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
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
. 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 . 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
Some proteins containing a ricin B lectin domain are listed below :
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 .
Serine protease I (RPI) (EC 3.4.21.-), from Rarobacter faecitabidus .
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 126.96.36.199), 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 188.8.131.52) (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
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 .
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 184.108.40.206)
(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.
December 2001 / First entry.
PROSITE method (with tools and information) covered by this documentation:
Hirabayashi J. Dutta S.K. Kasai K.
Novel galactose-binding proteins in Annelida. Characterization of 29-kDa tandem repeat-type lectins from the earthworm Lumbricus terrestris.
PROSITE is copyright. It is produced by the SIB Swiss Institute
Bioinformatics. There are no restrictions on its use by non-profit
institutions as long as its content is in no way modified. Usage by and
for commercial entities requires a license agreement. For information
about the licensing scheme send an email to
or see: prosite_license.html.