Deregulation of mitogen-activated protein kinase at low pH due to a structural rearrangement of activation segment

Insights into evolutionary interaction patterns of the 'Phosphorylation Activation Segment' in kinase

We are interested in studying the phosphorylation of the kinase activation loop, distinguishing the passage from the unphosphorylated to the phosphorylated form without allostery. We performed an interaction study to trace the change of interactions between the activation segment and the kinase catalytic core, before and after phosphorylation. Results show that the structural changes are mainly due to the attraction between the phosphate group and guanidine groups of the arginine side chains of RD-pocket, which are constituted mainly of guanidine groups of the catalytic loop, the β9, and the αC helix. This attraction causes propagation of structural variation of the activation segment, principally towards the N-terminal. The structural variations are not made on all the amino acids of the activation segment; they are conditioned by the existence of two beta sheets stabilizing the loop during phosphorylation. The first,β6-β9 sheet is usually present in most of the kinases; the second, β10-β11 is formed due to the interaction between the main chain amino acids of the activation loop and the αEF/αF loop.

Autophosphorylation of MAP kinase disables the MAPK pathway in apoptotic Xenopus eggs

Mitogen-activated protein kinases (MAPKs) are involved in the regulation of various cellular processes, including cell survival and apoptosis. Here, we report that Xenopus p42 MAPK becomes phosphorylated in apoptotic eggs, however this modification does not activate the enzyme. Using phosphorylation residue-specific antibodies, we demonstrate that this modification occurs on the Tyr residue in the MAPK activation segment, pinpointing the autophosphorylation mechanism. Notably, MAPK phosphorylation in apoptotic Xenopus eggs coincides with prominent intracellular acidification accompanying apoptosis in these cells. Furthermore, autophosphorylation of recombinant Xenopus MAPK is stimulated and phosphorylation of a protein substrate is inhibited under low pH conditions. Thus, acidic intracellular conditions inactivate MAPK and effectively disable the MAPK-mediated survival pathway in the apoptotic eggs. Given that cell acidification is a rather common feature of apoptosis, we hypothesize that stimulation of MAPK autophosphorylation and shutdown of the MAPK pathway may represent universal traits of apoptotic cell death.

Cloning, expression and characterization of a gene encoding mitogen activated protein kinase 2 (MPK2) from Tetrahymena thermophila

Environmental effects and mitogens determine cell phenotype in eukaryotes mainly through MAPK pathways. However, MAPK signaling pathways in T. thermophila have not been studied comprehensively. This study aims to express recombinant MPK2, a MAPK from T. thermophila, in E. coli to characterize its kinase activity. MPK2 was cloned by RT-PCR using degenerate oligonucleotide primers and RACE method. The full-length cDNA of the MPK2 gene is 1705bp that includes 1281bp ORF coding for a putative protein of 426 amino acids having a mass of 50.2kDa. The putative MPK2 protein contains all eleven conserved subdomains that are characteristics of serine/threonine protein kinases, and a TDY motif, which is a putative dual phosphorylation site common in Protista. MPK2 displays highest 48% overall identity to human ERK5 (MAPK7). The expression vector pGEX4T-1-MPK2 was constructed by inserting the coding region of MPK2 cDNA into pGEX4T-1 after introducing the nine point mutations, and then transformed into E. coli BL21(DE3). Autophosphorylation of 76kDa GST-MPK2 at tyrosine residues was confirmed not only by Western blot using anti-phosphotyrosine monoclonal antibody but also by in vitro kinase assay. GST-MPK2 was also able to phosphorylate the artificial substrate myelin basic protein. This study concludes that the free-living unicellular protist T. thermophila MPK2 has commonly conserved MAPK enzyme features, possibly involved in the regulation of cell survival responding to abiotic or biotic stressors, and the production and movement of haploid gametic nuclei between pairs during conjugation.

Intracellular acidification delays hormonal G2/M transition and inhibits G2/M transition triggered by thiophosphorylated MAPK in Xenopus oocytes

Xenopus oocyte maturation is analogous to G2/M transition and characterized by germinal vesicle breakdown (GVBD), spindle formation, activation of MPF and Mos-Xp42(Mpk1) pathways. It is accompanied prior to GVBD by a transient increase in intracellular pH. We determined that a well known acidifying compound, NH(4)Cl, delayed progesterone-induced GVBD in a dose-dependent manner. GVBD(50) was delayed up to 2.3-fold by 10 mM NH(4)Cl. Cyclin B2 phosphorylation, Cdk1 Tyr15 dephosphorylation as well as p39(Mos) accumulation, Xp42(Mpk1) and p90(Rsk) phosphorylation induced by progesterone were also delayed by incubation of oocyte in NH(4)Cl. The delay induced by NH(4)Cl was prevented by injection of MOPS buffer pH 7.7. In contrast to acidifying medium, alkalyzing treatment such as Tris buffer pH 9 injections, accelerated GVBD, MPF and Xp42(Mpk1) activation, indicating that pHi changes control early steps of G2/M dynamics. When injected in an immature recipient oocyte, egg cytoplasm triggers GVBD through MPF auto-amplification, independently of protein synthesis. In these conditions, GVBD and Xp42(Mpk1) activation were delayed by high concentration of NH(4)Cl, which never prevented or delayed MPF activation. Strickingly, NH(4)Cl strongly inhibited thiophosphorylated active MAPK-induced GVBD and MPF activation. Nevertheless, Tris pH 9 did not have any effects on egg cytoplasm- or active MAPK-induced GVBD. Taken together, our results suggest that dynamic of early events driving Xp42(Mpk1) and MPF activation induced by progesterone may be negatively or positively regulated by pH(i) changes. However Xp42(Mpk1) pathway was inhibited by acidification alone. Finally, MPF auto-amplification loop was not sensitive to pH(i) changes.