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Biologists are just beginning to delve into the functions of those molecules, called microproteins, micropeptides, or miniproteins. But their small size seems to allow them to jam the intricate workings of larger proteins, inhibiting some cellular processes while unleashing others. Early findings suggest microproteins bolster the immune system, control destruction of faulty RNA molecules, protect bacteria from heat and cold, dictate when plants flower, and provide the toxic punch for many types of venom. "There's probably going to be small [proteins] involved in all biological processes. We just haven't looked for them before," says biochemist Alan Saghatelian of the Salk Institute for Biological Studies in San Diego, California.
Many appear to be encoded in stretches of DNA—and RNA—that were not thought to help build proteins of any sort. Some researchers speculate that the short stretches of DNA could be newborn genes, on their way to evolving into larger genes that make full-size proteins. Thanks in part to small proteins, "We need to rethink what genes are,"
BEING SMALL LIMITS a protein's capabilities. Larger proteins fold into complex shapes suited for a particular function, such as catalyzing chemical reactions. Proteins smaller than about 50 to 60 amino acids probably don't fold, says chemist Julio Camarero of the University of Southern California in Los Angeles. So they probably aren't suited to be enzymes or structural proteins.
However, their diminutive size also opens up opportunities. "They are tiny enough to fit into nooks and crannies of larger proteins that function as channels and receptors," Olson says. Small proteins often share short stretches of amino acids with their larger partners and can therefore bind to and alter the activity of those proteins. Bound microproteins can also shepherd bigger molecules to new locations—helping them slip into cell membranes, for instance.
UNLIKE HULKING PROTEINS such as antibodies, microproteins delivered by pill or injection may be able to slip into cells and alter their functions. Captopril, the first of a class of drugs for high blood pressure known as angiotensin-converting enzyme inhibitors was developed from a small protein in the venom of a Brazilian pit viper. But the drug, which the Food and Drug Administration approved for sale in the United States in 1981, was discovered by chance, before scientists recognized small proteins as a distinct group. So far, only a few microproteins have reached the market or clinical trials.
The existence of nanobacteria was reported in 1997 by researchers Kajander and Ciftcioglu of the University of Kuopio in Finland, who claimed to have isolated these tiny organisms from human blood. After partially mapping the genetic sequence of the nanobacteria, the researchers announced them to be a new species, Nanobacterium sanguineum.