ASSIGNMENT

IMMUNOLOGY

TOPIC:-CYTOKINES RECEPTOR

SUBMITED TO:- SUBMITED BY:-DEEPAK SIR MOIN KHAN HUSSAIN

General Introduction of Cytokine Receptors

Cytokine receptors such as the EpoR, prolactin, and thrombopoietin

receptors function as ligand-induced or ligand-stabilized homodimers. They

interact with the Janus (JAK) family of cytoplasmic tyrosine kinases to participate

in signal transduction. Erythropoietin and other cytokine receptors are activated

through hormone–induced receptor dimerization and autophosphorylation of JAK

kinases that are associated with the cytoplasmic domains of the receptors. In the

case of cytokine receptors such as Epo-like receptors and growth hormone

receptors, the receptor consists of an extracellular ligand-binding domain, a short

single-pass transmembrane domain, and a cytoplasmic domain that lacks

tyrosine kinase activty. In the extracellular domain, there are about 200 to 250

amino acid residues comprising two subdomains (1 and 2), each predicted to

consist of seven beta-strands and to be structurally related to fibronectin type III

(FN III) domains (Barzan, J.F., 1990). The amino-terminal FNIII-like subdomain

contains a pair of spatially conserved cysteine bridges, while the carboxyl-

terminal FNIII-like subdomain contains a conserved β-stand F and a highly

conserved WSXWS motif. A four-residue hinge region links subdomains 1 and 2.

Many of these structural characteristics can be seen in figure 4.

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Figure 4. Dimerized cytokine

receptor with ligand bound.

Endocrinology 143(1): 2-10.

The single transmembrane

domain and part of the

juxtamembrane domain are proposed to be alpha-helical. Three hydrophobic

amino acids, L253, I257, and W258, found in the juxtamembrane domain are crucial

for receptor signaling. These three hydrophobic residues are predicted to form a

hydrophobic patch on the alpha-helix (Constantinescu et al., 2001). This

segment is also specifically required to switch on JAK activation. It has been

proposed that the juxtamembrane domain is important both in activation of JAK

and in positioning of the cytoplasmic domain of Epo-like receptors in the

conformation to be an acceptable JAK substrate (Constantinescu et al., 2001).

The importance of the juxtamembrane domain helix and the three hydrophobic

residues mentioned are schematically represented in figure 5.

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Figure 5. Importance of L253

positioning in the

juxtamembrane domain for JAK

activation. A) wild-type,

functional. B) L253A knockout,

disabled. C) juxtamenbrane

domain lengthened 1 aa so

L253 is not positioned in the

hydrophobic patch, disabled. D)

lengthened 3 aa so L253 is

again positioned in the

hydrophobic patch, functional.

Mol. Cell Biol. 7: 377-385.

The cytoplasmic domains of the receptors have JAK binding domains and

several tyrosines involved in recruiting other signaling proteins such as STAT by

providing docking sites. When the ligand binds to the extracellular domain, it

triggers transphosphorylation of the JAKs bound to each of the receptor

monomer cytoplasmic domains activating the tyrosine kinase activity of the JAKs.

The JAKs induce phosphorylation of tyrosines in the cytoplasmic domains in

each of the receptor monomers. This subsequently leads to docking of signalling

proteins such as STAT, PI-3’ kinase, the protein tyrosine phosphatases SHP1

and SHP2, and Shc to the tyrosine residues. Several of the tyrosines of these

bound proteins, in turn, become phosphorylated by the JAKs.

Conformational Changes in Cytokine Receptors Induced by Ligand Binding

The erythropoietin receptor (EPOR) is activated by ligand-induced

homodimerization. One ligand binds to the heterodimerized extracellular

domains of the receptor so that it has a 1:2 ligand:receptor stoichiometry. An

interesting feature of this ligand–receptor interaction is that the ligand has no axis

of symmetry. Two distinct sites, site 1 and site 2, in the ligand molecule each

have different affinities for the receptor. Site 1 has a higher binding affinity to the

receptor than does site 2. However, the ligand molecules were shown to engage

the two extracellular domains of the receptor monomers at similar contact points

on each dimerized receptor (Stuart J. Frank, 2002).

Cytokine receptors exist as preformed dimers. Livnah et al. reported that

the crystal structure of unliganded EpoR is a dimer, but with a dramatically

different arrangement of the two subunits from the ligand-bound EpoR. Ligand

binding to the extracellular domain of two cytokine receptors induces formation of

a receptor dimer of very specific conformation. Unliganded receptor dimers exist

in a conformation that prevents activation of JAK, but then undergo a ligand-

induced conformational change that allows JAK to be activated (Ingrid Remy et

al., 1999). The unliganded receptor is in an open-scissors-like configuration with

the dimerization interface consisting of self-association of the two ligand-binding

sites on the extracellular binding proteins (EBPs). In this case, the C-terminal

ends of the subdomain 2 regions of the EBPs are quite far apart (over 70 Å). In

the ligand-bound EBP structures, however, these C-terminal regions are much

closer (30 Å for the Epo-engaged EpoR) as can be seen in figure 6. Therefore

the preformed dimer, which is in an inactive state, by keeping the cytoplasmic

domains apart brings the extracellular and cytoplasmic domains into proximity

and allows signalling upon ligand binding. After reorganization of the EBPs

induced by ligand binding, this specific conformation is transmitted through the

two transmembrane alpha-helices to the two receptor juxtamembrane domains.

Residues in the first eleven amino acids of this juxtamembrane domain appear to

have an alpha-helical orientation that is functionally continuous with that of the

transmembrane domain. The segment of JAK is bound, at least in part, to

specific amino acids in this eleven amino acid juxtamembrane domain. In this

case, the ligand-triggered, receptor-reorganized dimerization brings the two

bound JAK proteins together in such a way that they can phosphorylate and

activate one other. The activated JAK can then phosphorylate multiple tyrosines

in the two receptor cytoplasmic domains leading to the phosphorylation of other

signaling proteins involved in signal transduction.

Figure 6. Large-

scale structural

changes in the

cytoplasmic domains

of cytokine receptors

upon ligand binding.

Endocrinology 143(1): 2-10.

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RTK Versus Cytokine Receptors

Both RTK and cytokine receptors have been discussed in substantial

detail in this article so far. Next, a comparison of the similarities and differences

between the two receptor classes will be undertaken. Though the particulars of

various receptors in each class have been mentioned, this comparison will only

consider those characteristics more and less common to each receptor class.

RTKs and cytokines are cell surface signal receptors that share numerous

features in common. However, they also possess many unique features that

differentiate between the two receptor classes. The monomeric units of each

receptor class follow a similar three domain layout: a variable glycosylated

extracellular N-terminus for ligand binding, a single transmembrane alpha-helix

for membrane anchoring, and a conserved cytoplasmic C-terminus that takes

part in signaling (though the C-terminal domains are different between the

classes, they are somewhat conserved within them). Both RTKs and cytokines

exist as dimers in the active state, yet only cytokines are dimers in the inactive

state. RTKs exist as monomers when in the inactive state and dimerize only

upon ligand binding. In cytokines, ligand binding changes the conformation of

the complex of dimer and noncovalently bound JAKs to activate the JAKs. This

brings up a very significant difference. RTKs possess intrinsic tyrosine kinase

activity, whereas cytokines depend on the tyrosine kinase activity of JAKs to

serve the same role. In either case, transphosphorylation between the

monomers (intrinsic or JAK-mediated) activates the ability of the complex to carry

out tyrosine phosphorylation of other signaling proteins. Moreover, some of the

phosphotyrosine residues generated by transphosphorylation play a part in

forming docking sites for recruiting and binding these signaling proteins to the

complex. Once the signaling proteins are phosphorylated, both receptors

release them into the cytoplasm where they can carry out their roles in signaling

pathways. There is some variability within each class of receptor, but most

receptors in each class share the features just discussed. Since the two different

classes share many features in common, it is evident that RTKs and cytokines

are functionally related cell surface receptors.