InterPro domain: IPR043159

The D-galactoside binding lectin purified from sea urchin (Anthocidaris crassispina) eggs exists as a disulphide-linked homodimer of two subunits; the dimeric form is essential for hemagglutination activity [ 1 ]. The sea urchin egg lectin (SUEL) forms a new class of lectins. Although SUEL was first isolated as a D-galactoside binding lectin, it was latter shown that it bind to L-rhamnose preferentially [ 2 , 2 ]. L-rhamnose and D-galactose share the same hydroxyl group orientation at C2 and C4 of the pyranose ring structure.

A cysteine-rich domain homologous to the SUEL protein has been identified in the following proteins [ 3 , 4 , 5 ]:

• Plant beta-galactosidases ( 3.2.1.23 ) (lactases).
• Mammalian latrophilin, the calcium independent receptor of alpha-latrotoxin (CIRL). The galactose-binding lectin domain is not required for alpha-latratoxin binding [ 6 ].
• Human lectomedin-1.
• Rhamnose-binding lectin (SAL) from catfish (Silurus asotus, Namazu) eggs. This protein is composed of three tandem repeat domains homologous to the SUEL lectin domain. All cysteine positions of each domain are completely conserved [ 6 ].
• The hypothetical B0457.1, F32A7.3A and F32A7.3B proteins from Caenorhabditis elegans.
• The human KIAA0821 protein.

Structurally, the rhamnose-binding lectin domain (also known as the N-terminal lectin domain, Lec) is composed of five beta-strands , a single, long alpha-helix, and two small helical elements. The overall fold is that of a beta-sandwich with two antiparallel sheets [ 6 ].

1. Amino acid sequence and molecular characterization of a D-galactoside-specific lectin purified from sea urchin (Anthocidaris crassispina) eggs. Biochemistry 30, 2391-4
2. Tandem repeat structure of rhamnose-binding lectin from catfish (Silurus asotus) eggs. Biochim. Biophys. Acta 1472, 668-75
3. Alpha-latrotoxin receptor, latrophilin, is a novel member of the secretin family of G protein-coupled receptors. J. Biol. Chem. 272, 21504-8
4. Isolation and characterization of rhamnose-binding lectins from eggs of steelhead trout (Oncorhynchus mykiss) homologous to low density lipoprotein receptor superfamily. J. Biol. Chem. 273, 19190-7
5. Structural requirements for alpha-latrotoxin binding and alpha-latrotoxin-stimulated secretion. A study with calcium-independent receptor of alpha-latrotoxin (CIRL) deletion mutants. J. Biol. Chem. 274, 3590-6
6. Solution structure and sugar-binding mechanism of mouse latrophilin-1 RBL: a 7TM receptor-attached lectin-like domain. Structure 16, 944-53