Ca 2+ -Dependent Maturation of Subtilisin from a Hyperthermophilic Archaeon, Thermococcus kodakaraensis : the Propeptide Is a Potent Inhibitor of the Mature Domain but Is Not Required for Its Folding

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Abstract
Subtilisin from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 is a member of the subtilisin family. T. kodakaraensis subtilisin in a proform ( T. kodakaraensis pro-subtilisin), as well as its propeptide ( T. kodakaraensis propeptide) and mature domain ( T. kodakaraensis mat-subtilisin), were independently overproduced in E. coli , purified, and biochemically characterized. T. kodakaraensis pro-subtilisin was inactive in the absence of Ca 2+ but was activated upon autoprocessing and degradation of propeptide in the presence of Ca 2+ at 80°C. This maturation process was completed within 30 min at 80°C but was bound at an intermediate stage, in which the propeptide is autoprocessed from the mature domain ( T. kodakaraensis mat-subtilisin*) but forms an inactive complex with T. kodakaraensis mat-subtilisin*, at lower temperatures. At 80°C, approximately 30% of T. kodakaraensis pro-subtilisin was autoprocessed into T. kodakaraensis propeptide and T. kodakaraensis mat-subtilisin*, and the other 70% was completely degraded to small fragments. Likewise, T. kodakaraensis mat-subtilisin was inactive in the absence of Ca 2+ but was activated upon incubation with Ca 2+ at 80°C. The kinetic parameters and stability of the resultant activated protein were nearly identical to those of T. kodakaraensis mat-subtilisin*, indicating that T. kodakaraensis mat-subtilisin does not require T. kodakaraensis propeptide for folding. However, only ∼5% of T. kodakaraensis mat-subtilisin was converted to an active form, and the other part was completely degraded to small fragments. T. kodakaraensis propeptide was shown to be a potent inhibitor of T. kodakaraensis mat-subtilisin* and noncompetitively inhibited its activity with a K i of 25 ± 3.0 nM at 20°C. T. kodakaraensis propeptide may be required to prevent the degradation of the T. kodakaraensis mat-subtilisin molecules that are activated later by those that are activated earlier.

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