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Structural Mechanics of the Alpha-2-Macroglobulin Transformation

Yasuhiro Arimura and Hironori Funabiki


Published in Journal of Molecular Biology, Mar 2022 (Link)


- 3.6–4.1 Å resolution cryo-EM structure of native-form A2M tetramer was determined

- 6.6 Å resolution cryo-EM structure of intermediate-form A2M tetramer was determined

- Cross-like native A2M tetramer structure explains how A2M captures large proteases

- The intact bait region works as a latch lock to block the A2M conformational change


Alpha-2-Macroglobulin (A2M) is the critical pan-protease inhibitor of the innate immune system. When proteases cleave the A2M bait region, global structural transformation of the A2M tetramer is triggered to entrap the protease. The structural basis behind the cleavage-induced transformation and the protease entrapment remains unclear. Here, we report cryo-EM structures of native- and intermediate-forms of the Xenopus laevis egg A2M homolog (A2Moo or ovomacroglobulin) tetramer at 3.7–4.1 Å and 6.4 Å resolution, respectively. In the native A2Moo tetramer, two pairs of dimers arrange into a cross-like configuration with four 60 Å-wide bait-exposing grooves. Each bait in the native form threads into an aperture formed by three macroglobulin domains (MG2, MG3, MG6). The bait is released from the narrowed aperture in the induced protomer of the intermediate form. We propose that the intact bait region works as a “latch-lock” to block futile A2M transformation until its protease-mediated cleavage.


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