like.” j A division of labor in cells’ protein factories Ribosomes may specialize to produce select proteins MOLECULAR BIOLOGY

DOI: 10.1126/science.356.6344.1218

Cite this paper

@article{Leslie2017ADO, title={A division of labor in cells' protein factories.}, author={Mitch Leslie}, journal={Science}, year={2017}, volume={356 6344}, pages={1218-1219} }