Cross-linking of CEACAM1 D(1–4)-His and D(2–4)-His. (A and C) Purified D(1–4)-His (A) or D(2–4)-His (C) were cross-linked with BS³ in the presence or absence of 2.5 mM Ca²⁺, 2.5 mM Mg²⁺, 3 mM EDTA, and Ni-NTA liposomes at a high protein/lipid ratio (hp; 1:10 [wt/wt]) or a low protein/lipid ratio (lp; 1:90 [wt/wt]) in various combinations. The samples were analyzed by Western blotting for CEACAM1 monomers/dimers/multimers. The black line indicates that intervening lanes have been spliced out. (B) Quantification of D(1–4)-His dimers and higher order multimers. (D) Quantification of D(2–4)-His dimers. Data show mean ± standard deviation of three independent experiments. Statistical analyses (t test) were made with reference to cross-linked protein with no extra additions (Untr.). Significant p-values (α level 0.05) are shown above the histogram bars.