| Abstract: | 125I-Labeled colony-stimulating factor (CSF) 2 alpha (interleukin 3, multi-CSF, and mast cell growth factor) was used to characterize receptors specific for this lymphokine on the cell surface of the factor-dependent cell line FDC-P2. CSF-2 alpha binding to these cells was specific and saturable. Among a panel of lymphokines and growth factors, only unlabeled CSF-2 alpha was able to compete for the binding of 125I-labeled CSF-2 alpha to cells. Equilibrium binding studies revealed that CSF-2 alpha bound to 434 +/- 281 receptors/cell with a Ka of 8.7 +/- 3.9 X 10(9) M-1. Affinity cross-linking experiments with the homobifunctional cross-linking reagents disuccinimidyl suberate, disuccinimidyl tartrate, and dithiobis(succinimidyl propionate) produced a radiolabeled band of Mr = 97,000 on intact cells and in purified cell membranes, while an additional band of Mr = 138,000 was produced upon cross-linking to intact cells only. The relationship between these two bands is discussed. The results indicate that the receptor for CSF-2 alpha on FDC-P2 cells consists at a minimum of a subunit of Mr = 72,500. |
