title: Protein core adaptability: crystal structures of the cavity-filling variants of Escherichia coli RNase HI.
abstract: It has generally been accepted that an increase in protein stability is proportional to the increase in hydrophobicity. When a cavity is created by large-to-small substitutions of amino acid residues in protein cores, protein stability decreases 5.3 kJ/mol per single methyl(ene) group removal. In contrast, many reported cavity-filling mutations either failed to increase stability or produced marginal increases in stability; even in successful cases, the increase in stability was much lower than expected from the cost of single methyl(ene) group removal in cavity-creating mutations. Previously it was found that some cavity-filling mutant proteins at Ala52 in E. coli RNase HI increased stability, but decreased activity and they did not increase the stability to the degree expected by the hydrophobic effect alone. The present study attempted to structurally analyze these variant proteins, and it was found that substitutions have little effect on the overall fold but cause conformational strains with the neighboring residues. The present results and literature on cavity-creating/-filling variants provide insight into protein architecture, indicating that natural protein cores are able to accommodate larger side-chain residue by substitution; in other words, excess-packing may not be chosen in natural selection.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
authors.forename: Masaki
authors.initials: M
authors.lastname: Tanaka
authors.forename: Hyongi
authors.initials: H
authors.lastname: Chon
authors.forename: Clement
authors.initials: C
authors.lastname: Angkawidjaja
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: D018360:Crystallography, X-Ray / Q000379:methods*; D029968:Escherichia coli Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D057927:Hydrophobic and Hydrophilic Interactions; D055550:Protein Stability; D017433:Protein Structure, Secondary; D016914:Ribonuclease H / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D014961:X-Ray Diffraction
chemical_list: D029968:Escherichia coli Proteins; D016914:Ribonuclease H; C078856:ribonuclease HI