Interaction with protocadherin-gamma regulates the cell surface expression of protocadherin-alpha

The protocadherin-alpha (CNR/Pcdhalpha) and protocadherin-gamma (Pcdhgamma) proteins, members of the cadherin superfamily, are putative cell recognition/adhesion molecules in the brain. Overexpressed cadherins are generally expressed on the cell surface and elicit cell adhesion activity in several cell lines, although hardly any overexpressed CNR/Pcdhalpha proteins are expressed on the cell surface, except on HEK293T cells, which show low expression. We analyzed the expression of CNR/Pcdhalpha and Pcdhgamma in HEK293T cells and found that they formed a protein complex and that Pcdhgamma enhanced the surface expression of CNR/Pcdhalpha. This enhanced surface expression was confirmed by flow cytometry analysis and by marking cell surface proteins with biotin. The enhancement was observed using different combinations of CNR/Pcdhalpha and Pcdhgamma proteins. The surface expression activity was enhanced by the extracellular domains of the proteins, which could bind each other. Their cytoplasmic domains also had binding activity and influenced their localization. Their protein-protein interaction was also detected in extracts of mouse brain and two neuroblastoma cell lines. Thus, interactions between CNR/Pcdhalpha and Pcdhgamma regulate their surface expression and contribute to the combinatorial diversity of cell recognition proteins in the brain.