Molecular tomography of CEACAM1 ectodomains attached to liposomes. Segmentation was performed by gray-level thresholding. (A) Tomogram of D(1–4)-His–decorated liposomes. The z direction is parallel to the electron beam. (B and C) Free surfaces of D(1–4)-His–decorated (B) and D(2–4)-His–decorated (C) liposomes. (D and E) Monomeric bridges (C-dimers) connecting two liposomes (L1 and L2) are shown at two angles. In the colored panels, D1–D1 contact zones are indicated by a black dotted line, and individual Ig domains are numbered 1–4. Arrowheads point to bridge positions, and arrows point to the binding areas. (F and G) Multimeric bridges connecting two liposomes (L1 and L2) are shown. The bridge in F is composed of seven molecules anchored to L1 (orange) and L2 (green). (G) Two bridging clusters of 6 and ∼10 molecules. (H and I) D(1–4) (n = 130) and D(2–4) (n = 163) monomers/clusters on free-liposome surfaces and D(1–4) monomers/clusters (n = 63) bridging two membranes were analyzed and classified according to size. The differences in each cluster class compared with D(1–4) (free) populations were analyzed by a two-proportion z test. P-values are shown above the histogram bars. Arrows indicate bridge positions. The overviews in B and C and the bridges in E and F are presented in 3D in Videos 2 and 3, respectively. (D–F) Liposome surfaces are indicated by white dashed lines.