Folded proteins occur frequently in libraries of random amino acid sequences

A library of synthetic genes encoding 80- to 100-residue proteins composed mainly of random combinations of glutamine (Q), leucine (L), and arginine (R) has been expressed in Escherichia coli. These genes also encode an epitope tag and six carboxyl-terminal histidines. Screening of this library by immunoblotting showed that 5% of these QLR proteins are expressed at readily detectable levels. Three well-expressed QLR proteins were purified and characterized. Each of these proteins has significant alpha-helical content, is largely resistant to degradation by Pronase, and has a distinct oligomeric structure. In addition, one protein unfolds in a highly cooperative manner. These properties of the QLR proteins demonstrate that they possess folded structures with some native-like properties. The QLR proteins differ from most natural proteins, however, in being remarkably resistant to denaturant-induced and thermal-induced unfolding and in being relatively insoluble in the absence of denaturants.