4F2 antigen (CD98) was identified as a heterodimer consisting of an 80 kDa type II glycosylated integral membrane protein 4F2 heavy chain (4F2 hc) linked by disulfide bonds to a hydrophobic, non-glycosylated, 37 kDa protein 4F2 light chain (4F2 lc). The 4F2 hc colocalizes with cadherins at cell adhesion sites and is required for the intracellular targeting of 4F2 heterodimers. 4F2 hc also associates with β1 integrins and overexpression of 4F2 reverses the suppression of β1 integrin activation caused by overexpression of β1 cytoplasmic domains. 4F2 has also been identified as a fusion regulatory protein FRP-1, since anti-FRP-1 antibodies can induce HIV-mediated cell fusion via an integrin system. 4F2 heterodimers are also important for activation of amino acid transport. 4F2 hc is 30% homologous with the amino acid transport activator D2/rBAT and expression of 4F2 hc in Xenopus oocytes induces system y+L amino acid transport. Thus, 4F2 hc may participate in intracellular trafficking and activation of amino acid transporters, as well as in the regulation of integrin signaling.