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, CD81/tetraspanin-28 and CD9/tetraspanin-29. Also interacts with integrin alpha-3/beta-1 and integrin alpha-4/beta-1. Expressed in brain, kidney, testis, liver and placenta with moderate expression in all other tissues. Detected on a majority of B-cells, T-cells, and natural killer cells but not on monocytes, polynuclear cells and platelets. 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.