reinhardtii in which an Amt gene was inactivated. In this new study, Kustu and her research team isolated strains of C. reinhardtii in which the Rh protein was missing did not grow well in environments with high levels of carbon dioxide, suggesting that Rh was necessary for the algae's ability to benefit from high CO2 levels. When the algae were grown in air, expression of the Rh gene was low. reinhardtii was high for cells grown in air supplemented with 3 percent CO2, about 100 times the concentration normally found in air. Kustu and other researchers had previously shown that expression of the Rh gene in C. Algae use sunlight energy to capture CO2 by photosynthesis. The researchers suggested that green algae have Rh proteins because they thrive in aquatic environments with high concentrations of CO2. Ammonia is toxic to vertebrates and, not surprisingly, Amt proteins are absent in this class of animals. In contrast, Rh proteins are rare among microbes, yet common in vertebrates. Amt proteins are widespread among microbes and plants, which use ammonia as the preferred source of nitrogen, a critical nutrient. The researchers reached their conclusions by studying the humble green alga, Chlamydomonas reinhardtii, one of the few microorganisms known to have both Rh and Amt proteins. "Other experiments that suggest that Rh proteins transport ammonium ion have involved cloning Rh genes into microorganisms or cells that do not have them naturally." "Our research corrects that assumption by showing that Amt proteins are working with ammonia gas, and Rh proteins are working with carbon dioxide" said Kwang-Seo Kim, a research specialist at the Kustu Lab and lead author of the paper. Because of this, many scientists have suggested that Rh proteins also function as active transporters for charged ammonium ions. Notably, Rh and Amt proteins are more closely related to each other than any other proteins. They also believed Amt was an active transporter and used energy to move the ion molecules in or out of cells against a gradient. Until a few years ago, scientists thought that Amt handled the charged ion for ammonia (NH4+), which is the major form found in water. Unlike active transporters, channels allow multiple molecules of gas to move through at the same time, an important distinction for gases that need to move across membranes quickly. ![]() ![]() ![]() Recent evidence indicates that ammonium/methylammonium transporter (Amt) proteins act as gas channels for NH3. They both dissolve readily in water, which can slow their passage across oily membranes. This is because gases typically have no trouble crossing cell membranes unaided, so it was not suspected that the Rh protein would play such a role.īut among gases, carbon dioxide (CO2) and ammonia (NH3) are exceptional, the researchers explained. Kustu noted that scientists had long doubted the presence of protein channels for CO2 or any gases. "This has implications for understanding how humans breathe, how we control the acidity (pH) of various fluids in our bodies and how our kidneys function, all of which rely upon movement of CO2 across cell membranes," said Sydney Kustu, professor of plant and microbial biology at UC Berkeley's College of Natural Resources and principal investigator of the study. The study's conclusions, the researchers said, will lead to new directions in human and animal physiology research, as well as generate lively debate among biochemists and hematologists. CO2 can also pass through the cell membrane unaided (above right), but not quickly enough, said UC Berkeley researchers. Image: Rh proteins act as gas channels that help speed the transfer of carbon dioxide (CO2) in and out of red blood cells.
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