CD81
Description
The CD81 (CD81 molecule) is a protein-coding gene located on chromosome 11.
CD81 molecule, also known as CD81 (Cluster of Differentiation 81), is a protein which in humans is encoded by the CD81 gene. It is also known as 26 kDa cell surface protein, TAPA-1 (Target of the Antiproliferative Antibody 1), and Tetraspanin-28 (Tspan-28).
== Gene == The gene is located on the plus strand of the short arm of chromosome 11 (11p15.5). It is 20,103 bases in length and encodes a protein of 236 amino acids (predicted molecular weight 25.809 kDa). The protein does not appear to be post translationally modified and has four transmembrane domains. Both the N-terminus and C-terminus lie on the intracellular side of the membrane. The gene is expressed in hemopoietic, endothelial, and epithelial cells. It is absent from erythrocytes, platelets, and neutrophils.
== Function == The protein encoded by this gene is a member of the transmembrane 4 superfamily, also known as the tetraspanin family. Most of these members are cell-surface proteins that are characterized by the presence of four hydrophobic domains.
CD81 is a key component of specialized membrane microdomains known as tetraspanin-enriched microdomains (TERMs). These platforms facilitate the clustering and signaling of receptors, playing a vital role in immune responses and cell-cell interactions. In B cells, CD81 is essential for the trafficking and localization of the CD19 receptor, which is crucial for B cell activation and antibody production. CD81 enables the formation of signaling complexes at TERMs upon encountering pathogens, leading to a lower threshold of antigen required for B cell activation. CD81 also influences the formation of antigen-induced synapses between T cells and B cells, supporting T helper type 2 polarization and costimulation. Beyond B cell functions, CD81 is involved in antigen presentation by localizing within MHC class II compartments. CD81 can act as a regulator of cell fusion, both promoting and inhibiting fusion events depending on the cellular context. It positively regulates sperm-egg fusion, potentially participating in acrosome reactions. However, CD81 inhibits myotube fusion during muscle regeneration and prevents macrophage fusion into multinucleated giant cells, ultimately regulating the formation of specialized immune cells. CD81 also plays a role in the regulation of enzymatic activity, specifically controlling the subcellular localization and degradation of SAMHD1, an enzyme involved in intracellular dNTP levels. Further, CD81 contributes to cell adhesion and motility, positively regulating integrin-mediated adhesion of macrophages, especially during inflammatory responses in the lung.
CD81 is also known as CVID6, S5.7, TAPA1, TSPAN28.
