5.7 Flashcards
Note 1 —-»
Peter Agre received the 2003 Nobel Prize in Chemistry for this sort of discovery. Dr. Agre, a faculty member at the Johns Hopkins School of Medicine, was studying Rh blood antigens. These proteins are of medical importance when Rh-negative mothers have Rh-positive babies. Membrane-spanning proteins are challenging to work with, and the samples that he and his team isolated seemed to consist of two proteins. They were certain that the smaller one was just a breakdown product of the larger Rh protein—and they were completely wrong.
Note 2 —-»
The researchers made antibodies that would specifically bind to and label this smaller protein. They found two interesting results: The antibody did not bind to any part of the Rh protein, indicating that the smaller protein wasn’t part of the Rh protein. And the antibody did bind in huge quantities to red blood cells, showing that this previously unknown protein is one of the most abundant proteins in blood cell membranes. Agre and his team also determined that an identical protein was even more abundant in certain kidney cells.
Note 3 —-»
Agre searched the scientific literature and consulted other researchers for ideas and advice. A colleague suggested that the protein might be the elusive water channel that physiologists had predicted would explain the rapid transport of water in some cells. To test this hypothesis, the researchers produced messenger RNA that coded for the mystery protein and injected the mRNA into frog eggs. (The plasma membranes of frog eggs are known to be quite water-impermeable, a beneficial trait as they naturally develop in hypotonic pond water.) Biochemical tests showed that within 72 hours, the frog egg cells had translated the mRNA into the new protein.
Note 4 —-»
The researchers then transferred a group of RNA-injected frog eggs and a control group of eggs injected with only a buffer solution to a hypotonic solution and monitored the eggs with videomicroscopy. As you can see by the red curve on the graph, the experimental egg cells exploded in 3 minutes. The control eggs showed minimal swelling, even for time periods exceeding an hour. The researchers concluded that the newly discovered protein enabled the rapid movement of water into the cells.
Note 5 —-»
Since the results of that experiment were reported in 1992, much research has been done on aquaporins, determining their structure and dynamic functioning. The chapter introduction presented a model of aquaporin structure. Molecular biophysicists have produced computer simulations that show water molecules flipping their way single file through an aquaporin. Such simulations have revealed how aquaporins allow only water molecules to pass through them. Aquaporins have been found in bacteria, plants, and animals, and evolutionary biologists are tracing the relationships of these various aquaporins. Medical researchers study the function and occasional malfunction of aquaporins in the human kidney, lungs, brain, and lens of the eye. The serendipitous discovery of aquaporins has led to a broad range of scientific research and medical applications.
Why did the researchers use frog eggs to test the function of this unknown protein? Why did they also monitor the behavior of control eggs in the hypotonic solution?
Frog eggs are quite impermeable to water. The control eggs confirmed this trait and provided a comparison to the bursting of the eggs that were making aquaporins.
