Yale researchers solve structure
of the ribosome

Crystallography confirms a long-held notion the RNA, not protein, sparks protein synthesis on the ribosome.

In a landmark achievement, Yale researchers have determined the atomic structure of the ribosome’s large subunit, paving the way for more effective drugs to fight infection.

The findings, published in two separate articles covering 25 pages in the Aug. 11 issue of the journal Science, were derived in Yale laboratories led by Thomas Steitz, the Eugene Higgins Professor of Molecular Biophysics and Biochemistry and an investigator at the Howard Hughes Medical Institute, and Peter Moore, the Eugene Higgins Professor of Chemistry.

“This is like climbing Mt. Everest or running the four-minute mile,” Steitz said. “We have solved the structure of the ribosome’s large subunit, which is the largest unique structure determined. We have established that the ribosome is a ribozyme, an enzyme in which catalysis is done by RNA, not protein.”

The ribosome is the cellular structure responsible for synthesizing protein molecules in all organisms. In addition to enhancing the understanding of protein synthesis, the research offers new clues about evolution and has significant medical implications because the ribosome is a major target for antibiotics.

Many antibiotics cure disease by selectively inhibiting the protein-synthesizing activity of large ribosomal subunits in disease-causing bacteria, while leaving human ribosomes alone. Unfortunately, over the years, many bacteria have become resistant to these agents, and the possibility exists that the devastating bacterial diseases that were brought under control by antibiotics in the 1940s and 1950s will once again become scourges.

“Now that we know the structure of the large ribosomal subunit,” Steitz said, “we can determine its exact structure with antibiotics bound to it.” The same methods of “structure-based drug design” that led to the development of HIV protease inhibitors for aids can now be used on the ribosome.

“The information that emerges should enable pharmaceutical companies to devise new inhibitors of ribosome function that can be used to control bacterial diseases that have become resistant to older antibiotics,” said Moore.

Although the ribosome is microscopic, it is gigantic in molecular terms. The larger of its two subunits is about 50 times larger than the average enzyme. Its function is to read the genetic information encoded in messenger RNA and generate the protein molecules that those messenger RNA molecules specify. The proteins made by an organism’s ribosomes are responsible for virtually all of its properties, including how it looks and behaves.

The structure of the ribosome’s large subunit was determined using X-ray crystallography, a technique that can produce three-dimensional images at resolutions so high that individual atoms can be positioned. The 3,000 nucleotides of RNA in the large ribosomal subunit form a compact, complexly folded structure, and its 31 proteins permeate its RNA.

Enzymes composed entirely of protein promote virtually all chemical reactions that occur in living organisms. One of the most remarkable findings to emerge from this research is that the protein synthesis reaction that occurs on the ribosome derives from the two-thirds of its mass that is RNA, not from the one-third that is protein.

“It was suspected for many years that the RNA of the ribosome was the enzymatic component. We now know that for certain,” Steitz said. “This means that in the very early days of evolution, protein synthesis evolved using RNA molecules because there were no protein molecules.”

Stem cells from bone marrow can help to repair liver

Much of the power of self-repair in the liver comes, of course, from the liver’s own cells, but a substantial portion is derived from a previously unsuspected source outside the liver, according to a paper published in Hepatology.

That novel source of liver cells is bone marrow, the producer of multipotent stem cells, which can develop into many different kinds of cells throughout the body. In the human adult, bone marrow stem cells have long been known for their unusual ability to give rise to both white and red blood cells, but their potential also to become nerve or muscle or epithelial cells has only been discovered in the last several years. Investigators including Yale’s Diane Krause, M.D., found that stem cells that travel through the bloodstream to the liver can develop into both hepatocytes and bile-duct cells, which are responsible for normal liver function.

The Hepatology paper describes the analysis of tissue samples from transplants in male patients who had received livers (but no bone marrow) from females, and in female patients who had received bone marrow (but not their livers) from males. That is, both groups of transplant patients had bone marrow from one sex (male) and livers from the other (female).

In each case, some of the new cells that developed in the liver contained a Y chromosome, indicating their male origin. The researchers reason that in the male transplant recipients, these new cells could only have come from the males’ own bone marrow, and in the females, the new cells must have come from the (male) donated bone marrow.

“This is an exciting finding, and it is incredibly surprising, because the bone marrow has never been considered as a source of liver cells,” says senior author Krause, an assistant professor of laboratory medicine. Not only does the finding open up new possibilities for treating many kinds of liver disease, it also indicates that fully functional stem cells with a remarkable plasticity can be found within adult bone marrow.

How nicotine may buffer the brain

Smokers who claim that tobacco relaxes them are reporting a documented biochemical effect. Nicotine, the main active compound of tobacco, lowers the perception of pain and physical stress by reducing the amount of the neurotransmitter dopamine that is broken down by neurons in the prefrontal cortex, a region of the brain that lies just behind the forehead. But Yale psychiatrist Tony P. George, M.D., and his colleagues report in the July issue of Neuropsychopharmacology that the dopamine pathways are not acting on their own. It appears that they are regulated by the brain’s system of endogenous opioid peptides—the brain’s own pain relievers.

The Yale group performed experiments in which rats were given small amounts of nicotine; some were also given naloxone, which blocks the action of endogenous opioid peptides, while others received an inactive saline solution. When the animals were tested by brief electrical shocks to the foot, the response of the saline group showed that nicotine had acted on the endogenous opioid peptides to reduce the amount of dopamine metabolized by prefrontal neurons, while the response of the naloxone group showed normal (non-nicotine) levels of dopamine metabolism. This indicates that the endogenous opioid system must participate in order for nicotine to be able to alter levels of dopamine uptake and thereby reduce perceptions of pain and stress.

Understanding the molecular basis of nicotine’s effects in the brain may give scientists new tools for developing ways to treat nicotine dependence. “Furthermore,” says George, “our results may have implications for our understanding of neuropsychiatric disorders such as schizophrenia,” in which smoking, excessive responses to stress, and some dysfunction of the prefrontal cortex all may be linked.

A genetic cause for hypertension during pregnancy

Blood pressure normally dips slightly during pregnancy but, as every obstetrician knows, a spike in pressure can lead to a serious and potentially life-threatening complication. The reasons for this type of hypertension, which occurs in about 6 percent of pregnancies, remain mysterious but research by a Yale team has shed the first light on a likely molecular cause.

Working with a family predisposed to a rare form of hypertension, David Geller, M.D., Ph.D., and colleagues identified a mutation in a protein in kidney cells that normally regulates salt balance. The protein, known as the mineralocorticoid receptor, is normally activated by the steroid aldosterone. The Yale scientists found that, in patients with the mutation, it is also activated by the hormone progesterone. “The consequence is that when women with this mutation become pregnant, the 100-fold rise in progesterone levels activates the receptor, causing increased salt balance and a marked increase in blood pressure,” said the paper’s senior author, Richard P. Lifton, M.D., Ph.D., a Howard Hughes Medical Institute (HHMI) investigator and professor of genetics, medicine, and molecular biophysics and biochemistry. Hypertension during pregnancy can lead to preeclampsia, which may be fatal to mother, fetus or both. The team’s findings were reported in the July 7 issue of Science.

“Our findings demonstrate that a normal hormone of pregnancy can have abnormal effects that can cause hypertension to worsen. This raises the possibility that more common forms of pregnancy-related hypertension may be attributable to similar mechanisms,” Lifton said. This information, he said, will motivate careful examination of the possibility that progesterone is acting to promote increased salt balance in other forms of pregnancy-related hypertension and may lead to clinical trials of salt restriction in selected groups of women whose blood pressure rises with pregnancy.

The paper was dedicated to the memory of the late Paul B. Sigler, M.D., who died in January 2000. Sigler, a noted Yale and HHMI structural biologist, created a computer model of the mineralocorticoid receptor and demonstrated how the mutated receptor might be activated by progesterone. The group then used this model to perform further experiments that proved the mechanism of action of the mutation.

Facial recognition is impaired
in autism

The developmental disorder autism interferes with social functioning—even with the recognition of faces, as a functional magnetic resonance imagery study now shows in detail. The study, which appeared in April in the Archives of General Psychiatry, was the work of a Yale research team headed by Robert T. Schultz, Ph.D., an associate professor at the Yale Child Study Center.

A decade of investigation has established that people with autism have more difficulty than unaffected individuals in recognizing faces. Instead, they rely on perceptual processes typically used to recognize non-face objects. The Yale study confirms these observations in terms of brain activity patterns. When a person with autism examines a face, his or her brain reacts differently from the brain of a normal person. Instead of bursting into activity at a site called the fusiform gyrus, which normally responds preferentially to faces, individuals with autism display increased activity in the inferior temporal gyrus, which normally responds most strongly to objects. In addition, people with autism tend to process faces by focusing on a few salient features rather than on the overall configuration, as if they were processing an object.

The new findings lead to a riddle. Could this abnormal brain activity be a cause of autism, or the result of a long-standing disinterest in social interactions that dates back to early childhood? “With our data, it is not possible to know,” says Schultz, but he and many of his fellow researchers look forward to finding out.

Also in Findings:

Solving the structure of the ribosome  |  Stem cells can help to repair liver  |  Nicotine may buffer the brain  |  Hypertension during pregnancy  |  Facial recognition is impaired in autism    

Chronicle  |  Rounds  |  Et cetera    

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Originally published in Yale Medicine, Fall 2000/Winter 2001.
Copyright © 2000-2001 Yale University School of Medicine. All rights reserved.