Solution structures and backbone dynamics of the ribosomal protein S6 and its permutant P54‐55

The ribosomal protein S6 from Thermus thermophilus has served as a model system for the study of protein folding, especially for understanding the effects of circular permutations of secondary structure elements. This study presents the structure of a permutant protein, the 96‐residue P^54‐55, and the structure of its 101‐residue parent protein S6^wt in solution. The data also characterizes the effects of circular permutation on the backbone dynamics of S6. Consistent with crystallographic data on S6^wt, the overall solution structures of both P^54‐55 and S6^wt show a β‐sheet of four antiparallel β‐strands with two α‐helices packed on one side of the sheet. In clear contrast to the crystal data, however, the solution structure of S6^wt reveals a disordered loop in the region between β‐strands 2 and 3 (Leu43‐Phe60) instead of a well‐ordered stretch and associated hydrophobic mini‐core observed in the crystal structure. Moreover, the data for P^54‐55 show that the joined wild‐type N‐ and C‐terminals form a dynamically robust stretch with a hairpin structure that complies with the in silico design. Taken together, the results explain why the loop region of the S6^wt structure is relatively insensitive to mutational perturbations, and why P^54‐55 is more stable than S6^wt: the permutant incision at Lys54‐Asp55 is energetically neutral by being located in an already disordered loop whereas the new hairpin between the wild‐type N‐ and C‐termini is stabilizing.