Dimerization of receptor Protein-Tyrosine Phosphatase alpha in living cells

Background: Dimerization is an important regulatory mechanism of single membrane-spanning receptors. For instance, activation of receptor protein-tyrosine kinases (RPTKs) involves dimerization. Structural, functional and biochemical studies suggested that the enzymatic counter arts of RPTKs, the receptor protein-tyrosine phosphatases (RPTPs), are inhibited by dimerization, but whether RPTPs actually dimerize in living cells remained to be determined. Results: In order to assess RPTP dimerization, we have assayed Fluorescence Resonance Energy Transfer ( FRET) between chimeric proteins of cyan- and yellow-emitting derivatives of green fluorescent protein, fused to RPTP, using three different techniques: dual wavelength excitation, spectral imaging and fluorescence lifetime imaging. All three techniques suggested that FRET occurred between RPTP-CFP and -YFP fusion proteins, and thus that RPTP dimerized in living cells. RPTP dimerization was constitutive, extensive and specific. RPTP dimerization was consistent with cross-linking experiments, using a non-cell-permeable chemical cross-linker. Using a panel of deletion mutants, we found that the transmembrane domain was required and sufficient for dimerization. Conclusions: We demonstrate here that RPTP dimerized constitutively in living cells, which may be mediated by the transmembrane domain, providing strong support for the model that dimerization is involved in regulation of RPTPs.