Peptidase A8, signal peptidase II (Peptidase_A8)
Type: Family

This group of aspartic endopeptidases belong to the
MEROPS
peptidase family A8 (signal peptidase II family). The type example is the Escherichia coli lipoprotein signal peptidase or SPase II (

, MEROPS identifier A08.001), which removes the signal peptide from the N terminus of the murein prolipoprotein, an essential step in production of the bacterial cell wall. This enzyme recognises a conserved sequence known as the "lipobox sequence" (Leu-Xaa-Yaa+Cys, in which Xaa is Ala or Ser and Yaa is Gly or Ala) and cleaves on the amino side of the cysteine residue to which a glyceride-fatty acid lipid is attached. SPase II is an integral membrane protein with four transmembrane regions, with the active site on the periplasmic side and close to the membrane surface. The active site aspartic acid residues have been identified by site-directed mutagenesis and occur in the motifs GNXXDRX and FNXAD, where X is a hydrophobic residue . The enzyme is inhibited by the cyclic pentapeptide antibiotic globomycin and also by pepstatin . Although no tertiary structure has been solved, proteins in this family are unlikely to have similar folds to any other aspartic peptidase, and family A8 is assigned to is own clan, AC.

Homologues are found only in bacteria. Most bacteria have one homologue, but a few bacteria, including
Pseudomonas fluorescens
and
Staphylococcus epidermidis
, have two family members. Predicted homologues in eukaryotes are probably derived from contaminants.

Aspartic peptidases, also known as aspartyl proteases ([ec:3.4.23.-]), are widely distributed proteolytic enzymes known to exist in vertebrates, fungi, plants, protozoa, bacteria, archaea, retroviruses and some plant viruses. All known aspartic peptidases are endopeptidases. A water molecule, activated by two aspartic acid residues, acts as the nucleophile in catalysis. Aspartic peptidases can be grouped into five clans, each of which shows a unique structural fold .

Peptidases in clan AA are either bilobed (family A1 or the pepsin family) or are a homodimer (all other families in the clan, including retropepsin from HIV-1/AIDS) . Each lobe consists of a single domain with a closed β-barrel and each lobe contributes one Asp to form the active site. Most peptidases in the clan are inhibited by the naturally occurring small-molecule inhibitor pepstatin .

Clan AC contains the single family A8: the signal peptidase 2 family. Members of the family are found in all bacteria. Signal peptidase 2 processes the premurein precursor, removing the signal peptide. The peptidase has four transmembrane domains and the active site is on the periplasmic side of the cell membrane. Cleavage occurs on the amino side of a cysteine where the thiol group has been substituted by a diacylglyceryl group. Site-directed mutagenesis has identified two essential aspartic acid residues which occur in the motifs GNXXDRX and FNXAD (where X is a hydrophobic residue) . No tertiary structures have been solved for any member of the family, but because of the intramembrane location, the structure is assumed not to be pepsin-like.

Clan AD contains two families of transmembrane endopeptidases: A22 and A24. These are also known as "GXGD peptidases" because of a common GXGD motif which includes one of the pair of catalytic aspartic acid residues. Structures are known for members of both families and show a unique, common fold with up to nine transmembrane regions . The active site aspartic acids are located within a large cavity in the membrane into which water can gain access .

Clan AE contains two families, A25 and A31. Tertiary structures have been solved for members of both families and show a common fold consisting of an α-β-α sandwich, in which the β sheet is five stranded .

Clan AF contains the single family A26. Members of the clan are membrane-proteins with a unique fold. Homologues are known only from bacteria. The structure of omptin (also known as OmpT) shows a cylindrical barrel containing ten β strands inserted in the membrane with the active site residues on the outer surface .

There are two families of aspartic peptidases for which neither structure nor active site residues are known and these are not assigned to clans. Family A5 includes thermopsin, an endopeptidase found only in thermophilic archaea. Family A36 contains sporulation factor SpoIIGA, which is known to process and activate sigma factor E, one of the transcription factors that controls sporulation in bacteria .