Sodium‐Dodecyl‐Sulfate‐Resistant Complex Formation of Epimorphin Monomers and Interaction of the 150‐kDa Complex with the Cell Surface

Epimorphin was previously identified as a mesenchymal factor essential for epithelial/mesenchymal interaction. Since a monoclonal antibody, MC‐1, inhibits the biological activity of epimorphin when added to the extracellular medium, the location of this factor is thought to be extracellular. However, sequence analysis of the isolated epimorphin cDNA revealed that its product resembled other proteins that are involved in intracellular vesicular transport. Furthermore, the molecular size of epimorphin predicted from the cloned cDNA was not consistent with the size observed by immunoblot analyses. In the present study, an attempt was made to resolve these inconsistencies in the nature and function of epimorphin. Evidence is presented for the following: (a) monomeric epimorphin forms various sodium‐dodecyl‐sulfate‐resistant complexes, each displaying a unique immunoreactivity, of which MC‐1 recognizes only a certain form; (b) the MC‐1‐reactive form of epimorphin is sensitive to extracellular trypsin whereas the other forms are not; (c) cells selectively attach to surfaces coated with recombinant epimorphin fragments containing the MC‐1 epitope in vitro. These results suggest that a certain population of epimorphin molecules is involved in cell/cell interaction through a process of complex formation, transportation to extracellular regions, and direct binding to the cell surface.