SSF50370

Ricin B-like lectins

SUPERFAMILY entry
Member databaseSUPERFAMILY
SUPERFAMILY typehomologous superfamily

Description
Imported from IPR035992

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
[4, 3, 5]
. 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
[2, 1]
. 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
[3, 5]
. A disulphide bond is also conserved in some of the QxW repeats
[3]
.

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
[2]
. The ricin B lectin domain has no major segments of a helix or β-sheet but each of the QxW repeats contains an ω-loop
[1]
. An idealized ω-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.

References
Imported from IPR035992

1.Structure of ricin B-chain at 2.5 A resolution. Rutenber E, Robertus JD. Proteins 10, 260-9, (1991). View articlePMID: 1881882

2.Structure and evolution of ricin B chain. Rutenber E, Ready M, Robertus JD. Nature 326, 624-6, (1987). View articlePMID: 3561502

3.A mosquitocidal toxin with a ricin-like cell-binding domain. Hazes B, Read RJ. Nat. Struct. Biol. 2, 358-9, (1995). View articlePMID: 7664090

4.Novel galactose-binding proteins in Annelida. Characterization of 29-kDa tandem repeat-type lectins from the earthworm Lumbricus terrestris. Hirabayashi J, Dutta SK, Kasai K. J. Biol. Chem. 273, 14450-60, (1998). View articlePMID: 9603958

5.The (QxW)3 domain: a flexible lectin scaffold. Hazes B. Protein Sci. 5, 1490-501, (1996). View articlePMID: 8844840

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