Oligomeric state of the colon carcinoma-associated glycoprotein GA733-2 (Ep-CAM/EGP40) and its role in GA733-mediated homotypic cell-cell adhesion

The GA733-2 antigen (GA733) is a homotypic calcium-independent cell adhesio n molecule (CAM) present in most normal human epithelial cells and gastroin testinal carcinomas. Because oligomerization of some CAMs regulates cell ad hesion and signal transduction, the correlation between GA733 oligomeric st ate and cell-cell adhesion was investigated. Sedimentation equilibrium stud ies showed that full-length ( FL) GA733 exists as dimers and tetramers in s olution, whereas the GA733 extracellular domain (-EC) is a monomer, The K-d of GA733-FL is less than 10 nM for the monomer-dimer association, whereas the dimer-tetramer association is about 1000-fold weaker (K-d similar to 10 muM). Chemical crosslinking of purified GA733-FL in solution resulted in a major product corresponding to GA733 dimers, and minor amounts of trimers and tetramers. However, GA733-EC cross-linked under the same conditions was consistently a monomer. Chemical cross linking of dissociated colon carcin oma cells produced predominantly GA733 dimers, whereas cross-linking of cel ls in monolayers yielded some tetramers as well. GA733-FL retained its cell -cell adhesion function as shown by inhibition of cell aggregation, whereas monomeric GA733-EC was inactive. These data show that GA733 exists predomi nantly as high affinity noncovalent cis-dimers in solution and on dissociat ed colon carcinoma cells, The lower affinity association of dimers to form tetramers is most likely the head-to-head interaction between GA733 cis-dim ers on opposing cells that represents its cell-cell adhesion activity.