<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>ORF2 of <taxon tax_id="12045">Potato leafroll virus</taxon> (PLrV) encodes a polyprotein which is translated following a -1 frameshift. The polyprotein has a putative linear arrangement of membrane achor-VPg-peptidase-polmerase domains. The serine peptidase domain which is found in this group of sequences belongs to MEROPS peptidase family S39 (clan PA(S)). It is likely that the peptidase domain is involved in the cleavage of the polyprotein [<cite idref="PUB00011866"/>].</p><p>The nucleotide sequence for the RNA of PLrV has been determined [<cite idref="PUB00003126"/>, <cite idref="PUB00001567"/>]. The sequence contains six large open reading frames (ORFs). The 5' coding region encodes two polypeptides of 28K and 70K, which overlap in different reading frames; it is suggested that the third ORF in the 5' block is translated by frameshift readthrough near the end of the 70K protein, yielding a 118K polypeptide [<cite idref="PUB00003126"/>]. Segments of the predicted amino acid sequences of these ORFs resemble those of known viral RNA polymerases, ATP-binding proteins and viral genome-linked proteins. The nucleotide sequence of the genomic RNA of <taxon tax_id="12042">Beet western yellows virus</taxon> (BWYV) has been determined [<cite idref="PUB00004354"/>]. The sequence contains six long ORFs. A cluster of three of these ORFs, including the coat protein cistron, display extensive amino acid sequence similarity to corresponding ORFs of a second luteovirus: <taxon tax_id="12037">Barley yellow dwarf virus</taxon> [<cite idref="PUB00004354"/>].</p> Peptidase S39B, luteovirus