Variants of human thymidylate synthase with loop 181–197 stabilized in the inactive conformation

Loop 181–197 of human thymidylate synthase (hTS) populates two major conformations, essentially corresponding to the loop flipped by 180°. In one of the conformations, the catalytic Cys195 residue lies distant from the active site making the enzyme inactive. Ligands stabilizing this inactive conformation may function as allosteric inhibitors. To facilitate the search for such inhibitors, we have expressed and characterized several mutants designed to shift the equilibrium toward the inactive conformer. In most cases, the catalytic efficiency of the mutants was only somewhat impaired with values of k_cat/K_m reduced by factors in a 2–12 range. One of the mutants, M190K, is however unique in having the value of k_cat/K_m smaller by a factor of ～7500 than the wild type. The crystal structure of this mutant is similar to that of the wt hTS with loop 181–197 in the inactive conformation. However, the direct vicinity of the mutation, residues 188–194 of this loop, assumes a different conformation with the positions of C_α shifted up to 7.2 Å. This affects region 116–128, which became ordered in M190K while it is disordered in wt. The conformation of 116–128 is however different than that observed in hTS in the active conformation. The side chain of Lys190 does not form contacts and is in solvent region. The very low activity of M190K as compared to another mutant with a charged residue in this position, M190E, suggests that the protein is trapped in an inactive state that does not equilibrate easily with the active conformer.     

Maturation hormone induced an increase in the translational activity of starfish oocytes coincident with the phosphorylation of the mRNA cap binding protein,eIF-4E,and the activation of several kinases

The stimulation of translation in starfish oocytes by the maturation hormone, 1-methyladenine (1-MA), requires the activation or mobilization of both initiation factors and mRNAs [Xu and Hille, Cell Regul. 1:1057, 1990]. We identify here the translational initiation complex, eIF-4F, and the guanine nucleotide exchange factor for eIF-2, eIF-2B, as the rate controlling components of protein synthesis in immature oocytes of the starfish, Pisaster orchraceus. Increased phosphorylation of eIF-4E, the cap binding subunit of the eIF-4F complex, is coincident with the initial increase in translational activity during maturation of these oocytes. Significantly, protein kinase C activity increased during oocyte maturation in parallel with the increase in eIF-4E phosphorylation and protein synthesis. An increase in the activities of cdc2 kinase and mitogen-activated myelin basic protein kinase (MBP kinase) similarly coincide with the increase in eIF-4E phosphorylation. However, neither cdc2 kinase nor MBP kinase phosphorylates eIF-4E in vitro. Casein kinase II activity does not change during oocyte maturation, and therefore, cannot be responsible for the activation of translation. Treatment of oocytes with phorbol 12-myristate 13-acetate, an activator of protein kinase C, for 30 min prior to the addition of 1-MA resulted in the inhibition of 1-MA-induced phosphorylation of eIF-4E, translational activation, and germinal vesicle breakdown. Therefore, protein kinase C may phosphorylate eIF-4E, after very early events of maturation. Another possibility is that eIF-4E is phosphorylated by an unknown kinase that is activated by the cascade of reactions stimulated by 1-MA. In conclusion, our results suggest a role for the phosphorylation of eIF-4E in the activation of translation during maturation, similar to translational regulation during the stimulation of growth in mammalian cells. © 1993 Wiley-Liss, Inc.     

Glycogen synthase kinase 3beta together with 14-3-3 protein regulates diabetic cardiomyopathy: effect of losartan and tempol

Glycogen synthase kinase (GSK) 3beta is a multifunctional protein that positively regulates myocardial apoptosis and negatively regulates hypertrophy. However, the role of GSK3beta in the diabetic myocardium is largely unknown. We found that GSK3beta became more active (less phosphorylated at serine 9) via decreased Akt phosphorylation, in parallel to c-Jun NH2 terminal kinase activation, which correlated with increased activated caspase 3 and myocardial apoptosis 3 days after streptozotocin (STZ) injection in mice. However, 28 days after STZ injection, GSK3beta became inactive, which correlated with the enhanced protein kinase C beta2 and p38 mitogen activated protein kinase expression, nuclear translocation of nuclear factor of activated T cells c3, cardiac hypertrophy and fibrosis. All of the above parameters were exacerbated in dominant-negative 14-3-3 transgenic mice. Our results suggest that GSK3beta together with 14-3-3 protein plays essential roles in the signaling of diabetic cardiomyopathy, and treatment with either losartan or tempol prevents these changes.     

NMR analysis of partially folded states and persistent structure in the alpha subunit of tryptophan synthase: implications for the equilibrium folding mechanism of a 29-kDa TIM barrel protein

Structural insights into the equilibrium folding mechanism of the alpha subunit of tryptophan synthase (αTS) from Escherichia coli, a (βα)₈ TIM barrel protein, were obtained with a pair of complementary nuclear magnetic resonance (NMR) spectroscopic techniques. The secondary structures of rare high-energy partially folded states were probed by native-state hydrogen-exchange NMR analysis of main-chain amide hydrogens. 2D heteronuclear single quantum coherence NMR analysis of several ¹⁵N-labeled nonpolar amino acids was used to probe the side chains involved in stabilizing a highly denatured intermediate that is devoid of secondary structure. The dynamic broadening of a subset of isoleucine and leucine side chains and the absence of protection against exchange showed that the highest energy folded state on the free-energy landscape is stabilized by a hydrophobic cluster lacking stable secondary structure. The core of this cluster, centered near the N-terminus of αTS, serves as a nucleus for the stabilization of what appears to be nonnative secondary structure in a marginally stable intermediate. The progressive decrease in protection against exchange from this nucleus toward both termini and from the N-termini to the C-termini of several β-strands is best described by an ensemble of weakly coupled conformers. Comparison with previous data strongly suggests that this ensemble corresponds to a marginally stable off-pathway intermediate that arises in the first few milliseconds of folding and persists under equilibrium conditions. A second, more stable intermediate, which has an intact β-barrel and a frayed α-helical shell, coexists with this marginally stable species. The conversion of the more stable intermediate to the native state of αTS entails the formation of a stable helical shell and completes the acquisition of the tertiary structure.     

Natural products as modulators of the nuclear receptors and metabolic sensors LXR,FXR and RXR

Nuclear receptors (NRs) represent attractive targets for the treatment of metabolic syndrome-related diseases. In addition, natural products are an interesting pool of potential ligands since they have been refined under evolutionary pressure to interact with proteins or other biological targets.This review aims to briefly summarize current basic knowledge regarding the liver X (LXR) and farnesoid X receptors (FXR) that form permissive heterodimers with retinoid X receptors (RXR). Natural product-based ligands for these receptors are summarized and the potential of LXR, FXR and RXR as targets in precision medicine is discussed.