Chemists Characterize Regulators Of Zinc Intake And Export In Cells

Inorganic elements, such as zinc, copper and iron, are vital to the healthy functioning of cells in living organisms, but little is known about how cells use these heavy metals. Each cell in the human body, for instance, requires an enormous amount of zinc, but that amount must be carefully controlled because zinc can be destructive in excess. How does the cell monitor the amount of zinc inside its walls?

Thomas V. O’Halloran, professor of chemistry, and Caryn Outten, a former Ph.D. student of O’Halloran’s, have solved an important part of the puzzle. They have described the mechanisms by which two sensor proteins regulate two pumps embedded in the cell membrane. One pump draws zinc into the cell when it is needed, and the other acts as a bouncer, ejecting zinc when the cell is saturated. These are the first — and appear to be the primary — zinc pumps, along with their regulatory proteins, identified in a cell.

The researchers also have shown that, contrary to current thinking, cells have no free-floating zinc; instead, all zinc is accounted for when in the cell, either bound to proteins or acting as a catalyst in biochemical reactions. This suggests the existence of zinc "chaperone" proteins whose role would be to escort the metal safely to the specific site where it is needed.

"Our work with zinc, copper and other metals involved in human health and disease is helping us to better understand the role these metals play in our body," said O’Halloran, who discovered the first copper chaperone protein in 1997.

O’Halloran and Outten studied the zinc regulatory proteins in the bacteria E. coli, which provides a good model of zinc metabolism in animal cells. They found that when the amount of zinc in the cell increases to a point where zinc is no longer needed for cellular tasks, the extra zinc binds tightly to the protein Zur. This reaction in turn shuts down the zinc intake pump.

In the case of the regulatory protein ZntR, the opposite happens when it combines with zinc. When ZntR senses any extra zinc after the intake pump has been shut off, ZntR binds with the zinc, turning on the export pump. Any unwanted and potentially dangerous zinc is pumped out of the cell.

The researchers determined the concentrations of zinc required within the cell in order for the proteins to turn off and on the pumps. They showed that the regulatory system is so sensitive and finely tuned that zinc does not have the opportunity to float freely in the cell’s cytoplasm before it binds to either Zur or ZntR, depending on the cycle.

"The zinc concentration is so low in between pump activity that free-floating zinc just doesn’t exist," said Outten, now a post-doctoral researcher at Johns Hopkins Bloomberg School of Public Health. "It’s a kinetic process — any zinc that shows up in the cell is immediately taken away, to work reactions in the cell, to bind to proteins such as Zur or ZntR or, if no zinc is needed, to be sent packing out of the cell."

The research was funded by the National Institutes of Health.