RICIN

Ricin-type beta-trefoil
RICIN
SMART accession number:SM00458
Description: Carbohydrate-binding domain formed from presumed gene triplication.
Interpro abstract (IPR000772): Ricin is a legume lectin from the seeds of the castor bean plant, Ricinus communis. The seeds are poisonous to people, animals and insects and just one milligram of ricin can kill an adult.

Primary structure analysis has shown the presence of a similar domain in many carbohydrate-recognition proteins like plant and bacterial AB-toxins, glycosidases or proteases [(PUBMED:9603958), (PUBMED:7664090), (PUBMED:8844840)]. This domain, known as the ricin B lectin domain, can be present in one or more copies and has been shown in some instance to bind simple sugars, such as galactose or lactose.

The ricin B lectin domain is composed of three homologous subdomains of 40 amino acids (alpha, beta and gamma) and a linker peptide of around 15 residues (lambda). It has been proposed that the ricin B lectin domain arose by gene triplication from a primitive 40 residue galactoside-binding peptide [(PUBMED:3561502), (PUBMED:1881882)]. The most characteristic, though not completely conserved, sequence feature is the presence of a Q-W pattern. Consequently, the ricin B lectin domain as also been refered as the (QxW)3 domain and the three homologous regions as the QxW repeats [(PUBMED:7664090), (PUBMED:8844840)]. A disulphide bond is also conserved in some of the QxW repeats [(PUBMED:7664090)].

The 3D structure of the ricin B chain has shown that the three QxW repeats pack around a pseudo threefold axis that is stabilised by the lambda linker [(PUBMED:3561502)]. The ricin B lectin domain has no major segments of a helix or beta sheet but each of the QxW repeats contains an omega loop [(PUBMED:1881882)]. An idealized omega-loop is a compact, contiguous segment of polypeptide that traces a 'loop-shaped' path in three-dimensional space; the main chain resembles a Greek omega.

Family alignment:
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There are 2458 RICIN domains in 2231 proteins in SMART's nrdb database.

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