Assessing computational amino acid β-turn propensities with a phage-displayed combinatorial library and directed evolution

Structure propensities of amino acids are important determinants in guiding proteins' local and global structure formation. We constructed a phage display library-a hexa-HIS tag upstream of a CXXC (X stands for any of the 20 natural amino acids) motif appending N-terminal to the minor capsid protein pill of M13KE filamentous phage-and developed a novel directed-evolution procedure to select for amino acid sequences forming increasingly stable beta-turns in the disulfide-bridged CXXC motif. The sequences that emerged from the directed-evolution cycles were in good agreement with type 11 beta-turn propensities derived from surveys of known protein structures, in particular, Pro-Gly forming a type II beta-turn. The agreement strongly supported the notion that beta-turn formation plays an active role in initiating local structure folding in proteins.