Identification of regions within the third FnIII-like domain of the IL-5R alpha involved in IL-5 interaction

Previously, two binding sites for interleukin 5 (IL-5) were identified on t he IL-5 receptor alpha chain (IL-5R alpha). They are located within the CD loop of the first fibronectin type III (FnIII)-like domain and the EF loop of the second FnIII-like domain. The first binding site was identified by e xploiting the different abilities of human IL-5R alpha (hIL-5R alpha) and m ouse IL-5R alpha (mIL-5R alpha) to bind hIL-5. Here we show that ovine IL-5 (oIL-5) has the ability to activate the hIL-5R alpha but not the mIL-5R al pha. By using chimeras of the mIL-5R alpha and hIL-5R alpha we demonstrate that residues within the first and third FnIII-like domains of mIL-5R alpha are responsible for this lack of activity. Furthermore, mutation of residu es on hIL-5R alpha to mIL-5R alpha within the predicted DE and FG loop regi ons of the third FnIII domain reduces oIL-5 activity, These results show th at regions of the third FnIII domain of IL-5R alpha are involved in binding , in addition to the regions in domains one and two of the IL-5R alpha that were identified in an earlier study. (C) 2000 Academic Press.