Class I Cytokine Receptors: Towards The Inside

The Growth Hormone Receptor (GHR) is a member of the Class I Cytokine Receptor family and comprises three different domains, an extracellular domain (ECD), a cell membrane spanning helix constituting the transmembrane domain (TMD) and an intracellular domain (ICD). Growth Hormone (GH) binds extracellularly to two receptors thereby activating two Janus Kinases 2 (JAK2) bound to the ICDs. Despite these receptors being studied for decades it remains an enigma how the signal of the extracellular binding event is translated into the cell. It has been suggested that the TMDs rearrange upon receptor activation, but the TMD dimer conformation of the inactive or active states is not known. We have investigated the structure and membrane interaction of the GHR TMD and ICD by NMR exploiting our new method for generation of isotopically labelled TMD. Reconstitution in DHPC detergent micelles allowed for assignments of the NMR backbone chemical shifts of the TMD and chemical shift analysis of the isolated GHR ICD indicates a fully intrinsically disordered ICD, including the 9 residues separating the TMD and the JAK2 binding site. Thus the question arises how the mechanical force of a hormone induced ECD and TMD rearrangement is translated to the JAK2 binding site, leading to its activation. Lipid binding sites in the GHR ICD juxtamembrane region have been proposed which would provide the required stability for a rigid body movement. In the present work we will present data that addresses these questions.