Summary: Ricin-type beta-trefoil lectin domain
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Ricin-type beta-trefoil lectin domain Provide feedback
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Internal database links
|SCOOP:||ET CDtoxinA Botulinum_HA-17 RicinB_lectin_2 Inhibitor_I66|
|Similarity to PfamA using HHSearch:||RicinB_lectin_2 RicinB_lectin_2|
External database links
This tab holds annotation information from the InterPro database.
InterPro entry IPR000772Ricin 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.
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This family corresponds to a large set of related beta-trefoil proteins . The beta-trefoil is formed by six two-stranded hairpins . Three of these form a barrel structure and the other three are in a triangular array that caps the barrel. The arrangement of the secondary structures gives the molecules a pseudo 3-fold axis.
The clan contains the following 16 members:AbfB Agglutinin Botulinum_HA-17 CDtoxinA DUF569 Fascin FGF FRG1 IL1 Inhibitor_I66 Ins145_P3_rec Kunitz_legume MIR Ricin_B_lectin RicinB_lectin_2 Toxin_R_bind_C
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1Cannot generate PP/Heatmap alignments for seeds; no PP data available
Key: available, not generated, — not available.
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Curation and family details
|Number in seed:||85|
|Number in full:||7775|
|Average length of the domain:||119.30 aa|
|Average identity of full alignment:||25 %|
|Average coverage of the sequence by the domain:||29.34 %|
|HMM build commands:||
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 80369284 -E 1000 --cpu 4 HMM pfamseq
|Family (HMM) version:||18|
|Download:||download the raw HMM for this family|
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For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the Ricin_B_lectin domain has been found. There are 217 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein seqence.
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