Fig. 5. Force spectra of ALCAM-mediated interactions. (A) The mean rupture forces for the homotypic (circle) and heterotypic (square) ALCAM-mediated interactions were found to increase linearly with the natural logarithm of the loading rate. This behavior is consistent with the Bell model (see text, R²>0.95). The results obtained using KG1a (white symbol) were similar to those using K562-ALCAM cells (black). At loading rates >1500 pN/second, the forces associated with ALCAM-CD6 bond rupture were significantly higher than those for the ALCAM-ALCAM interaction (n>20, P<0.05; error bars indicate s.e.m.). (B) Schematic representation of the significance of the Bell model parameters in terms of the energy barrier between the bound and unbound state. The situation when no force is applied (solid lines) or when external forces are applied to the bonds (dotted lines, see Discussion) is represented. (C) Comparison of the histograms of rupture forces (K562-ALCAM cells, n>70; three loading rates) and the theoretical probability density distributions for the failure of single ALCAM-ALCAM and ALCAM-CD6 bonds.