IL-20 receptor alpha (IL-20 R alpha ), also named IL-20 R1, CRF2-8, and ZCYTOR7, belongs to the class II cytokine receptor family, which includes 12 members. These receptors are characterized by the patterns of conserved amino acid (aa) residues in their extracellular domains, which are composed of tandem fibronectin type III domains (1). Class II cytokine receptors form heterodimeric signaling receptor complexes that mediate class II cytokine signals. Subunits of the different receptor complexes are shared and serve multiple functions (1).

The gene for human IL-20 R alpha is mapped to chromosome 6 and encodes a 553 aa glycoprotein with a 29 aa signal peptide, a 221 aa extracellular domain, a 24 aa transmembrane region and a 279 aa intracellular domain (2). IL-20 R alpha is widely expressed and is detected at high levels in multiple tissues including skin, testis, heart, placenta, salivary gland and prostate gland (1). The expression of IL-20 R alpha, together with that of IL-20 R beta, is upregulated in psoriatic skin lesions on keratinocytes, immune cells, and endothelial cells (1, 2).

IL-20 R alpha heterodimerizes with IL-20 R beta to form the functional receptor that mediates IL-19, IL-20 and IL-24 signals (3, 4). IL-20 R alpha also heterodimerizes with IL-10 R beta to form the functional receptor complex for IL-26 (5). Binding of these IL-10 family class II cytokines to their functional receptors induces activation of the JAK-STAT signal transduction pathway. At low ligand concentrations, STAT3 has been shown to be the predominant STAT proteins activated through either complexes (3 - 5).