Human Interferon gamma (IFN-g)
IFN-g Protein Structure:
As a cytokine, interferon-gamma is a highly versatile homodimeric protein that plays an essential role in cell mediated immune responses to viral and mycobacterial infections. The IFN-g gene is located on chromosome 12 and codes for a 17 kDa protein, which then undergoes posttranslational glycolysis converting the IFN-g to a 20-25 kDa glycoprotein1. The molecule that binds to the IFN-g receptor is a homodimer as depicted in Fig.1 and has an intricate degree of linkage between the two subunits2. Each subunit contains six alpha helices and low amounts of beta pleated sheets. It is a secondary function of IFN-g to decrease the concentration of intracellular tryptophan, thereby starving intracellular parasites. Other tryptophan inhibitors have been reported that share the same characteristic subunit interlocking and high degree of alpha helices found in IFN-g2. Though there is significant homology of IFN-g between species, the region between helices A-B and likewise A'-B' is highly variable and may explain why IFN-g is species specific2.
Figure 1: a) IFN-g homodimer glycoprotein structure
b) IFN-beta protein structure
The cylindrical portions represent alpha helices and are color coded orange and white
to distinguish between the two separate subunits of the IFN-g homodimer.
This image was borrowed from the following web address pending approval from its
author Steven Falick.
IFN-g signal transduction pathway:
IFN-g receptor, like IFN-g, is a homodimer with a binding dissociation constant of 10-11 to 10-10 M3. IFN-g receptors are found in high amounts on the cellular surface of many different cell types such as T cells, B cells, macrophages, NK cells and fibroblasts1. The receptor consists of an alpha subunit to which IFN-g binds to and a beta subunit necessary for signal transduction. As depicted in Fig.2, the alpha subunit is associated with Janus Kinase 1 (JAK1) and the beta subunit is associated with Janus Kinase 2 (JAK2). After IFN-g binds to its receptor the alpha subunit as well as JAK1 and JAK2 are tyrosine phosphorylated. JAK1 then phosphorylates a tyrosine on signal transducers and activators of transcription alpha (STAT alpha). After phosphorylation, two STAT alpha molecules dimerize via interaction between their SH2 domains4. This dimer, interferon-gamma activator factor (GAF), is able to to enter the nucleus and bind to interferon-gamma activator sequence. The end result is an elevated transcription of IFN-g5. Not shown in Fig.2 is a cofactor protein encoded on chromosome 21 that is necessary for the IFN-g receptor to function6.
Figure 2: IFN-g signaling pathway resulting in increased transcription
The above image was borrowed from the following address pending
approval from its authors.
IFN-g role in cell mediated immune responses:
IFN-g is produced by NK cells, dendritic cells, cytotoxic T cells, Progenitor Th0 cells and Th1 cells1. One of IFN-g's main roles in cell mediated immune responses is its antiviral activity. IFN-g has no viral specificity and inhibits the spread of viruses containing either RNA or DNA7. There are several mechanisms by which IFN-g reacts to viral infection. In conjunction with CD40, IFN-g binds to and activates macrophages, which are then able to kill intracellular pathogens such as viruses8. Furthermore, bound IFN-g causes the macrophage to produce elevated amounts of both MHC class I and II molecules, thus increasing the macrophage's presentation of foreign peptides. A second antiviral mechanism of INF-g is to shutdown the replication of viral DNA or RNA and to rid the cell of pathogen without killing it8. In addition to its antiviral activity, IFN-g also plays a role in delayed type hypersensitivity by binding to macrophages and eliciting the release of inflammatory mediators8.
IFN-g role in immunoregulation:
In accordance with its elevated syntheses in response to mycobacterial and viral infections, IFN-g directs several immunoregulatory mechanisms that enhance the cytokine's function. As previously mentioned, IFN-g increases the production of MHC class I and II, which in turn increase the likelihood that the infected cell will be recognized as such. To further facilitate the loading of peptide onto MHC, IFN-g also upregulates the transcription of HLA-DM, tapasin and TAP genes8. IFN-g also influences cell differentiation of the progenitor Th0. By increasing Th1 differentiation from Th0 progenitor cells, IFN-g in turn inhibits differentiation into Th2 cells9. Isotype switching in B cells to IgG is also enhanced by IFN-g, presumably because IgG activates the compliment system and apsonizes extracellular pathogens resulting in their uptake by phagocytic cells8,10.
Effects of IFN-g deficiencies and miscellaneous information:
For proteins with such diverse and profound effects as IFN-g it is not only important that it be synthesized when needed, it is equally important that IFN-g not linger around when its presence is unnecessary. There are two ways of preventing IFN-g from building up after its activating signal has been removed, both of which are self regulatory. Along IFN-g's mRNA 3' untranslated end is a sequence (AUUUA)n, that in turn reduces the mRNA's half-life. Secondly, activated T cells in addition to transcribing IFN-g also transcribe another protein that prompts cytokine mRNA destruction8. As a result, once the activating signal for increased IFN-g production is removed the levels of IFN-g quickly return to normal. Not surprisingly, a deficiency or mutant form of IFN-g has a wide range of effects including an elevated risk to viral and bacterial infections.
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