title: Extreme temperature tolerance of a hyperthermophilic protein coupled to residual structure in the unfolded state.
abstract: Understanding the mechanisms that dictate protein stability is of large relevance, for instance, to enable design of temperature-tolerant enzymes with high enzymatic activity over a broad temperature interval. In an effort to identify such mechanisms, we have performed a detailed comparative study of the folding thermodynamics and kinetics of the ribosomal protein S16 isolated from a mesophilic (S16(meso)) and hyperthermophilic (S16(thermo)) bacterium by using a variety of biophysical methods. As basis for the study, the 2.0 A X-ray structure of S16(thermo) was solved using single wavelength anomalous dispersion phasing. Thermal unfolding experiments yielded midpoints of 59 and 111 degrees C with associated changes in heat capacity upon unfolding (DeltaC(p)(0)) of 6.4 and 3.3 kJ mol(-1) K(-1), respectively. A strong linear correlation between DeltaC(p)(0) and melting temperature (T(m)) was observed for the wild-type proteins and mutated variants, suggesting that these variables are intimately connected. Stopped-flow fluorescence spectroscopy shows that S16(meso) folds through an apparent two-state model, whereas S16(thermo) folds through a more complex mechanism with a marked curvature in the refolding limb indicating the presence of a folding intermediate. Time-resolved energy transfer between Trp and N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl iodoacetamide of proteins mutated at selected positions shows that the denatured state ensemble of S16(thermo) is more compact relative to S16(meso). Taken together, our results suggest the presence of residual structure in the denatured state ensemble of S16(thermo) that appears to account for the large difference in quantified DeltaC(p)(0) values and, in turn, parts of the observed extreme thermal stability of S16(thermo). These observations may be of general importance in the design of robust enzymes that are highly active over a wide temperature span.
authors.affiliation: Department of Chemistry, University of Umeå, SE-901 87 Umeå, Sweden.
authors.forename: Marcus
authors.initials: M
authors.lastname: Wallgren
authors.forename: Jörgen
authors.initials: J
authors.lastname: Adén
authors.forename: Olena
authors.initials: O
authors.lastname: Pylypenko
authors.forename: Therese
authors.initials: T
authors.lastname: Mikaelsson
authors.forename: Lennart B-A
authors.initials: LB
authors.lastname: Johansson
authors.forename: Alexey
authors.initials: A
authors.lastname: Rak
authors.forename: Magnus
authors.initials: M
authors.lastname: Wolf-Watz
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D001896:Boron Compounds; D016993:Chlamydophila pneumoniae / Q000737:chemistry / Q000235:genetics; D018360:Crystallography, X-Ray; D005456:Fluorescent Dyes; D006358:Hot Temperature; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D012269:Ribosomal Proteins / Q000737:chemistry* / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D016963:Thermus thermophilus / Q000737:chemistry / Q000235:genetics; D014364:Tryptophan / Q000737:chemistry
chemical_list: C095489:4,4-difluoro-4-bora-3a,4a-diaza-s-indacene; D001426:Bacterial Proteins; D001896:Boron Compounds; D005456:Fluorescent Dyes; D011994:Recombinant Proteins; D012269:Ribosomal Proteins; C023509:ribosomal protein S16; D014364:Tryptophan