Forced unfolding modulated by disulfide bonds in the Ig domains of a cell adhesion molecule

Cell adhesion molecules (CAMs) mediate cell attachment and stress transfer through extracellular domains. Here we forcibly unfold the Ig domains of a prototypical Ig superfamily CAM that contains intradomain disulfide bonds. The Ig domains of all such CAMs have conformations homologous to cadherin e xtracellular domains, titin Ig-type domains, and fibronectin type-ill (FNII I) domains. Atomic force microscopy has been used to extend the five Ig dom ains of Mel-CAM (melanoma CAM)-a protein that is overexpressed in metastati c melanomas-under conditions where the disulfide bonds were either left int act or disrupted through reduction. Under physiological conditions where in tradomain disulfide bonds are intact, partial unfolding was observed at for ces far smaller than those reported previously for either titin's Ig-type d omains or tenascin's FNIII domains. This partial unfolding under low force may be an important mechanism for imparting elasticity to cell-cell contact s, as well as a regulatory mechanism for adhesive interactions. Under reduc ing conditions, Mel-CAM's Ig domains were found to fully unfold through a p artially folded state and at slightly higher forces. The results suggest th at, in divergent evolution of all such domains, stabilization imparted by d isulfide bonds relaxes requirements for strong, noncovalent, folded-state i nteractions.