Beta-amyloid peptide binds equivalently to binary and ternary alpha2-macroglobulin-protease complexes.
alpha2-Macroglobulin (alpha2M) is a protease inhibitor that has separate binding sites for transforming growth factor-beta (TGF-beta) and beta-amyloid peptide (Abeta), both of which have been identified in the beta2M sequence. In the 3D-structure of alpha2M, TGF-beta occupies the alpha2M central cavity, overlapping with the space that can accommodate up to two molecules of protease. As a result, ternary alpha2M-protease complexes (2 mol protease/mol alpha2M) have been reported to not bind TGF-beta. The goal of the present study was to test whether binding of Abeta to alpha2M is controlled by steric constraints imposed by associated proteases, similarly to TGF-beta. We confirmed that binary alpha2M-trypsin complex (1 mol trypsin/mol alpha2M) binds increased amounts of TGF-beta1, compared with native alpha2M, while ternary alpha2M-trypsin complex binds substantially decreased amounts of TGF-beta1. By contrast, Abeta-binding to binary and ternary alpha2M trypsin complex was equivalent. In both cases, binding was substantially increased compared with the negligible level observed with native alpha2M. Plasmin is a large protease (Mr approximately 82,000) that substantially occupies the alpha2M central cavity; however, alpha2M-plasmin complex also bound increased amounts of Abeta, compared with native alpha2M. We conclude that Abeta accesses its binding site, in alpha2M, from outside the alpha2M central cavity. The TGF-beta- and Abeta-binding sites are spatially separated not only in the primary sequence of alpha2M, but also in the 3D-structure.
Mettenburg
Joseph M
JM
Department of Biochemistry and Molecular Genetics University of Virginia School of Medicine, Charlottesville, Virginia, 22908, USA.
Gonias
Steven L
SL
D016229:Amyloid beta-Peptides / Q000737:chemistry / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D010447:Peptide Hydrolases / Q000378:metabolism*; D010455:Peptides / Q000378:metabolism*; D011485:Protein Binding; D016212:Transforming Growth Factor beta / Q000378:metabolism; D000511:alpha-Macroglobulins / Q000378:metabolism*
D016229:Amyloid beta-Peptides; D010455:Peptides; D016212:Transforming Growth Factor beta; D000511:alpha-Macroglobulins; D010447:Peptide Hydrolases