InterPro domain: IPR016187

Lectins occur in plants, animals, bacteria and viruses. Initially described for their carbohydrate-binding activity [ 1 ], they are now recognised as a more diverse group of proteins, some of which are involved in protein-protein, protein-lipid or protein-nucleic acid interactions [ 2 ]. There are at least twelve structural families of lectins, of which C-type (Ca+-dependent) lectins is one. C-type lectins can be further divided into seven subgroups based on additional non-lectin domains and gene structure: (I) hyalectans, (II) asialoglycoprotein receptors, (III) collectins, (IV) selectins, (V) NK group transmembrane receptors, (VI) macrophage mannose receptors, and (VII) simple (single domain) lectins [ 3 ].

This entry represents a structural domain found in C-type lectins, as well as in other proteins, including:

• The N-terminal domain of aerolysin [ 4 ] and the N-terminal domain of the S2/S3 subunit of pertussis toxin [ 5 ].
• The C-terminal domain of invasin [ 6 ] and intimin [ 7 ].
• Link domain, which includes the Link module of TSG-6 [ 8 ] (a hyaladherin with important roles in inflammation and ovulation) and the hyaluronan binding domain of CD44 (which contains extra N-terminal beta-strand and C-terminal beta-hairpin) [ 9 ].
• Endostatin [ 10 ] and the endostatin domain of collagen alpha 1 (XV) [ 11 ], these domains being decorated with many insertions in the common fold.
• The noncollagenous (NC1) domain of collagen IV, which consists of a duplication of the C-type lectin domain, with segment swapping within and between individual domains [ 12 ].
• Sulphatase-modifying factors (C-alpha-formyglycine-generating enzyme), where the fold is decorated with many additional structures [ 13 , 14 ].
• The C-terminal domain of the major tropism determinant (Mtd), where the fold is decorated with many additional structures, and has an overall similarity to the sulphatase modifying factor family but lacking the characteristic disulphide [ 15 ].

1. The structural basis for carbohydrate recognition by lectins. Adv. Exp. Med. Biol. 491, 1-16
2. Animal lectins: a historical introduction and overview. Biochim. Biophys. Acta 1572, 187-97
3. Divergent roles for C-type lectins expressed by cells of the innate immune system. Mol. Immunol. 41, 1109-21
4. Structure of the Aeromonas toxin proaerolysin in its water-soluble and membrane-channel states. Nature 367, 292-5
5. Crystal structure of the pertussis toxin-ATP complex: a molecular sensor. J. Mol. Biol. 258, 661-71
6. Crystal structure of invasin: a bacterial integrin-binding protein. Science 286, 291-5
7. Crystal structure of enteropathogenic Escherichia coli intimin-receptor complex. Nature 405, 1073-7
8. The link module from ovulation- and inflammation-associated protein TSG-6 changes conformation on hyaluronan binding. J. Biol. Chem. 278, 49261-70
9. Structure of the regulatory hyaluronan binding domain in the inflammatory leukocyte homing receptor CD44. Mol. Cell 13, 483-96
10. Zinc-dependent dimers observed in crystals of human endostatin. Proc. Natl. Acad. Sci. U.S.A. 95, 10443-8
11. Some questions related to melanocyte-stimulating hormone. Mayo Clin. Proc. 51, 632-6
12. The 1.9-A crystal structure of the noncollagenous (NC1) domain of human placenta collagen IV shows stabilization via a novel type of covalent Met-Lys cross-link. Proc. Natl. Acad. Sci. U.S.A. 99, 6607-12
13. De novo calcium/sulfur SAD phasing of the human formylglycine-generating enzyme using in-house data. Acta Crystallogr. D Biol. Crystallogr. 61, 1057-66
14. Crystal structure of human pFGE, the paralog of the Calpha-formylglycine-generating enzyme. J. Biol. Chem. 280, 15180-7
15. The C-type lectin fold as an evolutionary solution for massive sequence variation. Nat. Struct. Mol. Biol. 12, 886-92