Dimerization of the erythropoietin receptor transmembrane domain in micelles.

Erythropoietin receptor (EpoR) homodimerization is an initial regulatory step in erythrocyte formation. Receptor dimers form before ligand binding, suggesting that association between receptor proteins is dependent on the receptor itself. EpoR dimerization is an essential step in erythropoiesis, and misregulation of this dimerization has been implicated in several disease states, including multi-lineage leukemias; nevertheless, how EpoR regulates its own dimerization is unclear. In vivo experiments suggest the single-pass transmembrane helix is the strongest candidate for driving ligand-independent association. To address the self-association potential of this transmembrane segment, we studied its interaction energetics in micelles by utilizing a previously successful Staphylococcal nuclease (SN-EpoR TM) fusion protein. This fusion protein strategy allows expression of the EpoR transmembrane domain in Escherichia coli independent of the other EpoR domains. Sedimentation equilibrium analytical ultracentrifugation of the detergent-solubilized SN-EpoR TM demonstrated that the murine EpoR transmembrane domain self-associates to form dimers. Although this interaction is not as stable as the dimerization of the well-studied glycophorin A transmembrane dimer, the murine EpoR transmembrane domain dimer is more stable than the interactions of the colon carcinoma kinase 4 transmembrane domain. The same experiments with the human EpoR transmembrane domain, which differs from the mouse sequence by only three residues, revealed a less favorable interaction than that of the murine sequence and is only slightly more favorable than that expected for non-preferential binding. These results suggest that the mouse and human receptor proteins may differ in the roles they play in signaling.