PROSITE documentation PDOC50231
Lectin domain of ricin B chain profile


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 [1,2,3]. 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 (α, β and γ) 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 [4,5]. 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 [2,3]. A disulfide bond is also conserved in some of the QxW repeats [2].

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 [4]. The ricin B lectin domain has no major segments of a helix or β sheet but each of the QxW repeats contains an omega loop [5]. 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.

Some proteins containing a ricin B lectin domain are listed below [3]:

  • Ricin, from Ricinus communis (Castor bean). Ricin belongs to a group of plant AB-toxins, which also contains agglutinin and abrin. Ricin is composed of a sugar-binding subunit (B chain) that attaches to galactose residues presented by cell surface glycoproteins or glycolipids and a subunit (chain A) with enzymatic activity that attacks and inactivates ribosomes. The B chain contains two ricin B lectin domains [4].
  • Serine protease I (RPI) (EC 3.4.21.-), from Rarobacter faecitabidus [6]. RIP is a serine protease exhibiting lytic activity toward living yeast cells. It possess a ricin B lectin domain that is involved in mannose binding in its C terminal part.
  • The bacterial AHH1/ASH4/HlyA/VVHA family of hemolysins. Bacterial hemolysins are exotoxins that attack blood cell membranes and cause cell rupture by mechanisms not clearly defined. Proteins belonging to this family possess one ricin B lectin domain.
  • Glucan endo-1,3-β-glucanase (EC, from Oerskovia xanthineolytica. This yeast-lytic protein has a C-terminal ricin B lectin domain which appears to target the catalytic domain to yeast cell wall.
  • Endo-1,4-β-xylanase A (EC (gene xlnA), from Streptomyces. The C-terminal ricin B lectin domain has been proposed to bind to the polymeric substrate of this enzyme, but no carbohydrate binding data are available.
  • The macrophage mannose receptor (MRC1), from mammals. MRC1 is a Type I membrane receptor protein that is expressed at the surface of mature macrophages. It binds mannose- and fucose-rich carbohydrate polymers and appears to mediate phagocytic uptake of foreign microorganisms. Its extracellular region contains 8 copies of the C-type lectin domain (see <PDOC00537>) and one ricin B lectin domain to which no sugar binding function has been ascribed so far [7].
  • The AIM1 protein, from mammals. AIM1 is a member of the β- γ- crystallin superfamily and contains a C-terminal ricin B lectin domain. In human, it is associated with the control of tumorigenicity.
  • Phospholipase A2 receptor, from mammals. This proteins contains 8 copies of the C-type lectin domain and one N-terminal ricin B lectin domain whose function is unknown.
  • UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase (EC (gene GALNT1). This protein catalyzes the initial reaction in O-linked oligosaccharide biosynthesis, the transfer of an N-acetyl-D-galactosamine residue to a serine or threonine residue on the protein receptor. This protein binds carbohydrate as a soluble substrate and contains one copy of the ricin B lectin domain.
  • The 33-kDa hemagglutinin component of the botulinum neurotoxin complex. This protein has two copies of the ricin B lectin domain and agglutinates red blood cells.

This profile is directed against part of the lambda linker and all of the three QxW repeats.

Last update:

December 2001 / First entry.


Technical section

PROSITE method (with tools and information) covered by this documentation:

RICIN_B_LECTIN, PS50231; Lectin domain of ricin B chain profile  (MATRIX)


1AuthorsHirabayashi J. Dutta S.K. Kasai K.
TitleNovel galactose-binding proteins in Annelida. Characterization of 29-kDa tandem repeat-type lectins from the earthworm Lumbricus terrestris.
SourceJ. Biol. Chem. 273:14450-14460(1998).
PubMed ID9603958

2AuthorsHazes B. Read R.J.
TitleA mosquitocidal toxin with a ricin-like cell-binding domain.
SourceNat. Struct. Biol. 2:358-359(1995).
PubMed ID7664090

3AuthorsHazes B.
TitleThe (QxW)3 domain: a flexible lectin scaffold.
SourceProtein Sci. 5:1490-1501(1996).
PubMed ID8844840

4AuthorsRutenber E. Ready M. Robertus J.D.
TitleStructure and evolution of ricin B chain.
SourceNature 326:624-626(1987).
PubMed ID3561502

5AuthorsRutenber E. Robertus J.D.
TitleStructure of ricin B-chain at 2.5 A resolution.
SourceProteins 10:260-269(1991).
PubMed ID1881882

6AuthorsShimoi H. Iimura Y. Obata T. Tadenuma M.
TitleMolecular structure of Rarobacter faecitabidus protease I. A yeast-lytic serine protease having mannose-binding activity.
SourceJ. Biol. Chem. 267:25189-25195(1992).
PubMed ID1339445

7AuthorsHarris N. Peters L.L. Eicher E.M. Rits M. Raspberry D. Eichbaum Q.G. Super M. Ezekowitz R.A.
TitleThe exon-intron structure and chromosomal localization of the mouse macrophage mannose receptor gene Mrc1: identification of a Ricin-like domain at the N-terminus of the receptor.
SourceBiochem. Biophys. Res. Commun. 198:682-692(1994).
PubMed ID8297379

PROSITE is copyrighted by the SIB Swiss Institute of Bioinformatics and distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives (CC BY-NC-ND 4.0) License, see prosite_license.html.


View entry in original PROSITE document format
View entry in raw text format (no links)