Signal peptide peptidase A (SppA; Peptidase S49; Protease IV): SppA is an intramembrane enzyme found in all three domains of life and is involved in the cleavage of signal peptides after their removal from the precursor proteins by signal peptidases. Unlike the eukaryotic functional homologs that are proposed to be aspartic proteases, site-directed mutagenesis and sequence analysis have shown these bacterial, archaeal and thylakoid SppAs to be ClpP-like serine proteases. The predicted active site serine for members in this family occurs in a transmembrane domain, cleaving peptide bonds in the plane of the lipid bilayer. Mutagenesis studies also suggest that the catalytic center comprises a Ser-Lys dyad (both residues absolutely conserved within bacteria, chloroplast and mitochondrial signal peptidase family members) and not the usual Ser-His-Asp catalytic triad found in the majority of serine proteases. In addition to the carboxyl-terminal protease domain that is conserved in all the S49 family members, the E. coli SppA contains an amino-terminal domain (sometimes referred to as 67K type). Others, including sohB peptidase, protein C, protein 1510-N and archaeal signal peptide peptidase, do not contain the amino-terminal domain (sometimes referred to as 36K type). Interestingly, the single membrane spanning E. coli SppA carries out catalysis using a Ser-Lys dyad with the serine located in the conserved carboxy-terminal protease domain and the lysine in the non-conserved amino-terminal domain. This family also contains homologs that either have been found experimentally to be without peptidase activity, or lack amino acid residues that are believed to be essential for the catalytic activity of peptidases.
Comment:The N- and C-terminal halves of SppA in E. coli are tandem repeats and the Ser/Lys dyad is arranged such that the nucleophile Ser is located in the C-terminal half whereas the general base Lys is located in the N-terminal half.
Structure:3BEZ: Escherichia coli Signal peptide peptidase A (SppA) - View structure with Cn3D