Structural Biochemistry/Cytokine Receptors

Overview
Cytokines are important in growth and differentiation of cells. They are signaling molecules that lead to long-term genetic effects by activation of transcription factors. Cytosine receptors bind tightly to tyrosine kinases, the JAK kinases, which are members of a family of cytosolic protein. JAK kinases directly phosphorylate and activate transcription factors members of STAT (Signal Transduction and Activation of Transcription) family. Activation of cytokine receptors initiates the JAK/STAT pathway.



Cytokines Influence the Development of Different Cell Types
Cytokines form a small family of secreting signaling molecules that contain about 160 amino acids that control different parts of growth and differentiation of specific types of cells. Interleukins are cytokines that are essential for proliferation and functioning of T cells and antibody-producing B cells of the immune system. Interferons are another family of cytokines that are produced and secreted by cells after virus infections and act in nearby cells to induce enzymes that give these cells more resistance to virus infection. Many cytokines induce formation of important types of blood cells. Erythropoietin (Epo) for example, triggers the production of red blood cells by inducing the production and differentiation of erythroid progenitor cells in the bone marrow. Erythropoietin is synthesized by kidney cells and monitors the concentration of oxygen in the blood. A major function for erythropoietin is to transport oxygen to hemoglobin. Kidney cells respond to low amounts of oxygen by synthesizing large amounts of erythropoietin and delivering it to the blood through HIF-1α, which is an oxygen-sensitive transcription factor. When erythropoietin levels increase the level of erythroid progenitors increase and producing more red blood cells.

Cytokine Receptor Structures
All cytokines have a similar tertiary structure that consists of four long α helices that are folded together in a specific orientation. Cytokine receptors have the similar structures as well. They have an extracellular domain that is made of two subdomains. Each domain contains seven β strands folded together. An example of a cytokine binding to its receptor is the interaction of erythropoietin molecule with two identical erythropoietin receptor (EpoR) proteins. Cell response to a certain cytokine depends on the cell expressing the correct receptor. Even though cytokine receptors activate similar intercellular signal transduction pathways, cell response to a cytokine signal depends on the cell’s transcription factors, chromatin structures and other proteins responsible for the cell’s development. Eventually all activated pathways will lead to activation of transcription factors, which cause an increase or decrease in expression of some target genes.

JAK/STAT pathway
JAK2 kinase is tightly bounded to the cytosolic domain of all cytokine receptors. It contains an N-terminal receptor-binding domain, a C-terminal kinase domain, and a middle domain, which regulates kinase activity by an unknown mechanism. A mouse study showed that JAK2, erythropoietin, and the EpoR are all required for formation of adult-type erythrocytes. Embryonic mice lacking functional genes encoding the EpoR don’t form adult-type erythrocytes and die due to the inability to transport oxygen to the fetal organs. Similar results were seen in mice lacking functional genes encoding either Epo or JAK2.

Since erythropoietin binds to simultaneously to extracellular domains of two EpoR monomers in the cell surface, JAKs are brought close together in order for one to be phosphorylating the other on a tyrosine located in a region of the protein known as the activation lip. Phosphorylation of the activation lip leads to a conformational change that reduces the Km for ATP or leads to the substrate to be phosphorylated and increases the kinase activity. When the JAK kinases are activated they phosphorylate many tyrosine residues on the cytosolic domain of the receptor. The phosphotyrosine residues act as binding sites for SH2 domains (Src homology 2 domain), which are part of signal-transduction protein, including the STAT group of transcription factors. SH2 domains have three-dimensional structures, which bind to different sequences of amino acids that surround a phosphotyrosine residue. Differences in the hydrophobic socket in the SH2 domains of different STATs allow them to bind to phosphotyrosines adjacent to different sequences.

All STAT proteins contain an N-terminal DNA binding domain, an SH2 domain that binds to a specific phospotyrosine in a cytokine receptor’s cytosolic domain, and a C-terminal tyrosine that is phosphorylated by an associated JAK kinase. This ensures that in a specific cell only STAT proteins with an SH2 domain that can bind to a particular receptor protein will be activated. When a phosphorylated STAT dissociates spontaneously from the receptor, and two phosphorylated STAT proteins form a dimer, the SH2 domain on each binds to the phosphotyrosine in the other. Since dimerization also exposes the nuclear-localization signal (NLS), STAT dimers travel into the nucleus. It is here where they bind to specific enhancers controlling target genes.

Different STATS activate different genes in different cells. For example, stimulation of erythroid progenitor cells by erythropoietin (Epo) leads to activation of STAT5. Bcl-xL is a major protein induced by active STAT5, which prevents apoptosis of progenitors. They then proliferate and differentiate into erythroid cells. In a normal state, when Epo levels are low, bone marrow stem cells continuously create progenitor erythroid cells that are quickly destroyed. This process allows the body to respond quickly to the need for more erythrocyted in response to a rise in Epo levels.