C-type lectin (CLEC) family is a type of carbohydrate-binding protein domain named lectin. C-type lectins are the most diverse and prevalent lectin family in immunity with its requirement for calcium for binding. Proteins including a C-type lectin domain have diverse range of functions including cell-cell adhesion, immune response to pathogens and apoptosis. There are at least 14 types of C-type lectins: typeⅠto typeⅩⅣ. CLEC4D(CLECSF8) is a typeⅡ membrane glycoprotein belonging to the C-type lectin family, with restricted expression in the monocyte/macrophage lineage. It plays important roles in the function of macrophages.
CLEC4F, a member of C-type lectins, was firstly purified from rat liver extract with high binding affinity to fucose, galactose and N-acetylgalactosamine, and un-sialylated glucosphingolipids with GalNAc or Gal terminus. However, the biological functions of CLEC4F have not been elucidated. Histochemical staining showed that mouse CLEC4F（mCLEC4F） is only expressed on F4/80+ cells localized in liver, and is undetectable in bone marrow, spleen, lymph nodes, or other tissues in adult mice. However, mCLEC4F is detected in the liver of embryonic day 11.5 (E11.5), which is 1.5 day earlier than the formation of liver (E10) and is 3.5 day earlier than the formation of bone marrow (E15-16). Moreover, recombinant mCLEC4F.Fc binds to alpha-galactoceramide in a Ca++-dependent manner, and both galactose and ceramide can partially inhibit CLEC4F.Fc binding to alpha-galactoceramide. Interestingly, mCLEC4F-deficient (mCEC4F k/o) mice produced far less cytokines than wild type littermates after intravenous injection ofalpha-galactoceramide. This suggests that mCLEC4F is not only a specific marker for Kupffer cells, but is also critical for the presentation of glycolipid antigen to NKT cells.
CLEC5A, also known as MDL1 and MDL-1, is a member of the C-type lectin/C-type lectin-like domain (CTL/CTLD) superfamily. Members of this family share a common protein fold and have diverse functions, such as cell adhesion, cell-cell signalling, glycoprotein turnover, and roles in inflammation and immune response. CLEC5A with dnax-activation protein 12 and may play a role in cell activation. It also functions as a positive regulator of osteoclastogenesis. CLEC5A acts as a key regulator of synovial injury and bone erosion during autoimmune joint inflammation .The binding of dengue virus to CLEC5A triggers signaling through the phosphylation of TYROBP, this interaction does not result in viral entry, but stimulates proinflammatory cytokine release.
CLM-9, also known as TREM4, is a receptor which belongs to the TREM family. The TREM family of receptors regulates the activity of various cell types of the immune system including neutrophils, monocyte/macrophages, microglia, and dendritic cells. CLM-9 may mediate L-selectin-dependent lymphocyte rollings. It binds SELL in a calcium dependent manner. CLM-9 also binds lymphocyte which suggests that it functions in lymphocyte adhesion. The major CLM-9 transcript is expressed highly in human heart, skeletal muscle, and placenta. The mouse protein has been shown to be expressed in capillary endothelial cells. Human CLM-9 mediates the uptake of human IgA2 and mouse IgM.
CLP1, also known as COLEC12, is a scavenger receptor that displays several functions associated with host defense. It contains 1 C-type lectin domain and 3 collagen-like domains. CLP1 is strongly expressed in placenta and moderately expressed in heart, skeletal muscle, small intestine and lung. It promotes binding and phagocytosis of Gram-positive, Gram-negative bacteria and yeast. CLP1 mediates the recognition, internalization and degradation of oxidatively modified low density lipoprotein (oxLDL) by vascular endothelial cells. It binds to several carbohydrates including Gal-type ligands, D-galactose, L- and D-fucose, GalNAc, T and Tn antigens in a calcium-dependent manner and internalizes specifically GalNAc in nurse-like cells. It binds also to sialyl Lewis X or a trisaccharide and asialo-orosomucoid (ASOR). CLP1 may also play a role in the clearance of amyloid beta in Alzheimer disease.
Colipase belongs to the colipase family. Structural studies of the complex and of colipase alone have revealed the functionality of its architecture. It is a small protein with five conserved disulphide bonds. Structural analogies have been recognised between a developmental protein, the pancreatic lipase C-terminal domain, the N-terminal domains of lipoxygenases and the C-terminal domain of alpha-toxin. Colipase can only be detected in pancreatic acinar cells, suggesting regulation of expression by tissue-specific elements. Colipase allows lipase to anchor noncovalently to the surface of lipid micelles, counteracting the destabilizing influence of intestinal bile salts. Without colipase the enzyme is washed off by bile salts, which have an inhibitory effect on the lipase. Colipase is a cofactor needed by pancreatic lipase for efficient dietary lipid hydrolysis. It binds to the C-terminal, non-catalytic domain of lipase, thereby stabilising as active conformation and considerably increasing the overall hydrophobic binding site.
Clusterin is a secreted protein which belongs to the Clusterin family. Clusterin is expressed in adult testis, heart, ovary, adrenal gland, brain and liver. Clusterin has been suggested to be involved in several basic biological events such as cell death, tumor progression, and neurodegenerative disorders. In addition,Clusterin is up/ down regulated on the mRNA or protein level in many pathological and clinically relevant situations including cancer, organ regeneration, infection, Alzheimer disease, retinitis pigmentosa, myocardial infarction, renal tubular damage, autoimmunity and others.
CD110, also known as c-MPL, is a 635 amino acid transmembrane domain, with two extracellular cytokine receptor domains and two intracellular cytokine receptor box motifs. It is expressed at a low level in a large number of cells of hematopoietic origin. C-MPL is homologous with members of the hematopoietic receptor superfamily. Presence of anti-sense oligodeoxynucleotides of c-mpl inhibited megakaryocyte colony formation. Thrombopoietin is the ligand for c-mpl. It was shown to be the major regulator of megakaryocytopoiesis and platelet formation. Defects in c-MPL are a cause of congenital amegakaryocytic thrombocytopeniawhich is a disease characterized by isolated thrombocytopenia and megakaryocytopenia with no physical anomalies. Defects in c-MPL also cause thrombocythemia type 2 and myelofibrosis with myeloid metaplasia.
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