Schematic representation of the primary structures of SptA precursor and its derivatives (A) and amino acid sequence alignment of the CTEs of halolysins SptA (AAX19896) and SptB (AAX19897) from Natrinema sp. J7, 172P1 (P29143) from Nab. asiatica, Nep (AAV66536), a putative subtilisin (Hly natria) (ADD04299) from Nab. magadii, R4 (BAA10958) from Hfx. mediterranei, NRC1 (NP_281139) from Halobacterium sp. NRC-1, and thermitase (1THM_A) from Thermoactinomyces vulgaris (B).
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A, the signal peptide (SP), the N-terminal propeptide (Pro), the catalytic domain and the C-terminal extension (CTE) of SptA precursor are indicated. The locations of four cysteine residues are shown. The numbers on the right or left side of the boxes represent the positions of the C- or N-terminal residues of the proteins starting from the N-terminus of mature enzyme. B, the number (149) in the sequence (Hly natria) represents the number of inserted residues at the position indicated. The numbers on the right side represent the positions of the residues starting from the N-terminus of precursor or mature enzyme (in parentheses). Conserved cysteine residues are boxed. The vertical arrow indicates the cleavage site of the CTE of SptA under reducing conditions. The underlined sequence represents the N-terminal residues of the major autolysis products of SptA under lower salt conditions (Fig. 4A). The positions of C-termini of truncation mutants (SptAΔC125 and SptAΔC76) are indicated above the sequences. The eight β-strands (β1–8) in the CTE of SptA were predicted using DNAMAN software (Lynnon Biosoft Inc.).