This page provides a summary of the known lectin families, and an overview of the subcellular location and ligand binding specificities and evolution of the different families. The links below provide more detailed information on the structure, sugar-binding activity, biological function and evolution of proteins in each of the lectin families as well as annotated sequence alignments.
Complex oligosaccharide structures are displayed at cell surfaces, incorporated into the extracellular matrix and attached to secreted glycoproteins. These oligosaccharides can serve structural roles, mediate movement of glycoconjugates to the cell surface or act as markers that mediate cell-cell and cell-matrix recognition events. The non-structural roles of sugars generally require the participation of sugar-binding lectins (Drickamer and Taylor, 1998), in which sugar-binding activity can usually be ascribed to a single protein module within the lectin polypeptide. Such a module is referred to a carbohydrate-recognition domain (CRD).
CRDs in vertebrate lectins fall into a number of structurally distinct families. Of the eight well-established groups, four contain lectins that are predominantly intracellular and four contain lectins that generally function outside the cell. The intracellular lectins - calnexin family, M-type, L-type and P-type - are located in luminal compartments of the secretory pathway and function in the trafficking, sorting and targeting of maturing glycoproteins. The extracellular lectins - C-type, R-type, siglecs and galectins - are either secreted into the extracellular matrix or body fluids, or localized to the plasma membrane, and mediate a range of functions including cell adhesion, cell signalling, glycoprotein clearance and pathogen recognition. Recent findings point to the existence of additional new groups of animal lectins - F-box lectins, ficolins, chitinase-like lectins, F-type lectins and intelectins - some of which have roles complementary to those of the well-established lectin families.
|Model of mannose-binding
protein trimer with oligosaccharide ligands
Adapted from Weis WI and Drickamer K (1994) Structure, 2:1227-40
Summary of lectin families
|Lectin family||Typical saccharide ligands||Subcellular location||Examples of functions|
|Calnexin||Glc1Man9||ER||Protein sorting in the endoplasmic reticulum.|
|M-type lectins||Man8||ER||ER-associated degradation of glycoproteins.|
|L-type lectins||Various||ER, ERGIC, Golgi||Protein sorting in the endoplasmic reticulum.|
|P-type lectins||Man 6-phosphate, others||Secretory pathway||Protein sorting post-Golgi, glycoprotein trafficking, ER-associated degradation of glycoproteins, enzyme targeting.|
|C-type lectins||Various||Cell membrane, extracellular||Cell adhesion (selectins), glycoprotein clearance, innate immunity (collectins).|
|Galectins||b-Galactosides||Cytoplasm, extracellular||Glycan crosslinking in the extracellular matrix.|
|I-type lectins (siglecs)||Sialic acid||Cell membrane||Cell adhesion.|
|R-type lectins||Various||Golgi, Cell membrane||Enzyme targeting, glycoprotein hormone turnover.|
|F-box lectins||GlcNAc2||Cytoplasm||Degradation of misfolded glycoproteins.|
|Ficolins||GlcNAc, GalNAc||Cell membrane, extracellular||Innate immunity.|
|Chitinase-like lectins||Chito-oligosaccharides||Extracellular||Collagen metabolism (YKL-40).|
|F-type lectins||Fuc-terminating oligosaccharides||Extracellular||Innate immunity.|
|Intelectins||Gal, galactofuranose, pentoses||Extracellular/cell membrane||Innate immunity. Fertilization and embryogenesis.|
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Cellular location and ligand binding specificity of different lectin families
Reference : Drickamer, K. and Taylor, M.E. (1998) Evolving views of protein glycosylation. Trends Biochem. Sci., 23, 321-324.
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Evolution of lectin families
evolutionary patterns observed for different structural classes of lectins.
Orange bars indicate the presence of domains
that are structurally homologous and evolutionarily related to
the different types of CRDs found in animal lectins.
Blue bars denote the
existence of members of that structural category with demonstrated or predicted sugar-binding activity.
Green bars indicate known
conservation or similarity of biological activities in different species.
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