Fig. 3.

Native Ltn exchanges between two unrelated structures. (A) Ltn10 ↔ Ltn40 interconversion alters all tertiary contacts. Val¹⁵ and Ala⁴⁹ pack together in the Ltn10 hydrophobic core (Left) but are separated by 18 Å in Ltn40 (Right), whereas the converse is true for Leu¹⁴ and Leu⁴⁵. (B) Ltn10 and Ltn40 exhibit markedly different patterns of long-range contacts. NOE distance constraints for Ltn10 (blue circles) and Ltn40 (orange circles, intramolecular; red circles, intermolecular) are plotted below the diagonal. Close contacts (<2.5 Å) observed in >80% of the NMR structure ensemble are plotted above the diagonal for residues of Ltn10 (blue squares) and Ltn40 (white squares, intramolecular; red squares, intermolecular). (C) Relative changes in solvent-accessible surface (SAS) calculated as SAS_Ltn40 − SAS_Ltn10, expressed as the percentage of total surface area for each side chain. Highlighted residues are more solvent-exposed in either Ltn10 (cyan) or Ltn40 (orange). Residues on the Ltn10 surface (orange) reside in the core of the Ltn40 structure, whereas Ltn40 surface residues (cyan) are located in the Ltn10 interior. (D) (Left) The odd-numbered residues of β1 (orange), β2 (cyan), and β3 (orange) contribute to the Ltn10 core. (Right) In the Ltn40 native-state conformer, the odd-numbered side chains of β1 and β3 pack in the dimer interface, whereas the odd-numbered residues of β2 are reoriented to the opposite face of the β-sheet and reside on the surface. (E) Rearrangement of hydrogen bonds defining the Ltn secondary structure. Each bar denotes a pair of backbone N–H···O = C hydrogen bonds connecting β1–β2 (cyan), β2–β3 (orange), and β0–β3 (green). Ltn10 ↔ Ltn40 interconversion shifts β2 by one residue relative to β1 and β3, which rotate 180° and establish a new hydrogen bond pattern with residues of β0 and β2.