Effect of linker sequence on the stability of circularly permuted variants of ribonuclease T1

Circularly permuted variants of ribonuclease T1 were constructed with a library of residues covalently linking the original amino and carboxyl terminal ends of the wild-type protein. The library of linking peptides consisted of three amino acids containing any combination of proline, aspartate, asparagine, serine, threonine, tyrosine, alanine, and histidine. Forty two unique linker sequences were isolated and 10 of these mutants were further characterized with regard to catalytic activity and overall thermodynamic stability. The 10 mutants with the different linking sequences (HPD, TPH, DTD, TPD, PYH, PAT, PHP, DSS, SPP, and TPS), in addition to GGG and GPG, were 4.0–6.2 kcal/mol less stable than the wild-type ribonuclease T1. However, these circular permuted variants were only 0.4–2.6 kcal/mol less stable than the direct parent protein that is missing the disulfide bond connecting residues 2 and 10. The most stable linking peptide was HPD.