Oligomerization of uniquely folded mini-protein motifs: development of a homotrimeric betabetaalpha peptide.

The discovery of a discretely folded homotrimeric betabetaalpha motif (BBAT1) was recently reported (J. Am. Chem. Soc. 2001, 123, 1002-1003). Herein the design, synthesis, and analysis of a small library of peptides which led to the isolation of BBAT1 is described. betabetaalpha peptides based on the monomeric sequence of BBA5 (Folding Des. 1998, 120, 95-103) were synthesized to include the anthranilic acid/nitrotyrosine fluorescence quenching pair to rapidly detect interpeptide association. In the first generation of peptides synthesized, truncations in the loop region connecting the beta-hairpin to the alpha-helix revealed that a two-residue deletion in the loop promoted an interpeptide association as detected by fluorescence quenching. An additional library of 22 loop-truncated betabetaalpha peptides was subsequently synthesized to include a variety of sequence mutations in an effort to enhance the observed peptide-peptide binding. From the fluorescence quenching screen, peptide B2 was found to possess the strongest fluorescence-quenching response, indicative of a strong peptide-peptide association. Due the poor solubility of peptide B2, the S-methylated cysteine at position 9 in the loop was substituted with a glycine to generate peptide BBAT1 which possessed greatly improved water solubility and formed discrete trimers. The successful design of this oligomeric betabetaalpha structure will likely aid the design of more complex alpha-beta superstructures and further our understanding of the factors controlling protein-protein interactions at alpha-beta protein interfaces.