Peptidase S24/S26A/S26B/S26C <p>In the MEROPS database peptidases and peptidase homologues are grouped into clans and families. Clans are groups of families for which there is evidence of common ancestry based on a common structural fold:</p><ul> <li>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-, N-, 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.</li><li>Peptidase families are grouped by their catalytic type, the first character representing the catalytic type: A, aspartic; C, cysteine; G, glutamic acid; M, metallo; N, asparagine; 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. In the case of the asparagine endopeptidases, the nucleophile is asparagine and all are self-processing endopeptidases. </li></ul><p>In many instances the structural protein fold that characterises the clan or family may have lost its catalytic activity, yet retain its function in protein recognition and binding. </p><p>Proteolytic enzymes that exploit serine in their catalytic activity are ubiquitous, being found in viruses, bacteria and eukaryotes [<cite idref="PUB00003576"/>]. 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 [<cite idref="PUB00003576"/>]. 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 [<cite idref="PUB00003576"/>].</p><p>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 [<cite idref="PUB00003576"/>]. The geometric orientations of the catalytic residues are similar between families, despite different protein folds [<cite idref="PUB00003576"/>]. 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) [<cite idref="PUB00003576"/>, <cite idref="PUB00000522"/>].</p><p>This entry represents a stuctural domain superfamily found in serine peptidases belonging to MEROPS peptidase families: S24 (LexA family, clan SF); S26A (signal peptidase I), S26B (signalase) and S26C TraF peptidase. This domain has a complex fold made of several coiled beta-sheets, which contains an SH3-like barrel structure.</p><p>The S26 family includes <taxon tax_id="562">Escherichia coli</taxon> signal peptidase, SPase, which is a membrane-bound endopeptidase with two N-terminal transmembrane segments and a C-terminal catalytic region [<cite idref="PUB00014247"/>]. SPase functions to release proteins that have been translocated into the inner membrane from the cell interior, by cleaving off their signal peptides. In SPase proteins, this domain is disrupted by the insertion of an additional all-beta subdomain. </p><p> <ul> <li> <b>Note: This signature covers both the SH3-like barrel beta-ribbon domain (see <db_xref db="INTERPRO" dbkey="IPR011056"/>) and the all-beta subdomain (see <db_xref db="INTERPRO" dbkey="IPR019766"/>) inserted into it.</b> </li> </ul> </p><p> The S24 family includes:<ul> <li>the lambda repressor CI/C2 family and related bacterial prophage repressor proteins [<cite idref="PUB00014015"/>]. </li><li>LexA, the repressor of genes in the cellular SOS response to DNA damage [<cite idref="PUB00011869"/>]. </li><li>MucA and the related UmuD proteins, which are lesion-bypass DNA polymerases, induced in response to mitogenic DNA damage [<cite idref="PUB00014246"/>]. UmuD is self-processed by its own serine protease activity during the SOS response.</li><li>RulA, a component of the rulAB locus that confers resistance to UV.</li></ul> </p><p>All of these proteins, with the possible exception of RulA, interact with RecA, which activates self cleavage either derepressing transcription in the case of CI and LexA [<cite idref="PUB00011870"/>] or activating the lesion-bypass polymerase in the case of UmuD and MucA. UmuD'2, is the homodimeric component of DNA pol V, which is produced from UmuD by RecA-facilitated self-cleavage. The first 24 N-terminal residues of UmuD are removed; UmuD'2 is a DNA lesion bypass polymerase [<cite idref="PUB00011870"/>, <cite idref="PUB00011871"/>]. MucA [<cite idref="PUB00011872"/>, <cite idref="PUB00011873"/>], like UmuD, is a plasmid encoded a DNA polymerase (pol RI) which is converted into the active lesion-bypass polymerase by a self-cleavage reaction involving RecA [<cite idref="PUB00011874"/>].</p><p>This group of proteins also contains proteins not recognised as peptidases as well as those classified as non-peptidase homologues as they either have been found experimentally to be without peptidase activity, or lack amino acid residues that are believed to be essential for catalytic activity.</p>