Ricin B lectin Ricin is a legume lectin from the seeds of the castor bean plant, <taxon tax_id="3988">Ricinus communis</taxon>. The seeds are poisonous to people, animals and insects and just one milligram of ricin can kill an adult. <p>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 [<cite idref="PUB00006197"/>, <cite idref="PUB00006199"/>, <cite idref="PUB00006198"/>]. 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.</p><p>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 [<cite idref="PUB00006201"/>, <cite idref="PUB00006200"/>]. 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 [<cite idref="PUB00006199"/>, <cite idref="PUB00006198"/>]. A disulphide bond is also conserved in some of the QxW repeats [<cite idref="PUB00006199"/>].</p><p>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 [<cite idref="PUB00006201"/>]. 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 [<cite idref="PUB00006200"/>]. 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.</p>