Evidence for a Low-Affinity Interleukin-3 Receptor

Interleukin-3 (1L-3) regulates the proliferation of myeloid, erythroid, and lymphoid cells. Previous reports showed IL-3 binding restricted to a single high-affinity (K-d=50-200 PM) site. Here, we demonstrate by equilibrium studies an additional binding site for IL-3 with lower apparent affinity (K-d=5-20 nM). Furthermore, kinetic analysis shows that two binding sites for IL-3 exist: IL-3 dissociates slowly from the first site (T-1/2 = 4 hr; k(-1) = 2.7 x 10(-3) min(-1)), whereas it dissociates rapidly (T-1/2 = 4.0 min; k(-1) = 0.116 min(-1)) from the second site. Crosslinking showed that [I-125]IL-3 binding to the 115- and 140-kD proteins was not saturable at concentrations commensurate with high-affinity binding and IL-3 dissociated rapidly from these same molecules. Thus, the low affinity IL-3 receptor is a molecule(s) of 115-to 140-kD.