second ricin B-type lectin domain, beta-trefoil fold, found in Abrus precatorius abrin, agglutinin-I and similar proteins
This subfamily includes Abrus precatorius abrin (ABR) and agglutinin-I (AAG), which share a high degree of sequence similarity and belong to the type II ribosome-inactivating protein (RIP) family. Type II RIPs inhibit protein synthesis in eukaryotic cells and can also induce apoptosis. They are composed of two polypeptide proteins with a toxic A chain and a lectin-like B chain linked by a disulfide bond. The A chain of abrin and agglutinin-I is responsible for inhibiting protein synthesis through the catalytic inactivation of 60S ribosomal subunits by removing adenine from position 4,324 of 28S rRNA. Agglutinin-I is less toxic than abrin. The B chain is a galactose-specific lectin that facilitates the binding to the cell membrane that precedes endocytosis. The B-chain contains two ricin B-type lectin domains with a beta-trefoil fold, which is characterized by 12 beta strands folded into three similar trefoil subdomains (alpha, beta, and gamma) associated to give an overall structure with pseudo-3-fold symmetry. The model corresponds to the second ricin B-type lectin domain.
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