Role of disulfide bridges in determining the biological activity of interleukin 3.

Total chemical synthesis of analog proteins was used to examine the requirement for specific disulfide bridges for the biological activity of interleukin 3 (IL-3), a growth factor that stimulates multiple lineages of hemopoietic cells. Four structural analogs of the mature, 140 amino acid murine IL-3 molecule were synthesized in which specific cysteine residues were replaced by alanines. In a quantitative IL-3 assay, based on [3H]thymidine incorporation into factor-dependent cells, the IL-3 analog with alanines substituted for all four cysteines--i.e., [Ala17,79,80,140]IL-3--had 1/500th as much activity as the molecule synthesized according to the native sequence. The two analogs [Cys17,79,Ala80,140]IL-3 and [Cys17,140,Ala79,80]IL-3 had similarly low activity, whereas the [Cys17,80,Ala79,140]IL-3 analog had 2000-fold higher activity than these three analogs, and 3-fold higher than the molecule with the native sequence. This shows that in IL-3 a single disulfide bridge, between cysteines 17 and 80, is required for biological activity that approximates physiological levels. This disulfide probably stabilizes the tertiary structure of the protein to give a conformation that is optimal for function.