Effects of segment substitution on the structure and stability of immunoglobulin G binding domain of streptococcal protein G

Structural formation of segments plays pivotal roles in protein folding and stability, but how the segment influences the structural ensemble remains elusive. We engineered two hybrid proteins by replacing the central helical segment of immunoglobulin G binding domain of streptococcal protein G with an α-helix or β₂-strand element of a structural homologue, the immunoglobulin G binding domain of streptococcal protein L. The results show that substitution by the α-helical sequence retains a folded structure predominantly with a three-stranded β-sheet but slightly destabilizes the compact ensemble, while substitution by the β₂-strand sequence completely destroys the structural formation. The finding implies that the local segment may influence the tertiary structure and overall stability, and the tertiary interactions may modulate structural formation of the segment, which might be considered when studying protein folding, prediction, design, and engineering.