Cryo-EM structures reveal a dynamic transformation process of human alpha-2-macroglobulin working as a protease inhibitor

Human alpha-2-macroglobulin is a well-known proteases inhibitor against a broad spectrum of proteases. It also plays important roles in immunity, inflammation, and infections. Here, we report cryo-EM structures of human alpha-2-macroglobulin of the native state, the transformed state induced by its authentic substrate, human trypsin, and serial intermediate states between the native and the fully induced state. These structures exhibit distinct conformations, which reveal a dynamic transformation process of alpha-2-macroglobulin acting as a protease inhibitor. The results shed light on the molecular mechanism of alpha-2-macroglobulin entrapping substrates, and help to understand how alpha-2-macroglobulin possesses variant physiological functions. ### Competing Interest Statement The authors have declared no competing interest. * A2M : alpha-2-macroglobulin cryo-EM : cryo-electron microscopy hA2M : human alpha-2-macroglobulin nA2M : native alpha-2-macroglobulin iA2M : induced alpha-2-macroglobulin iA2M-trypsin : A2M induced by trypsin MA : methylamine iA2M-MA : A2M induced by MA n-monomer : native monomer i-monomer : induced monomer XL-MS : cross-linking mass spectrometry NSTEM : negative staining transmission electron microscopy SAXS : small-angle X-ray scattering BRD : bait region domain TED : thioester motif domain RBD : receptor binding domain MG1-MG7 : macroglobulin-type domains 1 to 7 CUB : complement C1r/C1s, Uegf, Bmp1 domain ITC : isothermal titration calorimetry CD : the circular dichroism 3D : three dimentional LRP1 : low-density lipoprotein receptor-related protein 1 ECAM : A2M in Escherichia coli SCAM : Salmonella enterica ser