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 multimers that were formed by D(1-4) in solution disappeared when the protein was attached to liposomes at a low protein/liposome ratio. This can be explained by a dilution of the ectodomains on the membrane surface, which will result in persistence only of the most stable dimer forms in which all four Ig domains are in register with each other. At a higher protein/liposome ratio, a significant increase in the amount of cross-linked D(1-4) A-dimers occurred. This was caused by the presence of domain D1 in the CEACAM1 ectodomain but was not a result of increased local concentration of membraneattached protein because a corresponding increase of crosslinked D(2-4) A-dimers did not occur. Instead, the enhanced A-dimer formation was most likely a function of D1-mediated, antiparallel (trans) C-dimerization because it only occurred at the higher protein/liposome ratio, which resulted in more efficient homophilic liposome adhesion. It was not simply a result of increased local concentration of CEACAM1 ectodomains in the membrane contact regions because the statistical analysis showed that the CEACAM1 clusters were at the same time rearranged to become significantly smaller. Rather, it suggests that formation of trans-C-dimers between liposomal membranes induced cis-A-dimer formation by an allosteric mechanism transmitted by Ig domain D1, which is in agreement with the tomographic results that showed an increased abundance of dimeric clusters in the liposomal bridges.