IgSF

INTRODUCTION The immunoglobulin superfamily (IgSF) is a large group of cell surface and soluble proteins that are involved in the recognition, binding, or adhesion processes of cells. 2 Molecules are categorized as members of this superfamily based on shared structural features with immunoglobulins (also known as antibodies); they all possess a domain known as an immunoglobulin domain or fold. Members of the IgSF include cell surface antigen receptors, co-receptors and co-stimulatory molecules of the immune system, molecules involved in antigen presentation to lymphocytes, cell adhesion molecules, certaincytokine receptors and intracellular muscle proteins. They are commonly associated with roles in the immune system. The sperm-specific protein Izumo, a member of the immunoglobulin superfamily, has also been identified as the only sperm membrane protein essential for sperm-egg fusion.

Immunoglobulin domains Proteins of the IgSF possess a structural domain known as an immunoglobulin (Ig) domain. Ig domains are named after the immunoglobulin molecules. They contain about 70-110 amino acids and are categorized according to their size and function.[2] Ig-domains possess a characteristic Ig-fold, which has a sandwich-like structure formed by two sheets of antiparallel beta strands. Interactions between hydrophobic amino acids on the inner side of the sandwich and highly conserved disulfide bonds formed between cysteine residues in the B and F strands, stabilize the Ig-fold. One end of the Ig domain has a section called the complementarity determining region that is important for the specificity of antibodies for their ligands.

Classification The Ig like domains can be classified as IgV, IgC1, IgC2, or IgI.[3] Most Ig domains are either variable (IgV) or constant (IgC). IgV: IgV domains with 9 beta strands are generally longer than IgC domains with 7 beta strands. IgC1 and IgC2: Ig domains of some members of the IgSF resemble IgV domains in the amino acid sequence, yet are similar in size to IgC domains. These are called IgC2 domains, while standard IgC domains are called IgC1 domains. IgI: Other Ig domains exist that are called intermediate (I) domains

Molecule function/category Examples Description 1.Antigen receptors Antibodies or immunoglobulins T cell receptor chains Antigen receptors found on the surface of T and B lymphocytes 2.Antigen presenting molecules Class I MHC Class II MHC beta-2 microglobulin The ligands for TCRs are major histocompatibility complex (MHC) proteins 3. Co-receptors CD4 CD8 CD19 4. Receptors on Natural killer cells Killer-cell immunoglobulin-like receptors (KIR) 5. IgSF CAMs NCAMs ICAM-1 CD2 subset Type IIa and Type IIb RPTPs, described in Receptor tyrosine kinases/phosphatases subsection below CD2 subset of IgSF represented large group of homologous cell adhesion molecules (CAMs), includes CD2, CD58, CD48,CD150, CD229 and CD244.

CAM

IgSF