Purification of the murine interleukin 3 receptor

Abstract

In this report we describe the purification of the murine interleukin 3 receptor (mIL-3R) to apparent homogeneity using a two-step procedure involving biotinylated mIL-3 (B-mIL-3) and affinity binding to immobilized antiphosphotyrosine and streptavidin agarose (SA). Purification was monitored using an assay for detergent solubilized-mIL-3Rs that utilized unglycosylated 125I-mIL-3 and concanavalin A (ConA)-Sepharose beads. The final material consisted of a 140-kDa tyrosine and serine phosphorylated protein that was greater than 98% pure as assessed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis of either [35S]methionine-labeled, silver-stained, or radioiodinated preparations. Characterization of the purified receptor revealed that it migrated identically under reducing and nonreducing conditions in SDS gels, possessed 10 kDa of N-linked carbohydrate, and was cleaved upon storage at 4 degrees C to a 70-kDa form. These properties suggested that the purified mIL-3R was identical to that identified by cross-linking studies. The KD of the purified receptor was 1-5 nM, similar to estimates obtained using intact normal mouse bone marrow cells and mIL-3-dependent cell lines. The two-step purification procedure also isolated a 120-kDa serine phosphorylated but nontyrosine phosphorylated mIL-3R species. Apart from phosphorylation differences, the 140- and 120-kDa species were apparently identical, yielding, after alkaline phosphatase treatment, the same molecular mass on SDS gels and similar chymotryptic peptide maps. Amino acid sequences and composition data obtained from the more abundant and more stable serine phosphorylated 120-kDa mIL-3R, further purified by SDS-polyacrylamide gel electrophoresis, suggested that the purified mIL-3R may be identical to the predicted sequence of the recently isolated cDNA clone AIC2A. This was further suggested by comparing chymotryptic maps of the 120-kDa mIL-3R with the Aic2A protein and using antibodies corresponding to the amino and carboxyl termini of the AIC2A cDNA product. However, the Aic2A protein, when expressed on the surface of COS or 3T3 cells or following detergent solubilization and partial purification with biotinylated mIL-3 and SA, displayed a substantially lower affinity for mIL-3.