<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>Cysteine peptidases have characteristic molecular topologies, which can be seen not only in their three-dimensional structures, but commonly also in the two-dimensional structures. These are peptidases in which the nucleophile is the sulphydryl group of a cysteine residue. Cysteine proteases are divided into clans (proteins which are evolutionary related), and further sub-divided into families, on the basis of the architecture of their catalytic dyad or triad [<cite idref="PUB00011704"/>]. </p><p>Nuclear inclusion A (NIA) proteases from potyviruses are cysteine peptidases belong to the MEROPS peptidase family C4 (NIa protease family, clan PA(C)) [<cite idref="PUB00003577"/>, <cite idref="PUB00004506"/>]. </p><p>Potyviruses include plant viruses in which the single-stranded RNA encodes a polyprotein with NIA protease activity, where proteolytic cleavage is specific for Gln+Gly sites. The NIA protease acts on the polyprotein, releasing itself by Gln+Gly cleavage at both the N- and C-termini. It further processes the polyprotein by cleavage at five similar sites in the C-terminal half of the sequence. In addition to its C-terminal protease activity, the NIA protease contains an N-terminal domain that has been implicated in the transcription process [<cite idref="PUB00003577"/>].</p><p>This peptidase is present in the nuclear inclusion protein of potyviruses.</p> Peptidase C4, potyvirus nuclear inclusion A