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

Tomography showed that CEACAM1-induced liposome adhesion was mediated by reciprocal binding between D1 domains presented both by single molecules and by small clusters of CEACAM1. The bridging clusters were both smaller and had different molecular packing compared with the freesurface clusters, and the cross-linking results indicated that this was a result of formation of D1-mediated antiparallel adhesion. An important piece of information for the interpretation of the cross-linking data is that we could show that antiparallel C-dimers mediating cell-cell adhesion are not stabilized by amine-reactive cross-linkers (see Müller et al. on p. 569 of this issue). Thus, the cross-linked dimers should exclusively represent parallel A-dimers. Also, the truncated D(2-4) ectodomains gave rise to a small proportion of crosslinked dimers, although the weaker interaction mediated by D(2-4) was not picked up by the SPR analysis. However, the A-dimerization contributed by Ig domain D1 can also be stabilized by cross-linking because the proportion of stabilized results demonstrate that the more complex oligomerization of D(1-4) is a function of the interactions of the D1 domain and suggest that the adhesion-promoting property of CEACAM1 is an important regulator of the organization and intermolecular interactions of CEACAM1 within the membranebound clusters.