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Fundamental Steps Along the Path Toward the Treatment of Disease
CHORI Scientist Solves Crystal Structure of Iron Regulatory Protein Complex

In an exciting new development in the field of mRNA research, CHORI Senior Scientist Elizabeth C. Theil, PhD and her colleagues have elucidated the structure of iron regulatory protein 1 (IRP1) complexed to the mRNA structure that regulates ferritin synthesis, iron responsive element (IRE).

"On a personal level, I've been tracking down this three-dimensional structure since about 1981," says Dr. Theil.

Dr. Theil had studied the RNA structure with ordinary and exotic chemical nucleases, mutagenesis, nuclear magnetic resonance spectroscopy, and all the tools of molecular and chemical biology and biochemistry, often in collaboration with William Walden, PhD, who is an expert with IRP1 - all to no avail.

"Getting the crystals of either the structured RNA with the binding protein, as we were just able to do," explains Dr. Theil, "required the magic of a three-way collaboration with x-ray crystallographer. Karl Volz.

A dual functioning protein, IRP1 is both an RNA binding protein, as it is found in the crystal structure in this study, as well as catalytic protein, when it binds an iron-sulfur cluster to become to a metabolic enzyme called aconitase. Understanding the structure of the IRE-RNA/IRP1 complex could reveal important clues toward the identification of potential drug targets for diseases caused by the mis-regulation of iron.

"The main reason I was interested in this structure," says Dr. Theil, "was because I wanted to know how the RNA looked when protein-bound and what changes occurred in the RNA and the protein when they bound."

While scientists have always known that all RNAs, including the ferritin IRE, had both "rigid" and "floppy" parts in solution alone, the only information scientists had about its bound structure was which parts of the RNA were covered up by the protein, not how they fit together.

While scientists have always known that all RNAs, including the ferritin IRE, had both "rigid" and "floppy" parts in solution alone, the only information scientists had about its bound structure was which parts of the RNA were covered up by the protein, not how they fit together.

"What we found is that in the presence of the protein, two bases of the structure, which were normally floppy, were sticking straight up into a large cavity of the protein, like the prongs on an electric plug," Dr. Theil explains. "Another section which had no structure at all in solution was sandwiched between two protein layers within a protein pocket created by the protein during RNA binding."

Not only are two binding sites between RNA and a protein unusual, but there also can be no mismatches between the binding between the IRE and IRP, since all the mRNAs with IRE structures use exactly the same bases to bind in the protein pocket and cavity. Thus, the floppy parts of the RNA are critical to its ability to bind to IRP1.

As Dr. Theil explains, "The flexibility in those regions is the key to the very high specificity and selectivity of this RNA."

While the study represents a landmark in basic research, identifying the structural elements of the IRE-RNA/IRP1 complex is also a significant milestone in the future development of disease treatments.

"Knowing the structure of the mRNA when it's bound to the protein means we have a target with which to manipulate the regulation of iron," explains Dr. Theil. "Even when people have too much iron in their bodies, there is still a lot of mRNA bound with protein that can't be translated. If we could find a small molecule that would knock of that protein, displace it some how, those mRNAs could make a lot more ferritin, which in turn, would synthesize the excess iron."

Such an approach could eventually transform the treatment of patients with thalassemia and sickle cell disease who suffer from iron overload as a byproduct of live-saving blood transfusions. Yet the study has myriad implications beyond iron regulatory diseases alone.

As Dr. Theil explains, "How this information relates to iron regulation specifically is very important, but if you think about it broadly, it can be applicable in all areas of disease in which the body makes too much of this or too little of that - and that's almost every disease. The real issue is finding out more about mRNA and how to manipulate it."

Of course, targeting mRNA instead of DNA is a whole new approach and, like any other scientific revolution, such as the development of antibiotics or cancer therapies, developing it will take a long, concerted effort and the continued scientific illumination that fundamental basic research studies as this one provide.

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Monday, May 16, 2011 11:33 PM

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