>TIGR00705 SppA_67K signal peptide peptidase SppA, 67K type; InterPro: IPR004634 Proteolytic enzymes that exploit serine in their catalytic activity are ubiquitous, being found in viruses, bacteria and eukaryotes . They include a wide range of peptidase activity, including exopeptidase, endopeptidase, oligopeptidase and omega-peptidase activity. Over 20 families (denoted S1 - S66) of serine protease have been identified, these being grouped into clans on the basis of structural similarity and other functional evidence . Structures are known for members of the clans and the structures indicate that some appear to be totally unrelated, suggesting different evolutionary origins for the serine peptidases . Not withstanding their different evolutionary origins, there are similarities in the reaction mechanisms of several peptidases. Chymotrypsin, subtilisin and carboxypeptidase C have a catalytic triad of serine, aspartate and histidine in common: serine acts as a nucleophile, aspartate as an electrophile, and histidine as a base . The geometric orientations of the catalytic residues are similar between families, despite different protein folds . The linear arrangements of the catalytic residues commonly reflect clan relationships. For example the catalytic triad in the chymotrypsin clan (PA) is ordered HDS, but is ordered DHS in the subtilisin clan (SB) and SDH in the carboxypeptidase clan (SC) , . Peptidases are grouped into clans and families. Clans are groups of families for which there is evidence of common ancestry. Each clan is identified with two letters, the first representing the catalytic type of the families included in the clan (with the letter 'P' being used for a clan containing families of more than one of the catalytic types serine, threonine and cysteine). Some families cannot yet be assigned to clans, and when a formal assignment is required, such a family is described as belonging to clan A-, C-, M-, S-, T- or U-, according to the catalytic type. Some clans are divided into subclans because there is evidence of a very ancient divergence within the clan, for example MA(E), the gluzincins, and MA(M), the metzincins. Families are grouped by their catalytic type, the first character representing the catalytic type: A, aspartic; C, cysteine; G, glutamic acid; M, metallo; S, serine; T, threonine; and U, unknown. The serine, threonine and cysteine peptidases utilise the amino acid as a nucleophile and form an acyl intermediate - these peptidases can also readily act as transferases. In the case of aspartic, glutamic and metallopeptidases, the nucleophile is an activated water molecule. This group of serine peptidases belong to MEROPS peptidase family S49 (protease IV family, clan S-). The predicted active site serine for members of this family occurs in a transmembrane domain. Signal peptides of secretory proteins seem to serve at least two important biological functions. First, they are required for protein targeting to and translocation across membranes, such as the eubacterial plasma membrane and the endoplasmic reticular membrane of eukaryotes. Second, in addition to their role as determinants for protein targeting and translocation, certain signal peptides have a signalling function. During or shortly after pre-protein translocation, the signal peptide is removed by signal peptidases. The integral membrane protein, SppA (protease IV), of Escherichia coli was shown experimentally to degrade signal peptides. The member of this family from Bacillus subtilis has only been shown to be required for efficient processing of pre-proteins under conditions of hyper-secretion . These enzymes have a molecular mass around 67 kDa and a duplication such that the N-terminal half shares extensive homology with the C-terminal half and was shown in E. coli to form homotetramers. E. coli SohB, which is most closely homologous to the C-terminal duplication of SppA, is predicted to perform a similar function of small peptide degradation, but in the periplasm. Many prokaryotes have a single SppA/SohB homolog that may perform the function of either or both. ; GO: 0009003 signal peptidase activity, 0006465 signal peptide processing, 0016021 integral to membrane.

Probab=92.88  E-value=0.56  Score=28.46  Aligned_cols=96  Identities=21%  Similarity=0.276  Sum_probs=66.9

Q ss_pred             CCCEEEEEEEE-ECEEEEEEEEECHHHCCCCCH-HHHHHHHHHHHHHHHHC-C-CEEEEECCCCCCCCCCHHHHHHHHHH
Q ss_conf             77169998787-041499999833031853577-89999999999998628-9-68999768887765212467777889
Q gi|254780820|r  107 IDSIVSAVGNV-RDFKLVAVVHEFSFIGGSIGI-AAGEAIVKSCERAIAEK-C-PLVMFTASGGARMQEGILSLMQLPRT  182 (284)
Q Consensus       107 ~davv~G~G~I-~G~~vvv~~~df~F~GGSmG~-~~geki~~a~e~A~~~~-~-PlI~~~~SGGaRMqEG~~sL~qMakt  182 (284)
T Consensus       321 ~iaiv~~~G~I~~G~~-------------t~~nG~ggD~~a~lLr~a~~D~~iKAvvLRinSPGG----sv~A---se~I  380 (614)
T TIGR00705       321 KIAIVYLEGSIVDGRD-------------TEENGVGGDTVAALLRKARSDPDIKAVVLRINSPGG----SVFA---SEII  380 (614)
T ss_pred             CEEEEEEECCEECCCC-------------CCCCCCHHHHHHHHHHHHCCCCCCEEEEEEEECCCC----CEEH---HHHH
T ss_conf             1699970064235775-------------667860036799999987079981289988638986----3428---7899


Q ss_pred             HHHHHHHHHCC-CCEEEEECCCCCCEEEEE----ECCCCCEEEEECCCEE
Q ss_conf             99999998629-988998567642011112----0146852555314211
Q gi|254780820|r  183 TIAINMLKDAG-LPYIVVLTNPTTGGVTAS----YAMLGDIHLAEPGAEI  227 (284)
Q Consensus       183 ~~a~~~l~~~~-lP~I~vl~~pt~GGv~AS----~a~lgDiiiaep~a~i  227 (284)
T Consensus       381 R~e~~~~~~~GkKPViv-----SMG~~AASGgYWiasaA~yIvA~p~TiT  425 (614)
T TIGR00705       381 RRELERLQARGKKPVIV-----SMGAMAASGGYWIASAADYIVADPNTIT  425 (614)
T ss_pred             HHHHHHHHHCCCCCEEE-----ECCHHHHCCCCHHCCCCCEEEECCCCCC
T ss_conf             99999998268997898-----4350232053002045571334787431