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

Fig. 5) showed that domains D2 and D3 do not bind to each other in an antiparallel manner, the D(1-4) ectodomains should be oriented in a parallel manner in these dimers. In addition, a small fraction of CEACAM1 trimers occurred. In all of the trimers, two of the ectodomains were closely associated in a parallel manner (representing A-dimers), whereas the third molecule was bound via its larger Ig domain to one of the larger end domains of the parallel A-dimer (Fig. 4 H) . Because the BIAcore experiments showed no D1-D4-or D4-D4-binding interactions, the binding domain in the parallel dimer must be one of the D1 domains. Thus, the structure of this trimeric assembly demonstrates in agreement with the SPR data that two different sites of the D1 domain can participate simultaneously in homophilic binding.