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Rat Calcineurin
Functionis a serine/threonine phosphatase that is activated by calmodulin(CaM) in a calcium-dependant manner. Also known as Serine threonine phosphatase 2B, it plays an important role in multiple pathways. Calcineurin can modulate the immune response, act on the muscle formation and remodeling as well as on the neuronal plasticity and the cell death. It is able to desphosphorylate numerous ion channels and thus modulate the basal excitability of neurons. Calcineurin has also an effect on the synaptic transmission since it affects the release of several neurotransmitters[1][2]. Calcineurin is responsible mainly for dephosphorylating several members of the NFAT transcription factor family (including NFATc1 through NFATc4). General structureis a heterodimeric Protein that consits of two subunits. They are called catalytic and regulatory subunit. Four molecules of Calcineurin form an asymmetric complex which leads to a total molecular weight of 370 kDa. The structure presented in this article is the catalytic subunit isoform of the serine/threonine-protein phosphatase 2B in rattus norvegicus (rat). It consists of 521 [1] aminoacids and has a molecular weight of 57 kDa [2]. The calatytic subunit is subdivided into functional domains which are a , a , a and an . The catalytic side includes a residue (green) at position 151 that acts as proton donor and metal binding sites. (shown in brown) binds at the position 118, 150, 199 and 281. Four residues are modified with a serine or a tyrosine. At position 2, a N-acetylserine has been found by similarity, as well as a nitrated tyrosine at position 224, and phosphoserine at position 469 and 492. Calcineurin is a cytoplasmic protein. It is widely expressed among tissues especially in the central nervous system (CNS) qnd in lymphoid cells qnd therefore involved in the mediation of the immune response [3]. Discovery of the structure:  The structure have been published in the year 2013 by Qilu Ye et al. [4] in Cellular Signaling. For their experiments they used x-ray diffraction. The PDB validation obtained a Resolutionof 3.0 Å, a free R-value of 0.273 and a work R-value of 0.241 [3].
Secondary structure:  Besides, there are six turns reported in the structure of one chain.
Principle of action As a response of receptor tyrosine kinase [4] activation as well as G protein-coupled receptor (GCPR) activation the Phospholipase C (PLC) catalyse the hydrolysis of PIP2 to IP3 and DAG. IP3 activates the Inositol 1,4,5-Trisphosphate Receptor and therby leads to an increasing amount of the second Messenger Ca2+ in the cytoplasma. Calcineurin is activated by Calmodulin, a calcium-binding protein. Calmodulin interacts with the calmodulin-binding/regulatory region of Calcineurin. That binding leads to a conformational change in the autoinhibitory domain and remove it from the active site [6]. It has been reported that Calcineurin activates the transcription factor NFAT by forming a complex and dephosporylation [7]. Following, the factor enters the nucleus and activates the expression of Interleukin-2.
Binding PartnersThe main partners of interaction are Calmodulin, NFATc1, NFATc2 and NFATc3. Many of the calcineurin substrates contain a PxIxIT motif. Among them, beside the phosphorylated forms of NFAT we can also mention cAMP response element binding protein (CREB), PP1, microtubule-associated protein tau and glycogen synthase kinase-3 beta (GSK- 3)[8][9][10][11]. is inhibited by the immunosuppressive drugs tacrolismus () or cyclosporine A (CsA). CsA and conduct their therapeutic role thought binding to the immunophilins cyclophilin and FK506 binding protein (FK506BP) respectively. The complexes CsA-cyclophilin and FK506-FK506BP bind then to calcineurin in a calcium-dependent manner thus inhibiting its phosphatase activity. Therefore the addition of these drugs to lymphocytes T prevent NFAT translocation to the nucleus and the subsequent activation its target gene.That's why FK506 and CsA are use in the treatment of various immune-mediated diseases. However since calcineurin is is widely expressed in non-haemopoietic tissues like the kidney and the hearth, both drugs present a long term toxicity and can lead to deleterious effect to these Organs [12][13]. Cofactors: Calcineurin belong to the family of metalloprotein. To conduct its activity it requires the presence of exposing Fe3+ and exposing Zn2+ ions in the active site (one per subunit). Superoxide dismutase has been shown to protect calcineurin from inactivation by preventing Fe3+ from oxidation. Thus after activation of calcineurin by calmodulin, the AID is displaced from the exposing Fe3+ to oxidation [14][15]. Related health defectsCalcineurin hyperactivation thought dysregulation of the Ca2+ dynamic have been show to play a critical role in several diseases like Rheumatoid arthritis (RA), Schizophrenia ,Diabetes, Systemic Lupus Erythematosus as well as Alzheimer diseases (AD) [16][17][18][19]. Taking the example of AD which is a age-related memory dysfunction, it it know that in older organism the brain is less plastic due to a dysregulation of Ca2+ dynamic. This in addition to the presence of oligomeric Aß is sufficient to explain an enhancement of CaN activity leading to severals symptoms like decreased neurotransmission, synaptic loss and neuroinflammation[20]. Therefore calmodulin inhibitors are potential alternatives against Alzheimer diseases. Human/Rat calcineurin comparisonHuman and Rat calcineurin have the same function and global structure The size (521 amino acids) and subunits of the linear structure are the same, as well as the 3D structure.
Indeed, Calcineurin is a highly conserved protein from yeast to mammals.
Evolutionary conservation Calcineurin A This catalytic subunit is highly conserved. Nevertheless it can be up to 20 % larger in lower eukaryotic species. In addition, the NH2 and COOH terminals are variable among species, as well as between calcineurin A genes within the same organism. The function of these variable domains is unknown.[21] Calcineurin B This regulatory subunit is also highly conserved throughout evolution. For instance, the sequence of the rat calcineurin B brain isoform is exactly the same as the human calcineurin B. This high degree of conservation allows functional interchange of calcineurin B subunits between eukaryotic species. [21]
Calcineurin historyCalcineurin was first detected by Wang and Desai in 1976 as a column fraction that inhibited the calmodulin-dependent cyclic nucleotide phosphodiesterase. Klee and Krinks did the first purification of calcineurin in 1978 and hypothesized that it might be a regulatory subunit of phosphodiesterase since it was demonstrated to inhibit phosphodiesterase activity. Klee et al. named it “calcineurin” on the basis of its Ca2+-binding properties (calci) and localization to neuronal tissue (neurin). From this time, functions and structure of Calcineurin have been discovered as well as medical applications (e.g. calcineurin inhibitors). [21] References
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