Mutation of lysine 1370 in full-length human α2-macroglobulin blocks binding to the low density lipoprotein receptor-related protein-1

α2-Macroglobulin (α2M) regulates cell physiology by binding to cellular receptors; however, residues that contribute to receptor-binding have not been elucidated in the full-length protein. In α2M fragments, expressed in bacteria, Lys1370 and Lys1374 are critical for binding to the low density lipoprotein receptor-related protein-1 (LRP-1) and a distinct α2M-signaling receptor. We expressed full-length recombinant human α2M (rα2M) and mutants in which Lys1370 or Lys1374 was converted to alanine in K-562 cells. The rα2M species demonstrated intact structure and function, as determined by subunit size, intersubunit disulfide bonds, reaction with trypsin or methylamine, and ability to undergo conformational change. Binding of transforming growth factor-β1 was unaltered. Mutation of Lys1370 almost entirely inhibited specific binding of methylamine-activated rα2M to RAW 264.7 cells. Mutation of Lys1374 had no effect. Binding of rα2M to RAW 264.7 cells was blocked by receptor-associated protein, indicating an essential role for LRP-1. These studies demonstrate that a single mutation in full-length rα2M is sufficient to block binding to LRP-1.