2 DISTINCT FUNCTIONAL HIGH-AFFINITY RECEPTORS FOR MOUSE INTERLEUKIN-3 (IL-3)

The human interleukin-3 receptor (IL-3R) is composed of an IL-3 specific a subunit (IL-3R-alpha) and a common beta-subunit (beta(c)) that is shared by IL-3, granulocyte/macrophage colony stimulating factor (GM-CSF) and IL-5 receptors. In contrast to the human, the mouse has two distinct but related genes, AIC2A and AIC2B, both of which are homologous to the human beta(c) gene. AIC2B has proved to encode a common beta-subunit between mouse GM-CSF and IL-5 receptors. AIC2A is unique to the mouse and encodes a low affinity IL-3 binding protein. Based on the observation that the AIC2A protein is a component of a high affinity IL-3R, we searched for a cDNA encoding a protein which conferred high affinity IL-3 binding when coexpressed with the AIC2A protein in COS7 cells. We obtained such a cDNA (SUT-1) encoding a mature protein of 70 kDa that has weak homology to the human IL-3R-alpha. The SUT-1 protein bound IL-3 with low affinity and formed high affinity receptors not only with the AIC2A protein but also with the AIC2B protein. Both high affinity IL-3Rs expressed on a mouse T cell line, CTLL-2, showed similar IL-3 binding properties and transmitted a growth signal in response to IL-3. Thus, the mouse has two distinct functional high affinity IL-3Rs, providing a molecular explanation for the differences observed between mouse and human IL-3Rs.