A possible new adhesive site in the cell-adhesion molecule uvomorulin.

The calcium-dependent cell-adhesion molecule uvomorulin is a member of the cadherin gene family. Recent studies on the homophilic binding of molecules from neighbouring cells have shown that the amino-terminal part of these proteins plays an important role in the adhesive mechanism. We show here that the epitope for monoclonal antibody DECMA-1, capable of blocking uvomorulin function, is located close to the membrane proximal part of the extracellular domain. To test the effect of structural changes in this membrane proximal region on the adhesive function of uvomorulin, we have studied the cluster of cysteine residues located in the vicinity of the DECMA-1 epitope. Treatment of cells with the reducing agent dithiothreitol (DTT) cleaved the di-sulphide bonds in uvomorulin and affected the adhesive properties of cells. Close cell-cell contacts accompanied by cell flattening and changes in cell shape were blocked by DTT; however, cell aggregation was not inhibited. Consistent with this, uvomorulin became more susceptible in its membrane proximal part to trypsin digestion after treatment with DTT, indicating that conformational changes in this region of the molecule affect the adhesive function. These results suggest that the membrane proximal region of uvomorulin is involved in the adhesive mechanism.