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CELL BIOLOGY Structure Turns Iron Entry Into Cells on Its Head Bottom Binding May Reorient Approach to Iron Overload Diseases, Cancer Quad scientists report the most finely detailed picture yet of how iron is delivered into cells, a finding that may help others harness the mechanism to dispense cancer-fighting drugs or work out the complex dynamics behind hereditary iron overload diseases. The human body has a love-hate relationship with iron. Iron is an essential nutrient, mostly for the fresh hemoglobin in the 200 billion new red blood cells made every day. Dividing cells also need iron to synthesize DNA. Yet, on its own, the highly reactive metal can wreak havoc by making toxic molecules that damage lipid membranes, proteins, and DNA. Revealing a secret molecular handshake. A new model unexpectedly shows that iron-transporting transferrins bind to the side and underside of the transferrin receptor, not to the top. The transferrin molecule straightens when it attaches to the receptor, which puts the C-lobe in a position to more quickly release its iron load, while the N-lobe may have a harder time letting go of its metal cargo. (Image courtesy of Thomas Walz and Cell Press) By necessity, cells have developed sophisticated machinery to safely shuttle iron through the body. In the pathway that supplies iron to cells, the molecule transferrin collects iron and binds to a receptor on the cell surface. The cell engulfs the receptor-transferrin complex, forming an endosome, where iron is released and pumped out into the cytoplasm. The complex stays intact until it returns to the cell surface. The pieces separate, and the transferrin can be used again to bind iron and take it into cells. The process is well studied, but the precise nature of these molecular interactions has remained elusive. Using high-resolution electron microscopy, researchers in the lab of Thomas Walz, HMS assistant professor of cell biology, have found a surprising aspect to the fit between the iron-transport molecule transferrin and its best-known cell surface receptor. The proposed structure, published in the Feb. 20 Cell, reveals the microscopic machinery that ushers iron into cells. A Surprising Bond "It was quite unexpected that transferrin binds its receptor from the bottom rather than from the top," said Yifan Cheng, HMS research associate and first author of the study. The proposed new model is "a breakthrough that resets the limit of the technique" and "that results in a more plausible mechanism of [iron] release," wrote Des Richardson of Children's Cancer Institute Australia for Medical Research, located in Sydney, in an accompanying Cell commentary. "A lot of cancer cells express a lot of transferrin receptors. There are groups trying to exploit the transferrin cycle for delivery of drugs into cancer cells." --Thomas Walz The newly discovered structure of the receptor-transferrin complex gives other researchers enough detail to effectively modify the interacting proteins, which may boost the efforts to deliver drugs to cancer cells, according to Walz. "A lot of cancer cells express a lot of transferrin receptors," he said. "There are groups trying to exploit the transferrin cycle for delivery of drugs into cancer cells." The findings also advance understanding about how the body maintains a healthy balance of iron and what goes wrong in several genetic diseases in which too much iron builds up and damages the liver, pancreas, and heart. The most common hereditary iron overload disease, hemochromatosis, affects up to 1 in 250 individuals of northern European descent. In it, a genetic mutation creates a malfunctioning HFE protein. HFE normally works to reduce intestinal absorption of the tiny bit of iron from the diet. It also slows down iron recycling by specialized macrophages, which eat old and damaged red blood cells and load the iron back onto transferrin, said Howard Hughes investigator Nancy Andrews, the Leland Fikes professor of pediatrics at Children's Hospital. "A better understanding of transferrin-transferrin receptor binding is likely to help us figure out more about the HFE-transferrin receptor complex, which is very likely important in how HFE acts to regulate iron homeostasis," Andrews said. The receptor structure alone was solved five years ago in the lab of Howard Hughes investigator Stephen Harrison, HMS professor of biological chemistry and molecular pharmacology and director of the HMS Center for Molecular and Cellular Dynamics. Harrison is a co-author on the new paper. Walz is a founding faculty member of the center, a research initiative launched last fall that seeks to reveal fresh avenues of medical intervention by turning static images of molecules into "movies" of fully functioning molecular machines. Using cryo-electron microscopy, Yifan Cheng (right), Thomas Walz, and their colleagues produced a detailed structural model of how an iron-loaded transferrin molecule binds to its receptor. (Photo by Phil Farnsworth) Based on the separate crystal structures of the receptor and unattached transferrin, scientists had theorized that transferrin molecules landed on the easily accessible top of the receptor. Several studies from a California Institute of Technology group led by Pamela Bjorkman, a former graduate student in the Harvard lab of Don Wiley, showed that the HFE protein competes with transferrin to bind to the receptor and that HFE binds to the side, not the top, of the molecule. Even then it was not clear how the receptor-transferrin complex compares to the receptor-HFE structure, and the complex defied repeated structural determination attempts in several labs to solve its structure, including those of Harrison and Bjorkman. Cool Technique Cheng took up the longstanding problem and decided to study the complex by cryo-electron microscopy, a technique not usually successful with such a small molecule. Cheng tinkered with the optimal freezing conditions. The ice layer had to be thin enough to minimize background noise from the frozen solution, but not so thin that the molecule collapsed. The researchers heap praise on their Brandeis University colleague Nikolaus Grigorieff for developing a fast and powerful computer program that enabled them to computationally refine the orientation of the imaged complexes and reconstruct the three-dimensional structure from two-dimensional images. Cheng and Walz used the existing crystal structures of iron-loaded and iron-free transferrin and the transferrin receptor to model the complex before and after iron release. The result is not the smallest complex imaged by electron microscopy, but it is close. The detailed image wows even the researchers themselves. "We can't see individual amino acids," Cheng said, "but we can visualize individual alpha helixes." --Carol Cruzan Morton