Cloning of Genes of the Aminopeptidase T Family from Thermus Thermophilus HB8 and Bacillus Stearothermophilus NCIB8924: Apparent Similarity to the Leucyl Aminopeptidase Family

To obtain genes with sequence similarity to aminopeptidase T (AP-T) of Thermus aquaticus YT-1, we cloned the genes encoding aminopeptidase Th (AP-Th) from Thermus thermophilus HB8 and aminopeptidase II (APII) from Bacillus stearothermophilus NCIB8924. The AP-Th gene encoded a polypeptide of 408 amino acid residues and the deduced molecular weight of this subunit was 45,015. The APII gene encoded a polypeptide of 413 amino acid residues with a deduced molecular weight of 46,207. The extent of amino acid sequence similarity between AP-Th and AP-T was 86%, and that between APII and AP-T was 43%. The substrate specificities of these expressed enzymes were similar, and each efficiently hydrolyzed leucyl- or phenyl-peptide substrates. Since the deduced amino acid sequence of these enzymes show no similarity to other known aminopeptidases, they appear to comprise an independent family of peptidases, designated the AP-T family. However, a conserved region within the enzymes of the AP-T family shows similarity to the active site signature of the leucyl aminopeptidase family, suggesting that these enzymes may belong to the leucyl aminopeptidase superfamily.