Double-duty proteins.

Double duty proteins

Consider the lens -- a sort of biological monocle perfectlysuspended inside the eye. It has no blood vessels, no nerves, not even any genetic material--yet it lives and grows for as long as we do, and in order to bend light as efficiently as possible, it has the most concentrated protein content of any tissue in the body. According to a recent report, there may be more to that protein than meets the eye.

Graeme Wistow and Joram Piatigorsky, of the National EyeInstitute in Bethesda, Md., report in the June 19 SCIENCE that certain proteins called crystallins, which are common in the lens and were heretofore regarded as simply structural, are actually enzymes or are closely related to enzymes. They confirmed this role, which had been suggested by previous, preliminary research, by comparing the amino acid sequences of the crystallins with the sequences found in several enzymes. The high degree of similarity leads them to suggest that there is an evolutionary advantage to materials that can do "double duty", at least in highly specialized parts of the body such as the lens.

The cells that make up a lens are indeed specialized. In orderto be transparent, they lose all their internal structures early in embryo development, leaving them unable to replicate or to produce their own energy. Crystallins, however, which make up as much as 60 percent of the vertebrate lens are sturdy enough to last a lifetime without replication, while as enzymes they are capable of converting sugars into energy for the lens. Piatigorsky told SCIENCE NEWS that such a perfect overlap of chores may be one of the best examples of "evolutionary pragmatism," in which a single gene may code for proteins that perform a number of functions.

"In the evolution of the lens, nature took a very practicalroute," he says. "It took the genes and proteins that were already around," and used them to fill both structural and functional needs.