Exploring hydrophilic sequence space to search for uncharted foldable proteins by AlphaFold2

Proteins typically fold into unique three-dimensional structures largely driven by interactions between hydrophobic amino acids. This understanding has helped improve our knowledge of protein folding. However, recent research has shown an exception to this idea, demonstrating that specific threonine-rich peptides have a strong tendency to form β-hairpin structures, even in the highly hydrophilic amino acid sequences. This finding suggests that the hydrophilic amino acid sequence space still leaves room for exploring foldable amino acid sequences. In this study, we conducted a systematic exploration of the repetitive amino acid sequence space by AlphaFold2 (AF2), with a focus on sequences composed exclusively of hydrophilic residues, to investigate their potential for adopting unique structures. As a result, the sequence space exploration suggested that several repetitive threonine-rich sequences adopt distinctive conformations and these conformational shapes can be influenced by the length of the sequence unit. Moreover, the analysis of structural dataset suggested that threonine contributes to the structural stabilization by forming non-polar atom packing that tolerates unsatisfied hydrogen bonds, and while also supporting other residues in forming hydrogen bonds. Our findings will broaden the horizons for the discovery of foldable amino acid sequences consisting solely of hydrophilic residues and help us clarify the unknown mechanisms of protein structural stabilization.