Erythropoietin receptor dimerization

Seubert, N., Royer, Y., Staerk, J., Kubatzky, K. F., Moucadel, V., Krishnakumar, S., Smith, S. O., and Constantinescu, S. N. (2003). Active and inactive orientations of the transmembrane and cytosolic domains of the erythropoietin receptor dimer. 

Figure 17.12 Ribbon diagram of EMPl bound to the extracellular domain of the erythropoietin receptor (EBP). Binding of EMPl causes dimerization of erythropoietin receptor. The x-ray crystal structure of the EMPl-EBP complex shows a nearly symmetrical dimer complex in which both peptide monomers interact with both copies of EBP. Recognition between the EMPl peptides and EBP utilizes more than 60% of the EMPl surface and four of six loops in the erythropoietin-binding pocket of EBP.

A variation on the basic theme of receptor Tyr kinases is seen in receptors that have no intrinsic protein kinase activity but, when occupied by their ligand, bind a soluble Tyr kinase. One example is the system that regulates the formation of erythrocytes in mammals. The cytokine (developmental signal) for this system is erythropoietin (EPO), a 165 amino acid protein produced in the kidneys. When EPO binds to its plasma membrane receptor (Fig. 12-9), the receptor dimerizes and can now bind the soluble protein kinase JAK (Janus kinase). This binding activates JAK, which phosphory-lates several Tyr residues in the cytoplasmic domain of the EPO receptor. A family of transcription factors, collectively called STATs (signal transducers and activators of transcription), are also targets of the JAK kinase activity. An SH2 domain in STATS binds (P)-Tyr residues in the EPO receptor, positioning it for this phosphorylation by JAK. When STATS is phosphorylated in re-... 

Crystallographic evidence for preformed dimers of erythropoietin receptor before ligand activation. Science 283(5404), 987-990. 

Livnah, O., Stura, E. A., Middleton, S. A., Johnson, D. L., Jolliffe, L. K., and Wilson, I. A. (1999). Crystallographic evidence for preformed dimers of erythropoietin receptor before ligand activation. Sdence 283, 987-990. 

JAKs and signal transducers and activators of transcription (STATs) are functionally analogous with IRS and PI3K. JAKs are physically associated with a cell surface receptor (e.g. for leptin, erythropoietin (EPO), growth factors or cytokines) STATs are free monomeric proteins within the cytosol but following phosphorylation by a JAK, individual proteins dimerize and then move into the nucleus of the cell where they control gene expression. 

These include the receptors for erythropoietin, somatotropin, and interferons. Their receptors are membrane spanning and on activation can activate a distinctive set of cytoplasmic tyrosine kinases (Janus kinases [JAKs]). JAKs phosphory-late signal transducers and activators of transcription (STAT) molecules. STATs dimerize and then dissociate, cross the nuclear membrane, and modulate gene transcription. 

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