May play a key role in diverse functions ascribed to CD81 and CD9 such as oocytes fertilization or hepatitis C virus function. May regulate proliferation and differentiation of keratinocytes. May be a negative regulator of cell motility: suppresses T-cell mobility coordinately with CD81, associates with CD82 to suppress prostate cancer cell migration, regulates epidermoid cell reaggregation and motility on laminin-5 with CD9 and CD81 as key linkers. May also play a role on integrin-dependent morphology and motility functions. May participate in the regulation of neurite outgrowth and maintenance of the neural network in the adult brain. Interacts directly with CD82 and CD9/tetraspanin-29. Also interacts with integrin alpha-3/beta-1 and integrin alpha-4/beta-1 (By similarity). Part of a complex composed of CD9, PTGFRN and CD81 (PubMed:23575678). Interacts with CD81/tetraspanin-28. Expressed in lymphocytes as well as in many tissues with higher expression in brain. Detected in all regions of the brain with weak expression in the pituitary. Expressed selectively by neurons but not by glial cells. Expressed in myoblasts (at protein level). Expressed at 9.5 dpc in the ventral domain of the neural tube, with dorsal expression apparent at 10.5 dpc, as well as in the dorsal bud of the pancreas. Detected at 11.5 dpc within the ventricular zone of the neural tube in the hindbrain, diencephalon and telencephalon; also detected in the epithelial lining of the bronchi and esophagus. At 12.5 dpc, expressed in the bronchi, in the atrial and ventricular myocardium, the dorsal root ganglia and the epithelial lining of the rectum and bladder. At 15.5 dpc, detected in the developing choroid plexus epithelium, neural retina, olfactory epithelium and developing vomeronasal organ. Induced by neurite outgrowth in neuroblastoma cell lines Neuro-2a. The Ig-like C2-type domains 3 and 4 are required for interaction with CD81. The short cytoplasmic domain is very basic, interacts with membrane PIPs, and mediates plasma membrane localization.