The CEACAM1 N-terminal Ig domain mediates cis- and trans-binding and is essential for allosteric rearrangements of CEACAM1 microclusters

Klaile, Esther; Vorontsova, Olga; Sigmundsson, Kristmundur; Müller, Mario M.; Singer, Bernhard B.; Öfverstedt, Lars-Göran; Svensson, Stina; Skoglund, Ulf; Öbrink, Björn

The structures of soluble rat CEACAM1 ectodomains containing all four Ig domains, D(1-4), or lacking the N-terminal Ig domain, D(2-4), were determined by molecular electron tomography of vitrified specimens. 3D images were reconstructed by filtered backprojection and refined by constrained maximum entropy tomography (COMET), which allowed visualization of molecular details at a resolution of 20 Å. Particles were selected for structural analysis by two independent procedures: (1) gray-level thresholding and (2) seeded watershed segmentation (SWS; Fig. 2, A and B ). The recombinant . The threshold was 1,000 voxels for D(1-4) dimers and 500 voxels for monomers. All volumes <500 voxels were identified as background noise. All but no additional structures that were identified by SWS were also identified by gray-level thresholding. (B) Comparison of two monomer and two dimer particles from the tomogram analyzed in A identified by SWS (top; surface rendered; volumes given in voxels) and gray-level thresholding (bottom; volume rendered). Bars, 5 nm. (C and D) Comparison of the CEACAM1 D(1-4)-His solution (C) and plain buffer (D), both scaled to the same intensity levels by gray-level thresholding. 30-pixel-deep slices (300 × 160 × 30; pixel size, 5.74 Å) of the total reconstructed volumes are shown. Bars, 50 nm. yet allow recognition of the contacting interfaces even at 20-Å resolution, which gives an idea of the structural details that, at best, would be seen in glycosylated CEACAM1 dimers/oligomers by electron tomography.