Human Siglec-15 is a transmembrane glycoprotein in the Siglec family. Siglecs are type I transmembrane proteins where the NH3+-terminus is in the extracellular space and the COO--terminus is cytosolic. Each Siglec contains an N-terminal V-type immunoglobulin domain (Ig domain) which acts as the binding receptor for sialic acid. These lectins are placed into the group of I-type lectins because the lectin domain is an immunoglobulin fold. All Siglecs are extended from the cell surface by C2-type Ig domains which have no binding activity. Siglecs differ in the number of these C2-type domains. Human Siglec-15 consists of a 244 amino acid (aa) extracellular domain (ECD) with two Ig-like domains, a 21 aa transmembrane segment, and a 44 aa cytoplasmic domain. Siglec-15 function is important for osteoclast formation and TRANCE/RANK Ligand signaling in osteoclasts.
Human Siglec-15 is a transmembrane glycoprotein in the Siglec family. Siglecs are type I transmembrane proteins where the NH3+-terminus is in the extracellular space and the COO--terminus is cytosolic. Each Siglec contains an N-terminal V-type immunoglobulin domain (Ig domain) which acts as the binding receptor for sialic acid. These lectins are placed into the group of I-type lectins because the lectin domain is an immunoglobulin fold. All Siglecs are extended from the cell surface by C2-type Ig domains which have no binding activity. Siglecs differ in the number of these C2-type domains. Human Siglec-15 consists of a 244 amino acid (aa) extracellular domain (ECD) with two Ig-like domains, a 21 aa transmembrane segment, and a 44 aa cytoplasmic domain. Siglec-15 function is important for osteoclast formation and TRANCE/RANK Ligand signaling in osteoclasts
Sialidase-1 belongs to the N-acetyl-a neuraminidase family. Sialidase-1 is expressed in many tissues; it is highly expressed in the pancreas, and weakly expressed in the brain. Sialidase-1 is a lysosomal enzyme, which cleaves terminal sialic acid residues from substrates such as glycoproteins and glycolipids. Deficiencies in the human enzyme Sialidase-1 leads to sialidosis, a rare lysosomal storage disease. Sialidase-1 has been shown to interact with Cathepsin A (protective protein), β-galactosidase and N-acetylgalactosamine-6-sulfate sulfatase in a multienzyme complex.
Single Ig IL-1-related receptor (SIGIRR) or TIR8 is a member of Toll-like receptor-interleukin 1 receptor signaling (TLR-IL-1R) receptor superfamily. Although SIGIRR/TIR8 shows the typical conserved motifs that characterize the IL-1R and Toll superfamily; it is structurally and functionally distinct from both. SIGIRR/TIR8 has only one Ig domain in its extracellular portion whereas the IL-1R family contains three Ig folds. An unusually long cytoplasmic domain is reminiscent of the structure of drosophila Toll; yet the SIGIRR peptide sequence is more closely related to IL-1RI. SIGIRR/TIR8 was mainly expressed in mouse and human epithelial tissues such as kidney; lung and gut. Resting and activated T and B lymphocytes and monocytes-macrophages expressed little or no SIGIRR/TIR8; with the exception of the mouse GG2EE macrophage line. Inflammation is enhanced in SIGIRR-deficient mice. SIGIRR negatively modulates immune responses. Inflammation is enhanced in SIGIRR-deficient mice; as shown by their enhanced chemokine induction after IL-1 injection and reduced threshold for lethal endotoxin challenge.
Siglecs (sialic acid binding Ig-like lectins) are I-type lectins that belong to the immunoglobulin superfamily. They are characterized by an N-terminal Ig-like V-type domain which mediates sialic acid binding, followed by a varying number of Ig-like C2-type domains. Siglecs 5-11 constitute the CD33/Siglec-3 related group, and are differentially expressed in the hematopoietic system. Siglec-G is the apparent ortholog of human Siglec-10. We describe here a novel member of the siglec protein family that shares a similar structure including five Ig-like domains, a transmembrane domain, and a cytoplasmic tail containing two ITIM-signaling motifs. Siglec-10 was identified through database mining of an asthmatic eosinophil EST library. Siglec-10 binds sialated proteins and lipids in alpha 2,3 or alpha 2,6 linkage and shows a preference for GT1b gangliosides. This binding can be modulated by cis interactions of Siglec-10 with sialated molecules expressed on the same cell. When tyrosine phosphorylated, the cytoplasmic ITIMs interact with phosphatases SHP-1 and SHP-2 to propagate inhibitory signals. The Siglec-10-VAP-1 interaction seems to mediate lymphocyte adhesion to endothelium and has the potential to modify the inflammatory microenvironment via the enzymatic end products.
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