Signal peptide peptidase A (SppA) 36K type, a serine protease, has catalytic Ser-Lys dyad
Signal peptide peptidase A (SppA; Peptidase S49; Protease IV) 36K type: SppA is 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. Members in this subfamily are all bacterial and include sohB peptidase and protein C. These are sometimes referred to as 36K type since they contain only one domain, unlike E. coli SppA that also contains an amino-terminal domain. Site-directed mutagenesis and sequence analysis have shown these SppAs to be serine proteases. The predicted active site serine for members in this family occurs in a transmembrane domain. Mutagenesis studies also suggest that the catalytic center comprises a Ser-Lys dyad and not the usual Ser-His-Asp catalytic triad found in the majority of serine proteases.
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.