An intracellular protease of the crenarchaeon Sulfolobus solfataricus, which has sequence similarity to eukaryotic peptidases of the CD clan

We purified from crude extracts of the hyperthermophilic crenarchaeon Sulfolobus solfataricus a protease that is able to hydrolyse proteins with a pH optimum of 7.5 and a temperature optimum of 70 degrees C. Assays in the presence of classical protease inhibitors showed that the hydrolytic activity is sensitive to thiol-blocking reagents. Fluorescence assays using synthetic peptides demonstrated that the protease has a preference for cleaving glutamic acid residues. The first 12 residues of the protease match the N-terminus residues of a hypothetical protein in the S. solfataricus genome of 95 amino acids in length and calculated molecular mass of 11072 Da. The whole sequence of the protease is not related to any known protein, but it bears a segment which is highly similar to one containing the active cysteine residue in eukaryotic peptidases known as legumains. This is the first protease isolated from S. solfataricus capable of degrading native proteins effectively. Our results add to the knowledge of the intracellular proteolytic machine in hyperthermophilic microorganisms.