Evidence for a Low-Affinity Interleukin-3 Receptor

Abstract

Interleukin-3 (IL-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 pin) site. Here, we demonstrate by equilibrium studies an additional binding site for IL-3 with lower apparent affinity (K_d=5-20 iym). Furthermore, kinetic analysis shows that two binding sites for IL-3 exist: IL-3 dissociates slowly from the first site (I_½=4hr; k₋₁=2.7x10⁻³ min⁻¹), whereas it dissociates rapidly (T_½=4.0 min; k₋₁ = 0.116 min⁻¹) from the second site. Cross-linking showed that [¹²⁵I]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.

Key Words:

• IL-3 receptor
• down-regulation
• mast cells
• lymphokine
• tyrosine kinase