(A) A1AT polymers were extracted from the explant liver tissue of a ZZ homozygote and an MZ heterozygote. The purified material was resolved by nondenaturing PAGE with M A1AT monomer and heat-induced polymer for reference and visualized by immunoblot with the anti-A1AT polymer mAb_2C1 (right panel) and anti–total A1AT polyclonal antibody after stripping and reprobing the membrane (left panel). (B) The purified heat-induced M polymers, as well as MZ and ZZ liver polymers, were imaged by uranyl acetate negative-stain EM in the absence (top panels) and presence (bottom panels) of complexed Fab_2H2. Representative micrographs are shown. Scale bars: 60 nm. (C) Polymers with (blue) or without (red) at least 1 Fab_2H2 protuberance were classified according to shape and the number of constituent subunits recorded. The mean polymer length is indicated by the central bar ± SD; linear polymers with and without a detectable M component were 7.4 ± 4.0 and 6.5 ± 4.2 subunits in length, respectively, and circular polymers had 8.1 ± 2.6 (with) and 6.6 ± 2.6 (without) subunits (±SD). Polymer length differences in the presence (n = 53) or absence (n = 159) of detectable M subunits were not statistically significant by a Mann-Whitney U test. (D) Single-particle analysis of micrograph images of Fab_2H2-labeled heat-induced polymers, showing class sums representing the average of 111–624 dimer particle images each (columns 1 and 3) and the corresponding optimally oriented 3-dimensional structures (columns 2 and 4). The A1AT subunits are shown in blue, the Fab heavy chain in red, and the light chain in green. (E) The relationship between the dihedral angle defined by the centers of mass of the 2 Fab_2H2 molecules and A1AT molecules in the dimer is shown, along with the distance between the A1AT centers of mass, as obtained from the structures in D.