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Biochemists Advance Knowledge of Transport through MembranesHow do nutrients and vitamins enter living cells? National Science Foundation (NSF)-funded biochemists at the University of Oklahoma at Norman have made a dramatic advance that largely answers this question. Researchers previously inferred that the proteins in biological membranes somehow undergo structural changes during the uptake of important molecules. Now, experiments conducted by biochemist Phillip Klebba and colleagues prove the correctness of that assumption by directly showing that membrane proteins act much like exterior doors or gates that regulate entry into the cell. Their results, published in the May 23 issue of the journal Science, reveal that these entrance portals recognize substances that the cell needs for growth, actively open to allow their uptake and then close. Cells obtain, in this way, the molecules they need while preventing the entry of unnecessary or toxic compounds. The upshot of these results is that membrane proteins do not necessarily form static, passive pores. Rather, they are dynamic entities capable of sensing their environment and actively acquiring the substances needed for cell growth, says Klebba. The study, funded by the NSF and the National Institute of General Medical Sciences, looked at the protective membrane surrounding bacterial cells, which permits the entry of essential nutrients and vitamins but excludes toxic substances like detergents and antibiotics. "The question of how nutrients and vitamins enter living cells has been answered by these experiments," says Marcia Steinberg, director of NSF's biomolecular structure program, which funded Klebba's research. Small molecules cross this outer membrane through open channels formed by proteins. However, iron complexes are too large to pass through the open pores and instead enter through larger channels that are normally closed. The necessity of iron in metabolism makes its acquisition a fundamental need of living cells, but until now little was known about the operation of iron transport channels. In a second paper recently published in the Proceedings of the National Academy of Sciences, the researchers found that iron-containing molecules attach to the surfaces of cells by binding to the outside of a closed membrane protein. After binding iron, the protein opens to internalize the metal. In addition, the experiments identify a methodology -- electron spin resonance spectroscopy -- suitable for studying these events in living cells. Living cells can be labeled and studied with this approach without disrupting their natural functions. The experiments thus make it possible to observe transport events as they happen, says Klebba, which will lead to new insights about the molecular mechanisms of membrane transport.
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