title: Directed evolution of Tk-subtilisin from a hyperthermophilic archaeon: identification of a single amino acid substitution responsible for low-temperature adaptation.
abstract: Tk-subtilisin from the hyperthermophilic archaeon Thermococcus kodakaraensis is synthesized in a prepro-form (prepro-Tk-subtilisin), secreted in a pro-form (pro-Tk-subtilisin), and matured to an active form (mat-Tk-subtilisin*; a Ca(2+)-bound active form of matured domain of Tk-subtilisin) upon autoprocessing and degradation of the propeptide [Tk-propeptide; propeptide of Tk-subtilisin (Gly1-Leu69)]. Pro-Tk-subtilisin exhibited halo-forming activity only at 80 degrees C, but not at 70 and 60 degrees C, because Tk-propeptide is not effectively degraded by mat-Tk-subtilisin* and forms an inactive complex with mat-Tk-subtilisin* at <80 degrees C. Random mutagenesis in the entire prepro-Tk-subtilisin gene, followed by screening for mutant proteins with halo-forming activity at 70 and 60 degrees C, allowed us to identify single Gly56 --> Ser mutation in the propeptide region responsible for low-temperature adaptation of pro-Tk-subtilisin. SDS-PAGE analyses and mat-Tk-subtilisin* activity assay of pro-G56S-subtilisin indicated more rapid maturation than pro-Tk-subtilisin. The resultant active form was indistinguishable from mat-Tk-subtilisin* in activity and stability, indicating that Gly56 --> Ser mutation does not seriously affect the folding of the mature domain. However, this mutation greatly destabilized the propeptide, making it unstructured in an isolated form. As a result, Tk-propeptide with Gly56 --> Ser mutation (G56S-propeptide) was more susceptible to proteolytic degradation and less effectively inhibited mat-Tk-subtilisin* activity than Tk-propeptide. These results suggest that pro-G56S-subtilisin is more effectively matured than pro-Tk-subtilisin at lower temperatures, because autoprocessed G56S-propeptide is unstructured upon dissociation from mat-Tk-subtilisin* and is therefore effectively degraded by mat-Tk-subtilisin*.
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan.
authors.forename: M A
authors.initials: MA
authors.lastname: Pulido
authors.forename: Y
authors.initials: Y
authors.lastname: Koga
authors.forename: K
authors.initials: K
authors.lastname: Takano
authors.forename: S
authors.initials: S
authors.lastname: Kanaya
mesh_terms: D019943:Amino Acid Substitution; D019020:Directed Molecular Evolution; D011498:Protein Precursors; D020860:Subtilisin / Q000235:genetics* / Q000502:physiology; D013696:Temperature; D019712:Thermococcus / Q000201:enzymology* / Q000235:genetics
chemical_list: D011498:Protein Precursors; D020860:Subtilisin