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

In this study, we show that CEACAM1 behaves as a molecular system characterized by dynamic homophilic binding interactions. Three different methods, SPR-binding analysis, molecular tomography, and chemical cross-linking, consistently demonstrated that CEACAM1 ectodomains occur as a mixture of monomers, dimers, and oligomers. The tomographic analyses showed that the CEACAM1 ectodomain is flexible, being able to adopt several different conformations as a result of hinge regions between all of the Ig domains. Antiparallel trans-dimers (C-dimers) and parallel cis-dimers (A-dimers) could be distinguished. Also, the SPR-binding analyses identified the C-and A-dimerization reactions and demonstrated a rapid transition between monomers and C-dimers. The N-terminal D1 domain participated both in C-and A-dimerization, whereas domains D2-D4 were involved only in A-dimerization. All three methods revealed a partial dependence of divalent cations, which favored decreased ectodomain flexibility and enhanced formation of multimeric complexes. Because both dimerization reactions were enhanced, extracellular Ca/Mg should contribute to formation of CEACAM1-mediated cell adhesion.