CD9 antigen is a glycoprotein expressed on the surface of developing B lymphocytes, platelets, monocytes, eosinophils, basophil, stimulated T lymphocytes and by neurons and glial cells in the peripheral nervous system. It belongs to a family of membrane proteins termed tetraspanins which transverse the membrane four times. In pre B cells and platelets, CD9 antigen regulates cell activation and aggregation possibly through an association with the integrin CD41 / CD61 (GPIIb / GPIIIa). It also regulates cell motility in a variety of cell lines, and appears to be an important regulator of Schwann cell behaviour in peripheral nerve.
Function: Involved in platelet activation and aggregation. Regulates paranodal junction formation. Involved in cell adhesion, cell motility and tumor metastasis. Required for sperm-egg fusion.
Subunit: Forms both disulfide-linked homodimers and higher homooligomers as well as heterooligomers with other members of the tetraspanin family. Associates with CR2/CD21 and with PTGFRN/CD9P1. Interacts directly with IGSF8.
Tissue Specificity: Expressed by a variety of hematopoietic and epithelial cells.
Post-translational modifications: Protein exists in three forms with molecular masses between 22 and 27 kDa, and is known to carry covalently linked fatty acids.
Similarity: Belongs to the tetraspanin (TM4SF) family.
