Functional studies on receptor-type protein tyrosine phosphatases of the R3 subgroup

The receptor-type protein tyrosine phosphatases (RPTPs) of the R3subgroup play key roles in the immune, vascular and nervous system. They are characterised by an extracellular domain (ECD), comprised of multiple FNIII-like repeats, a transmembrane domain and a single intracellular phosphatase domain. Although their phosphatase domains have been Fstudied in detail the functional roles of their extracellular regions have not been clearly defined. Potential roles in ligand interaction, dimerisation and cell-cell contacts have been reported. Here I used a bimolecular fluorescence complementation (BiFC) assay in live cells to examine the molecular basis for the interaction of one of the R3 RPTP members, VE-PTP, with VE-cadherin, and explored the potential of others to interact with this protein. The potential of R3 RPTPs to homo-dimerise via extracellular domains in live cells was also addressed. Quantitative BiFC analysis using sialophorin (SPN), an unrelated membrane protein, and a membrane anchored C-terminal Venus-YFP (Myr-VC) fragment as controls revealed a specific interaction between VE-PTP and VE-cadherin using constructs expressing only the extracellular and transmembrane domains. Use of a deletion mutant indicated that, in contrast to previous studies, removal of the 17th FNIII-like domain of VE-PTP is not sufficient to disrupt this interaction. Other members of the R3 RPTP family (DEP-1, GLEPP1 and SAP-1) also exhibited the potential to interact with VE-cadherin suggesting that specificity of this protein-protein interaction is not determined by the ECD alone. The direct interaction of DEP-1 with VE-cadherin is likely to be of physiological relevance since both proteins are expressed in endothelial cells. GLEPP1 and SAP-1 exhibited homo-dimerisation, whereas DEP-1 and VE-PTP did not form dimers via their extracellular and/or transmembrane domains. SPN was identified as a possible bona fide ligand for DEP-1 and their interaction is likely to be of physiological relevance since they were both shown to regulate T cell receptor activation. The interactions identified in the present study suggest a role for both the extracellular domain and transmembrane domain of R3-PTPs in interaction with VE-cadherin. The study also highlights the importance of using multiple controls in BiFC experiments and quantitative analysis of results.