Alpha 2-macroglobulin binding to cultured fibroblasts: identification by affinity chromatography of high-affinity binding sites

Binding sites having the properties of high-affinity receptors for activated alpha 2-macroglobulin (alpha 2M) have been purified over 100-fold from membranes of spontaneously transformed NIH-3T3 cells (J. A. Hanover, S.-y. Cheng, M. C. Willingham, and I. H. Pastan [1983] J. Biol. Chem. 258, 370-377). To identify the molecular species involved in high-affinity binding, the solubilized receptor has been purified 500-fold by conventional procedures and further purified by affinity chromatography. After radioiodination of the 500-fold-purified preparation, the detergent-solubilized extract was applied to alpha 2M-Sepharose and an 85,000 +/- 5000 Mr species was selectively retained by the column. Binding of the 85,000 +/- 5000 Mr species to the affinity resin was inhibited by EDTA and by excess alpha 2M. Elution from the affinity column could be accomplished with bacitracin, a competitive inhibitor of alpha 2M binding, or with EDTA. Consistent with the previously reported characteristics of the high-affinity alpha 2M receptor, the 85,000 Mr species bound much more efficiently to methylamine-activated alpha 2M-Affigel than to alpha 2M-Affigel which had not been amine-activated. The present data suggest that a protein with a subunit Mr of 85,000 +/- 5000 may represent a component of the high-affinity alpha 2M receptor present on cultured fibroblasts.