The orphan mouse receptor interleukin (IL)-8R beta binds N51. Structure-function analysis using N51/IL-8 chimeric molecules.

We have demonstrated that the orphan receptor representing the putative mouse (mu) homolog of the human (hu) interleukin-8 receptor beta (IL-8R beta) binds the mouse N51 cytokine, also known as KC. The muIL-8R beta gene was constitutively expressed in NIH 3T3 cells (NIH-muIL-8R beta). Cells and plasma membranes from the NIH-muIL-8R beta clone showed binding of 125I-N51 that was displaced by unlabeled N51. Other related cytokines were assayed for their ability to displace 125I-N51. MIP-2 and GRO alpha/MGSA competed as well as N51 for the receptor, but huIL-8 and NAP-2 did not compete at all. Chimeric molecules between IL-8 and N51 were used to extend the binding analysis. The segment between the conserved cysteines 2 and 3, named domain I; cysteines 3 and 4, domain II; and cysteine 4 and the C terminus, domain III of IL-8 were replaced by the corresponding domains of N51 and vice versa. When studying the binding of 125I-N51 and the hybrid molecules to the receptor, we observed that chimeras of N51 containing either domain I, II, or III of IL-8 were agonists of N51, and chimeras of IL-8 containing domain II or III of N51 were partial agonists of N51. These results demonstrate that domain I of N51 does not confer binding specificity and suggest that the region from the third cysteine to the C terminus of the N51 molecule is more important for binding to muIL-8R beta.