Research

Rhesus Proteins Transport Ions, Not Gas

30. 7. 2014 | Do these proteins carry the gas ammonia or the ammonium ion across cell membranes? Biochemists have long speculated about the details of the ammonium transport proteins (Amt), which include the Rhesus protein factors, known as part of the mammalian blood group system. Researchers from Freiburg, Germany were able to show experimentally that indeed the ion is carried across the membrane, not the gas.
It was previously known that Amt proteins extend across cellular membranes and transport nitrogen into bacteria and plant cells, essential nutrients for their growth and survival. In mammals, Rhesus proteins regulate the acid and ion balance in kidney and liver cells. The scientific debate on Amt/Rh proteins stems from the difficulty of distinguishing between ammonia and ammonium in measurements, as the two molecules are continually transformed into each other through the exchange of protons.
A team of scientists led by Prof. Dr. Susana Andrade from the Institute of Biochemistry of the University of Freiburg has now determined the transport properties of Amt proteins with great precision on the basis of electrophysiology tests on artificial lipid systems.
Proteins of the Amt family
Bild vergrößern
(© Susana Andrade)


Proteins of the Amt family | transport ammonium across the lipid membrane of the cell.

The scientists cloned membrane proteins from an archaea, a microorganism that lives under extreme temperature conditions, and isolated them. Already in 2005, the Freiburg researchers shed light on the crystalline three-dimensional structure of a protein from this protein family. Now they imbedded this cloned protein into a layer of lipid molecules. With this setup, the researchers were able to measure the ion currents directly. Prof. Andrade's team discovered that a positive charge travels through the membrane: The membrane proteins do not transport the gas ammonia NH3, but rather the ammonium ion NH4+. "Our in vitro method gives us a level of precision that finally allows us to draw valid conclusions concerning the transport process," stresses Susana Andrade.

"The results can, in a large part, be transferred to the Rhesus proteins from mammals," Andrade continues. Amt proteins bear a close resemblance to the Rhesus proteins found in humans. They are produced in the blood, in the kidney, and in the liver, where they regulate the intake of ammonium and thus the body's pH level. The researchers tested three Amt proteins that can be found in bacteria, and also determined the speed at which they allow ammonium to pass through the membrane. "In the future, we want to modify individual components of the transporter to improve our understanding of the exact molecular details involved," explains the biochemist, who is also a member of the Freiburg Cluster of Excellence "BIOSS Centre for Biological Signalling Studies".   (© University of Freiburg, AcademiaNet)
Rudolf-Werner Dreier

More information

Source

  • Tobias Wacker, Juan J. Garcia-Celma, Philipp Lewe, and Susana L. A. Andrade, Direct observation of electrogenic NH4+ transport in ammonium transport (Amt) proteins, PNAS 2014; published ahead of print June 23, 2014, doi:10.1073/pnas.1406409111

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