The waters of the Dead Sea, a landlocked lake between Israel and Jordan, are the saltiest on Earth, inhospitable to all but a few hardy species of bacteria and one species of alga. How do these microbes survive? Researchers in Israel and the United States have now found the secret of at least one of them. Haloarcula marismortui (which means, roughly, salt- loving boxlike bacterium that lives in the Dead Sea) has unique proteins that protect it from the depredations of salt.
To operate normally, proteins must stay in solution, bonded to water molecules. Take away the water, and proteins precipitate, binding instead to one another and forming a whitish, dysfunctional sludge. A high concentration of salts has that effect on most proteins: the salts pull water molecules away from the proteins.
In H. marismortui, though, the proteins win the molecular tug-of- war. Structural biologist Menachem Shoham of Case Western Reserve University and his colleagues at Tel Aviv University and the Weizmann Institute of Science in Israel used X-ray crystallography--in which X-rays are bounced and scattered off protein crystals like the one shown here--to reveal the molecular structure of a common H. marismortui protein, ferredoxin. The protein turned out to be shaped like a coffee mug with a handle spiked all over with glutamic and aspartic acid, two negatively charged amino-acid building blocks with a very strong affinity for water molecules. (Water molecules have a positive charge on one end and a negative charge on the other.) The negatively charged acids snatch water away from the salt, helping to keep the protein in solution.
Shoham’s research team one day hopes to be able to graft similar amino-acid sequences onto other, salt-intolerant proteins, thereby making them soluble in saline water. In countries like Israel, where freshwater is in short supply, such a process could find wide application. For instance, industrial enzymes that are designed to degrade toxins could be dissolved in seawater or even sewage--both of which are plentiful all over the planet.