alpha 2-macroglobulin traps a proteinase in the midregion of its arms. An immunoelectron microscopic study.
alpha 2-Macroglobulin, one of the major plasma proteinase inhibitors with Mr = 720,000, is known to inhibit proteinases of all four classes through the "trap mechanism" (Barrett, A. J., and Starkey, P. M. (1973) Biochem. J. 133, 709-724), but the proteinase binding site of alpha 2-macroglobulin has not been identified precisely. We localized bound proteinase molecules on the electron microscopic images of alpha 2-macroglobulin, using anti-proteinase IgG. Serratial Mr = 56,000 proteinase produced by Serratia marcescens was chosen as the antigenic probe in this study because its affinity to specific antibodies was retained in its bound state to alpha 2-macroglobulin. Dimers of alpha 2-macroglobulin/Mr = 56,000 proteinase complexes cross-linked with anti-Mr = 56,000 proteinase IgG were prepared and subjected to electron microscopic observations. The electron microscopic image of alpha 2-macroglobulin complexed with Mr = 56,000 proteinase had four straight arms with an overall shape looking like the character "H." From the way anti-Mr = 56,000 proteinase IgG linked two alpha 2-macroglobulins, it was concluded that the proteinase existed in the midregion of one of the arms. This result helps us to form a more concrete view of the trap mechanism in that one of the arms of alpha 2-macroglobulin wraps the trapped proteinase and holds it isolated from high molecular weight substrates in the surrounding medium.
Arakawa
H
H
Department of Biochemistry and Biophysics, Faculty of Science, University of Tokyo, Japan.
Nishigai
M
M
Ikai
A
A
D001665:Binding Sites; D006801:Humans; D007700:Kinetics; D046911:Macromolecular Substances; D008854:Microscopy, Electron; D008958:Models, Molecular; D008970:Molecular Weight; D010447:Peptide Hydrolases / Q000097:blood*; D011480:Protease Inhibitors / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D000511:alpha-Macroglobulins / Q000378:metabolism* / Q000648:ultrastructure
D046911:Macromolecular Substances; D011480:Protease Inhibitors; D000511:alpha-Macroglobulins; D010447:Peptide Hydrolases