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 the presence of Ca/Mg, model 3 (the trimer model) clearly gave the best fit to the experimental values with the lowest  2 (Fig. 1 B) , which demonstrated that both type 1 and 2 reactions were recorded under these conditions. Fitting according to the trimer model gave the following values of the binding constants in Ca/Mg: k a1 = 0.0890 ± 0.0034 µM 1 s 1 ; k d1 = 0.6806 ± 0.0074 s 1 ; K D1 = 7.65 µM; k a2 = 0.0000598 ± 0.0000033 µM 1 s 1 ; k d2 = 0.01241 ± 0.00038 s 1 ; and K D2 = 208 µM. Thus, these results demonstrate that CEACAM1 D(1-4) ectodomains participate in two different, simultaneously occurring homophilic binding reactions. From the kinetic rate constants, it could be determined that the equilibrium concentrations of type 1 dimers were significantly higher than those of type 2 dimers at all protein concentrations.