Abstract. Ligand affinity chromatography was used to purify a cell surface a2-macroglobulin (ot2M) receptor. Detergent extracts of human placenta were applied to an affinity matrix consisting of ot2M, previously reacted with methylamine, coupled to Sepharose. Elution with EDTA specifically released polypeptides with apparent molecular masses of 420 and 39 kD. In some preparations, small amounts of a 90-kD polypeptide were observed. The 420-and 39-kD polypeptides appear specific for the forms of ~2M activated by reaction with proteinases or methylamine and do not bind to an affinity matrix consisting of native ot2M coupled to Sepharose. Separation of these two polypeptides was accomplished by anion exchange chromatography, and binding activity was exclusively associated with the 420-kD polypeptide. The purified 420-kD protein binds to the conformationally altered forms of ot2M that are known to specifically interact with o~2M receptors and does not bind to native c~2M. Binding of the 420-kD polypeptide to immobilized wheat germ agglutinin indicates that this polypeptide is a glycoprotein. The cell surface localization of the 420-kD glycoprotein was confirmed by affinity chromatography of extracts from surface radioiodinated fibroblasts. These properties suggest that the 420-kD polypeptide is a cell surface receptor for the activated forms of ~zM.Ot2-MACROGLOBULIN (ot2M)' is a 718,000-mol wt glycoprotein that is capable of inhibiting enzymes from all four classes of proteinases (2) in a reaction that is essentially irreversible. The reaction of proteinases or methylamine with c~2M "activates" the molecule by inducing conformational changes in o~2M (3,4,16,49) that, in the case of the proteinase reaction, reduce the activity of the bound enzyme toward large molecular weight substrates. These conformational changes also generate sites on the inhibitor that result in specific recognition by cell surface receptors (31,52,54). It has been possible to prepare monoclonal antibodies that selectively recognize the activated forms of u2M (34, 50) by binding to these newly generated regions (25).In addition to regulating proteinase activity, studies have suggested that c~2M binds to TGFfl (41), PDGF (22), and bFGF (7). The association of these molecules with ot2M results in a reduction of their activity, suggesting that a2M may serve not only to modulate proteinase activity but also the activity of these molecules. Interestingly, it appears that several tumor-derived or virus-transformed cell lines are devoid of specific cell surface receptors for t~2M-proteinase complexes (52).The process of receptor-mediated endocytosis has been the subject of extensive study, and the mechanisms involved in I. Abbreviations used in this paper: ct2M, ~2-macroglobulin; t~2M:Me, methylamine-treated ~2-macroglobulin; NRK, normal rat kidney. endocytosis have been investigated using a variety of labeled ligands (6), including transferrin (20), low density lipoprotein (1), EGF (20), and ot2M (8,37,54,55). Early studies reported the specific bindi...