Fig. 1.

Unmasking alternative fold topologies. (A) Sequence alignment of 3α A₁ and α/β-plait S_a1, with corresponding secondary structure regions at the Top and Bottom of the alignment. The parent sequences from which A₁ and S_a1 are derived, GA and S6 respectively, are also shown. (B) A single amino acid mutation in S_a1 (Center) such as A26D leads to unfolding (Left), while a L79R mutation exposes the alternative 3α state (Right). Color coding is as follows: N-terminal residues 1 to 10 (blue); residues 11 to 66 aligning with the A₁ amino acid sequence (orange); C-terminal residues 67 to 95 (red). (C) Representative chemical shift patterns in 2D ¹H-¹⁵N HSQC spectra of the unfolded (Left), α/β-plait (Center), and 3α states (Right). The HSQC spectrum of S_a1L79R (Right, black) is overlaid with the 56-amino acid 3α-helical protein, A₁ (red).