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

An important feature of N-terminal D1 domains in the CEACAM family is that they, in contrast to other Ig domains, lack intradomain disulfide bridges (Watt et al., 2001) . This might be a prerequisite for the allosterically regulated homophilic binding interactions that we propose play an important role in CEACAM1 adhesion-induced transmembrane signaling. A similar lack of disulfides occurs in the N-terminal Ig domain of CD2, which also has been demonstrated to change conformation upon adhesion (Li et al., 1996) . Interestingly, the N-terminal domain of CD2 has been shown to be in a metastable state, which can result in exchange of  strands between the domains in a CD2-dimer, resulting in a dimeric structure in which each domain is formed by the intercalation of two polypeptide D(1-4) dimers was significantly larger than that of D(2-4) under all conditions. Importantly, these findings show that the D1 domain can participate in mutual cis-binding in addition to trans-binding, demonstrating that it has two different homophilic binding sites, which is in agreement with the interpretation of the trimers identified by electron tomography. The cross-linked multimers formed by the D(1-4) ectodomains are clearly caused by the presence of domain D1 because no complexes larger than dimers were formed by D(2-4) under any conditions. These multimers most likely form because of the flexibility of the ectodomains, which allows combinations of parallel, A-type interactions between Ig domains belonging to several D(1-4) ectodomains (Fig. 8) .