The β(2)-Subunit (AMOG) of Human Na(+), K(+)-ATPase Is a Homophilic Adhesion Molecule

The β(2) subunit of Na(+), K(+)-ATPase was originally identified as the adhesion molecule on glia (AMOG) that mediates the adhesion of astrocytes to neurons in the central nervous system and that is implicated in the regulation of neurite outgrowth and neuronal migration. While β(1) isoform have been shown to trans-interact in a species-specific mode with the β(1) subunit on the epithelial neighboring cell, the β(2) subunit has been shown to act as a recognition molecule on the glia. Nevertheless, none of the works have identified the binding partner of β(2) or described its adhesion mechanism. Until now, the interactions pronounced for β(2)/AMOG are heterophilic cis-interactions. In the present report we designed experiments that would clarify whether β(2) is a cell–cell homophilic adhesion molecule. For this purpose, we performed protein docking analysis, cell–cell aggregation, and protein–protein interaction assays. We observed that the glycosylated extracellular domain of β(2)/AMOG can make an energetically stable trans-interacting dimer. We show that CHO (Chinese Hamster Ovary) fibroblasts transfected with the human β(2) subunit become more adhesive and make large aggregates. The treatment with Tunicamycin in vivo reduced cell aggregation, suggesting the participation of N-glycans in that process. Protein–protein interaction assay in vivo with MDCK (Madin-Darby canine kidney) or CHO cells expressing a recombinant β(2) subunit show that the β(2) subunits on the cell surface of the transfected cell lines interact with each other. Overall, our results suggest that the human β(2) subunit can form trans-dimers between neighboring cells when expressed in non-astrocytic cells, such as fibroblasts (CHO) and epithelial cells (MDCK).