Epithelial cadherin is important in establishing and maintaining cell to cell interactions in epithelial cells, thereby playing an important role during morphogenesis. The epithelial cadherin molecules have three main regions: the N-terminal extracellular region, the transmembrane region that spans the cell membrane once, and the C-terminal cytoplasmic region that communicates with the cytoskeletal actin filaments through catenins. We report studies of the calcium-dependent stability of extracellular domains 1 and 2 of epithelial cadherin as a two-domain construct (MECAD12). Circular dichroism (CD) spectra of MECAD12 indicated a typical beta-sheet conformation in all solution conditions. Thermal- and denaturant-induced unfolding was monitored by CD. Distinct calcium stabilization was observed as a shift in T-m, from 40 (apo) to 65 degrees C (10 mM Ca2+). Spectroscopic experiments agreed well with calorimetric (DSC). In the absence of calcium, the unfolding transition was shallow (Delta H-m. = 40 kcal/mol) but not obviously three state. Model-dependent analysis indicated that a second transition could be assigned to the unfolding, of domain 2. A calcium-binding constant was derived from the calcium-dependent shift in temperature denaturation profiles. The K-d that was obtained (55 mu M) was consistent with literature values. Thus, the modular domains of epithelial cadherin exhibit context-dependent behavior in both the apo and calcium-bound states. This cooperativity between the modules is consistent with the physiological role of epithelial cadherin in signal transduction through cell-adhesive contacts.
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