title: Evolution and thermodynamics of the slow unfolding of hyperstable monomeric proteins.
abstract: BACKGROUND
  The unfolding speed of some hyperthermophilic proteins is dramatically lower than that of their mesostable homologs. Ribonuclease HII from the hyperthermophilic archaeon Thermococcus kodakaraensis (Tk-RNase HII) is stabilized by its remarkably slow unfolding rate, whereas RNase HI from the thermophilic bacterium Thermus thermophilus (Tt-RNase HI) unfolds rapidly, comparable with to that of RNase HI from Escherichia coli (Ec-RNase HI).
  
  
  RESULTS
  To clarify whether the difference in the unfolding rate is due to differences in the types of RNase H or differences in proteins from archaea and bacteria, we examined the equilibrium stability and unfolding reaction of RNases HII from the hyperthermophilic bacteria Thermotoga maritima (Tm-RNase HII) and Aquifex aeolicus (Aa-RNase HII) and RNase HI from the hyperthermophilic archaeon Sulfolobus tokodaii (Sto-RNase HI). These proteins from hyperthermophiles are more stable than Ec-RNase HI over all the temperature ranges examined. The observed unfolding speeds of all hyperstable proteins at the different denaturant concentrations studied are much lower than those of Ec-RNase HI, which is in accordance with the familiar slow unfolding of hyperstable proteins. However, the unfolding rate constants of these RNases H in water are dispersed, and the unfolding rate constant of thermophilic archaeal proteins is lower than that of thermophilic bacterial proteins.
  
  
  CONCLUSIONS
  These results suggest that the nature of slow unfolding of thermophilic proteins is determined by the evolutionary history of the organisms involved. The unfolding rate constants in water are related to the amount of buried hydrophobic residues in the tertiary structure.
authors.affiliation: Department of Material and Life Science, Osaka University, Suita, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Okada
authors.forename: Tomohiro
authors.initials: T
authors.lastname: Okamoto
authors.forename: Atsushi
authors.initials: A
authors.lastname: Mukaiyama
authors.forename: Takashi
authors.initials: T
authors.lastname: Tadokoro
authors.forename: Dong-Ju
authors.initials: DJ
authors.lastname: You
authors.forename: Hyongi
authors.initials: H
authors.lastname: Chon
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Koga
authors.forename: Kazufumi
authors.initials: K
authors.lastname: Takano
authors.forename: Shigenori
authors.initials: S
authors.lastname: Kanaya
mesh_terms: D019843:Archaeal Proteins / Q000235:genetics / Q000378:metabolism; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002942:Circular Dichroism; D019143:Evolution, Molecular; D017510:Protein Folding; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D016914:Ribonuclease H / Q000235:genetics / Q000378:metabolism*; D016944:Sulfolobus / Q000201:enzymology / Q000235:genetics; D013696:Temperature; D013816:Thermodynamics; D020124:Thermotoga maritima / Q000201:enzymology / Q000235:genetics
chemical_list: D019843:Archaeal Proteins; D001426:Bacterial Proteins; C091139:ribonuclease HII; D016914:Ribonuclease H; C078856:ribonuclease HI