Molecular tomography of CEACAM1 D(1–4)-His and D(2–4)-His ectodomains. Segmentation was performed by gray-level thresholding. (A–K) Molecules are shown unaltered, color coded, and displayed together with schematic models (individual Ig domains are numbered when unambiguously identified). (A–D) Monomeric D(1–4)-His adopting an extended (A and B), kinked (C), or back-folded (D) form. (E) D(1–4)-His dimer interacting exclusively via the D1 domains (C-dimer). (F and G) D(1–4)-His dimers interacting via three (F) or four (G) of their Ig domains (A-dimers). (H) D(1–4)-His trimer consisting of a monomer (green) binding via its D1 domain to an A-dimer (red/blue). Molecules displayed in A–H are presented in 3D in Video 1. (I and J) D(2–4)-His monomers adopting an extended (I) or kinked (J) form. (K) D(2–4)-His parallel dimer. (L) All ectodomains in several reconstructed volumes containing D(1–4)-His in the presence of Ca/Mg (12 vol; 1,091 molecules) or EDTA (14 vol; 888 molecules) and D(2–4)-His in the presence of Ca/Mg (5 vol; 211 molecules) were analyzed. The ectodomains were classified as extended monomers (mono e), kinked plus back-folded monomers (mono k+bf), C-dimers (di C), A-dimers (di A), and trimers (tri). The differences in each ectodomain class compared with D(1–4) (Ca/Mg) populations were analyzed by two-proportion z test. P-values are shown above the histogram bars. Bars, 5 nm.