That's good news for scientists interested in designing new proteins. The new results suggest that early life forms could have created functional proteins with only a few amino acids, says study coauthor David Baker. Baker and his colleagues focused on a 57-unit chain, containing 18 different amino acids, from a protein subunit called the SH3 domain. This sequence is found in many large proteins that act as chemical messengers within and between cells. The chain folds back and forth on itself to create a structure, called a beta sheet, that resembles accordion pleats.
Similar sheets occur in many proteins, helping define their three-dimensional forms.
The researchers created millions of candidate proteins by considering each amino acid position and choosing potential substitutes from a small set. They then screened the different combinations for ones that folded into the proper configuration. Isoleucine repels water, making it a good choice for the amino acids that point inward toward the protein core. Lysine and glutamate both attract water, making them ideal for the amino acids on the protein's outer surface. Previous work had shown that combinations of just three amino acids are sufficient to form bundles of spiral-shaped structures called alpha helices.