Importance of polypeptide chain length for the correct local folding of a β-sheet protein

Equine β-lactoglobulin is a 162-residue β-sheet protein. A partially folded form of equine β-lactoglobulin contains a β-hairpin and an α-helix. The β-hairpin converts into non-native α-helices at temperatures < 0 °C. CHIBL, a truncated equine β-lactoglobulin (residues 88-142), contains the low-temperature α-helical structure even at room temperature, indicating that the interactions responsible for the stability of the β-hairpin reside in non-CHIBL residues. For the study reported herein, we characterized two truncated mutants and their leucine103 → proline103 variants to identify residues that stabilize the β-hairpin. The dependence of their circular dichroism spectra on chloride ion concentration and temperature revealed that the ability to transition from the non-native α-helices to the β-hairpin depends on the polypeptide chain length and improves as the chain length increases despite the apparent absence of any ordered structure in the extended sequences.

Highlights► An α-helix is converted into a β-structure during equine β-lactoglobulin folding. ► This α → β transition can be induced by the unstructured polypeptide chain. ► The longer the chain, the more stable the β-structure.