Department of Molecular Biology and Biochemistry, Simon Fraser University, South Science Building, 8888 University Drive, Burnaby, British Columbia, Canada.
Signal peptide peptidase (Spp) is the enzyme responsible for cleaving the remnant signal peptides left behind in the membrane following Sec-dependent protein secretion. Spp activity appears to be present in all cell types, eukaryotic, prokaryotic and archaeal. Here we report the first structure of a signal peptide peptidase, that of the Escherichia coli SppA (SppA(EC)). SppA(EC) forms a tetrameric assembly with a novel bowl-shaped architecture. The bowl has a dramatically hydrophobic interior and contains four separate active sites that utilize a Ser/Lys catalytic dyad mechanism. Our structural analysis of SppA reveals that while in many Gram-negative bacteria as well as characterized plant variants, a tandem duplication in the protein fold creates an intact active site at the interface between the repeated domains, other species, particularly Gram-positive and archaeal organisms, encode half-size, unduplicated SppA variants that could form similar oligomers to their duplicated counterparts, but using an octamer arrangement and with the catalytic residues provided by neighboring monomers. The structure reveals a similarity in the protein fold between the domains in the periplasmic Ser/Lys protease SppA and the monomers seen in the cytoplasmic Ser/His/Asp protease ClpP. We propose that SppA may, in addition to its role in signal peptide hydrolysis, have a role in the quality assurance of periplasmic and membrane-bound proteins, similar to the role that ClpP plays for cytoplasmic proteins.