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JAK2 activation by growth hormone and other cytokines.

Waters MJ, Brooks AJ - Biochem. J. (2015)

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

Affiliation: *Institute for Molecular Bioscience, The University of Queensland Institute, QLD 4072, Australia.

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.

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

Mentions: The model derived from these experimental and in silico findings predicts that binding of a bivalent ligand to the GH receptor will bring the ECD site 3 structures into proximity and then association through subunit rotation, and this will facilitate closer apposition of the upper JM sequence by overcoming the electrostatic repulsion of the JM EED sequence. Apposition will be stabilized with the aid of hydrogen bonds between Glu-260 of receptor 1 and the amide of Trp-267 in receptor 2, and between the backbone carbonyl of Phe-257 and amide of Glu-260 in receptor 2 [44]. Apposition of the upper TMD will result in relative rotation of the helices from the parallel State 1 to the left hand crossover State 2, with widening of the distance between the C-termini of the TMD helices. The leads to the key question: how could lower TMD separation result in activation of the JAK2 tyrosine kinases associated with the membrane proximal receptor Box1 motifs? Of particular relevance here is the existence of a pseudokinase (PK) domain adjacent to the kinase domain, which is known to be inhibitory [63]. Interestingly, of the 518 protein kinases, 48 contain PK domains, but only five of those contain an additional kinase domain. These are the four JAKs and the serine/threonine kinase GCN2 [64]. Hence any model for activation must remove the PK domain away from the kinase domain, and it is notable that myeloproliferative neoplasms with constitutive JAK2 activation possess point mutations in the PK domain or the linker to the adjacent SH2 domain, rather than the kinase domain. Since Lodish's group has shown that the constitutively active V617F mutant of JAK2 does not manifest its activity unless the EPO receptor is present [65,66] (or if the mutant is overexpressed [67]), the activation process is likely a trans-mediated activation event within a receptor/JAK2 dimer rather than a cis-activation process. If the PK inhibitory domain of one JAK2 was blocking the kinase domain of the other JAK2, then pulling the complex apart (such as would occur with separation of the Box1 motifs) would remove this trans-inhibition, and place the two kinase domains into proximity which could induce trans-activation (Figure 4 and animation at http://web-services.imb.uq.edu.au/waters/hgh.html). Given the nature of this process, supporting evidence for this model would need to be obtained in vivo.


JAK2 activation by growth hormone and other cytokines.

Waters MJ, Brooks AJ - Biochem. J. (2015)

Activation model of homodimeric class I cytokine receptors shown for the archetypal GH receptor
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4325515&req=5

Figure 4: Activation model of homodimeric class I cytokine receptors shown for the archetypal GH receptor
Mentions: The model derived from these experimental and in silico findings predicts that binding of a bivalent ligand to the GH receptor will bring the ECD site 3 structures into proximity and then association through subunit rotation, and this will facilitate closer apposition of the upper JM sequence by overcoming the electrostatic repulsion of the JM EED sequence. Apposition will be stabilized with the aid of hydrogen bonds between Glu-260 of receptor 1 and the amide of Trp-267 in receptor 2, and between the backbone carbonyl of Phe-257 and amide of Glu-260 in receptor 2 [44]. Apposition of the upper TMD will result in relative rotation of the helices from the parallel State 1 to the left hand crossover State 2, with widening of the distance between the C-termini of the TMD helices. The leads to the key question: how could lower TMD separation result in activation of the JAK2 tyrosine kinases associated with the membrane proximal receptor Box1 motifs? Of particular relevance here is the existence of a pseudokinase (PK) domain adjacent to the kinase domain, which is known to be inhibitory [63]. Interestingly, of the 518 protein kinases, 48 contain PK domains, but only five of those contain an additional kinase domain. These are the four JAKs and the serine/threonine kinase GCN2 [64]. Hence any model for activation must remove the PK domain away from the kinase domain, and it is notable that myeloproliferative neoplasms with constitutive JAK2 activation possess point mutations in the PK domain or the linker to the adjacent SH2 domain, rather than the kinase domain. Since Lodish's group has shown that the constitutively active V617F mutant of JAK2 does not manifest its activity unless the EPO receptor is present [65,66] (or if the mutant is overexpressed [67]), the activation process is likely a trans-mediated activation event within a receptor/JAK2 dimer rather than a cis-activation process. If the PK inhibitory domain of one JAK2 was blocking the kinase domain of the other JAK2, then pulling the complex apart (such as would occur with separation of the Box1 motifs) would remove this trans-inhibition, and place the two kinase domains into proximity which could induce trans-activation (Figure 4 and animation at http://web-services.imb.uq.edu.au/waters/hgh.html). Given the nature of this process, supporting evidence for this model would need to be obtained in vivo.

Bottom Line: 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 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.

View Article: PubMed Central - PubMed

Affiliation: *Institute for Molecular Bioscience, The University of Queensland Institute, QLD 4072, Australia.

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.

Show MeSH
Related in: MedlinePlus