Family N6


Summary Holotypes Alignment Tree Genomes Structure Literature Architecture

Summary for family N6

Family type peptidaseN06.001 - YscU protein (Yersinia pseudotuberculosis), MEROPS Accession MER0123660 (peptidase unit: 1-354)
Content of familyFamily N6 includes autoprocessing endopeptidases.
History Identifier created: MEROPS 9.3 (7 September 2010)
Pathogenic and symbiotic Gram-negative bacteria can influence some host cells to their advantage by secreting proteins into the host cell cytoplasm. These proteins do not have signal peptides and are secreted by a special mechanism known as the type III secretion system or injectisome. The secretion is mediated by a structure similar to a flagellum, with a basal body and a multimeric filamentous protein structure which protrudes from the bacterial surface and forms a pore in the host cell membrane. The basal body sits in the bacterial cell membrane but opens to the cytoplasm, allowing unfolded proteins to pass along the hollow filament. The FlhB transmembrane protein (N06.002) and its homologues are important when secretion changes from one protein to another (Edqvist et al., 2003). This protein undergoes autoproteloysis at an AsnPro-Thr-His motif (Bjornfot et al. 2009). In Yersinia there are about eleven of these Yersinia Outer membrane Proteins (YOPS), some of which are peptidases in families C55 and C58.
Catalytic typeAsparagine
Active site residuesN263 
Active siteCleavage occurs within the well conserved tetrapeptide AsnPro-Thr-His. In the crystal structure, this tetrapeptide forms a type II beta-turn that is strained, and thus facilitates formation of a hydrogen bond between the carbonyl oxygen of Asn263 and the backbone amide of His266. It is thought that the side chain amide of Asn263 becomes activated and attacks and cleaves the Asn-Pro bond. Cyclization of Asn263 occurs forming a succinamide intermediate which is subsequently hydrolysed (Zarivach et al., 2008). Zarivach et al., 2008 suggest that this cleavage is intein-like.
Molecular structureThe EscU protein has two domains, an N-terminal domain involved in the association with the type III secretion system inner membrane basal ring, and a C-terminal domain where cleavage occurs. Tertiary structures have been determined for the C-terminal domain from several homologous proteins, notably EscU from Escherichia coli and SpaS from Salmonella typhimurium (Zarivach et al., 2008). Each structure shows a four-strand beta sheet surrounded by four helices, which is unique to this family. The structure of EscU is the type structure for clan NB.
Distribution of family Bacteria details  
Archaea -  
Protozoa -  
Fungi -  
Plants details  
Animals details  
Viruses -  
Biological functionsAutoproteolysis is essential for mediating the switch in the secretion of proteins in the type III secretion system.
Statistics for family N6Sequences:1881
Identifiers with PDB entries:4
Downloadable files Sequence library (FastA format)
Sequence alignment (FastA format)
Phylogenetic tree (Newick format)
Other databases INTERPRO IPR006307
PFAM PF01312
SCOP 160545
Peptidases and Homologues MEROPS ID Structure
YscU proteinN06.001Yes
SpaS protein (Salmonella sp.)N06.002Yes
EscU protein (Escherichia coli)N06.003Yes
HrcU protein (Xanthomonas sp.)N06.004-
FlhB protein (Escherichia coli)N06.A01Yes
Family N06 non-peptidase homologuesnon-peptidase homologue-
Family N06 unassigned peptidasesunassigned-