JAK2 activation by growth hormone and other cytokines

Abstract

Growth hormone (GH) and structurally related cytokines regulate a great number of physiological and pathological processes. They do this by coupling their single transmembrane domain (TMD) receptors to cytoplasmic tyrosine kinases, either as homodimers or heterodimers. Recent studies have revealed that many of these receptors exist as constitutive dimers rather than being dimerized as a consequence of ligand binding, which has necessitated a new paradigm for describing their activation process. In the present study, we describe a model for activation of the tyrosine kinase Janus kinase 2 (JAK2) by the GH receptor homodimer based on biochemical data and molecular dynamics simulations. Binding of the bivalent ligand reorientates and rotates the receptor subunits, resulting in a transition from a form with parallel TMDs to one where the TMDs separate at the point of entry into the cytoplasm. This movement slides the pseudokinase inhibitory domain of one JAK kinase away from the kinase domain of the other JAK within the receptor dimer-JAK complex, allowing the two kinase domains to interact and trans-activate. This results in phosphorylation and activation of STATs and other signalling pathways linked to this receptor which then regulate postnatal growth, metabolism and stem cell activation. We believe that this model will apply to most if not all members of the class I cytokine receptor family, and will be useful in the design of small antagonists and agonists of therapeutic value.

Figure 1. Domain structure of Class I cytokine receptors

Domain structures depicted for the homomeric signalling receptors, and examples of heteromeric receptors that interact with the shared γc, gp130, βc and LIF-R (LIFR).

Figure 2. Growth hormone and the growth hormone receptor

Crystal structure of the growth hormone and the growth hormone receptor extracellular domain with illustration representing the extracellular juxtamembrane linker, transmembrane domain, intracellular domain and Box1 motif. The growth hormone high affinity site 1 and lower affinity site 2 which are supplemented by receptor–receptor binding in the lower FNIII dimerization domain (site 3) are indicated.

Figure 3. Transition of the GH receptor TMDs

The interaction of the GH receptor TMDs in the inactive parallel State 1 and active crossover State 2 are shown.

Figure 4. Activation model of homodimeric class I cytokine receptors shown for the archetypal GH receptor

Figure 5. Model of JAK2 activation for the GH receptor

In the inactive state (top), two JAK2 molecules, each bound to a GH receptor intracellular domain, interact so that the kinase domain of one JAK2 is inhibited by the pseudokinase domain of the other JAK2. GH binding causes separation of the ICD Box1 motifs and leads to separation of the pseudokinase–kinase trans-interaction and results in kinase–kinase trans-interaction and activation (bottom).