This idea of a cascade of phosphorylation events was supported by a study where an IRAK4 knockout in mice showed a more severe phenotype than other IRAK knockout experiments and signalling through Toll/IL-1 receptor (TIR) is virtually eliminated.In 2007 it was found that IRAK4 activity was necessary for activating signal pathways which lead to mitogen-activated protein kinases (MAPK), or Toll-like receptor-mediated immune responses (TLR), but was not essential to T-cell Receptor (TCR) signalling as was originally proposed. Protein structure
IRAK4 is a threonine/serine protein kinase made up of 460 amino acids, which contains both a kinase domain and a death domain. Its kinase domain exhibits the typical bilobed structure of kinases, with the N-terminal lobe consisting of a five-stranded antiparallel beta-sheet and one alpha helix. The C-terminal lobe is composed mainly of a number of alpha helices. Also contained within IRAK4s N-terminal is an extension of twenty amino acids, which is unique to IRAK4 among kinases, even within the IRAK family. Situated where the two lobes meet is an ATP binding site, which is covered by a tyrosine gatekeeper. Tyrosine as a gatekeeper is believed to be unique to the IRAK family of kinases. The protein also contains three auto-phosphorylation sites, each of which when mutated results in a decrease in the kinase activity of IRAK4.A structure of the autophosphorylation of the activation loop has been determined in which the activation loop Thr345 of one monomer is sitting in the active site of another monomer in the crystal (PDB: 4U9A, 4U97).
Second, deletion of the IRAK4 gene results in various cytokine response defects and finally, patients with IRAK4 deficiency have displayed defective immunity in response to IL-1, IL-8 and other TLR binding ligands. Considering IRAK4s downstream position of these signalling events, it is an important drug therapy target for various inflammatory disorders including rheumatoid arthritis, inflammatory bowel disease and other autoimmune diseases. Prostate cancer
An important area of research currently being explored is the role the IRAK4 gene may play in the development of prostate cancer. There are several interacting factors that lead to the development of this disease however genetic susceptibility of chronic inflammation has been deemed one of the most important. It has been found that mutations in the IRAK4 gene can lead to dysfunctional TLR signalling and ultimately result in increased innate immune responses and therefore an increased inflammatory response. Over time, this can lead to the onset of prostate cancer. Melanoma
Another interesting application of the IRAK4 gene was found in a study involving human melanoma patients. This research found that patients with melanin-cell tumors displayed an increase in the phosphorylation state of IRAK4. The siRNA inhibition of IRAK4 in mice displayed greater programmed cell death (PCD) and slowed tumor growth.IRAK4 is higher levels in some lines of melanoma. By reducing IRAK4 activity it may be possible to identify new chemotherapeutic agents to treat patients with advanced melanoma for which no effective treatment is available.