CD9

This gene encodes a member of the transmembrane 4 superfamily, also known as the tetraspanin family. Tetraspanins are cell surface glycoproteins with four transmembrane domains that form multimeric complexes with other cell surface proteins. The encoded protein functions in many cellular processes including differentiation, adhesion, and signal transduction, and expression of this gene plays a critical role in the suppression of cancer cell motility and metastasis.

CD9 molecule

Cardiovascular

Integral membrane protein associated with integrins, which regulates different processes, such as sperm-egg fusion, platelet activation and aggregation, and cell adhesion (PubMed:14575715, PubMed:18541721, PubMed:8478605).
Present at the cell surface of oocytes and plays a key role in sperm-egg fusion, possibly by organizing multiprotein complexes and the morphology of the membrane required for the fusion (By similarity).
In myoblasts, associates with CD81 and PTGFRN and inhibits myotube fusion during muscle regeneration (By similarity).
In macrophages, associates with CD81 and beta-1 and beta-2 integrins, and prevents macrophage fusion into multinucleated giant cells specialized in ingesting complement-opsonized large particles (PubMed:12796480).
Also prevents the fusion between mononuclear cell progenitors into osteoclasts in charge of bone resorption (By similarity).
Acts as a receptor for PSG17 (By similarity).
Involved in platelet activation and aggregation (PubMed:18541721).
Regulates paranodal junction formation (By similarity).
Involved in cell adhesion, cell motility and tumor metastasis (PubMed:7511626, PubMed:8478605).

Cell adhesion
Cell population proliferation
Cellular response to low-density lipoprotein particle stimulus
Fusion of sperm to egg plasma membrane involved in single fertilization
Glial cell migration
Myoblast fusion involved in skeletal muscle regeneration
Negative regulation of cell population proliferation
Negative regulation of platelet aggregation
Paranodal junction assembly
Platelet activation
Receptor internalization
Regulation of macrophage migration
Sperm-egg recognition

Cell membrane
Membrane
Secreted, extracellular exosome
Note: Present at the cell surface of oocytes. Accumulates in the adhesion area between the sperm and egg following interaction between IZUMO1 and its receptor IZUMO1R/JUNO.

Cytoplasmic: 2-12
Helical: 13-33
Extracellular: 34-55
Helical: 56-76
Cytoplasmic: 77-87
Helical: 88-111
Extracellular: 112-195
Helical: 196-221
Cytoplasmic: 222-228

Palmitoylated at a low, basal level in unstimulated platelets. The level of palmitoylation increases when platelets are activated by thrombin (in vitro). The protein exists in three forms with molecular masses between 22 and 27 kDa, and is known to carry covalently linked fatty acids (PubMed:11959120).
Palmitoylation by ZDHHC2 regulates CD9 expression, association with other tetraspanin family proteins and function in cell adhesion (PubMed:18508921).