Receptor-Like Protein Tyrosine Phosphatase α Homodimerizes on the Cell Surface

We reported previously that the N-terminal D1 catalytic domain of receptor protein-tyrosine phosphatase α (RPTPα) forms a symmetrical, inhibited dimer in a crystal structure, in which a helix-turn-helix wedge element from one monomer is inserted into the catalytic cleft of the other monomer. Previous functional studies also suggested that dimerization inhibits the biological activity of a CD45 chimeric RPTP and the catalytic activity of an isolated RPTPς D1 catalytic domain. Most recently, we have also shown that enforced dimerization inhibits the biological activity of full-length RPTPα in a wedge-dependent manner. The physiological significance of such inhibition is unknown, due to a lack of understanding of how RPTPα dimerization is regulated in vivo. In this study, we show that transiently expressed cell surface RPTPα exists predominantly as homodimers, suggesting that dimerization-mediated inhibition of RPTPα biological activity is likely to be physiologically relevant. Consistent with our published and unpublished crystallographic data, we show that mutations in the wedge region of D1 catalytic domain and deletion of the entire D2 catalytic domain independently reduced but did not abolish RPTPα homodimerization, suggesting that both domains are critically involved but that neither is essential for homodimerization. Finally, we also provide evidence that both the RPTPα extracellular domain and the transmembrane domain were independently able to homodimerize. These results lead us to propose a zipper model in which inactive RPTPα dimers are stabilized by multiple, relatively weak dimerization interfaces. Dimerization in this manner would provide a potential mechanism for negative regulation of RPTPα. Such RPTPα dimers could be activated by extracellular ligands or intracellular binding proteins that induce monomerization or by intracellular signaling events that induce an open conformation of the dimer.