As Extracellular Glutamine Levels Decline,Asparagine Becomes an Essential Amino Acid

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Measurements of Na+-occluded intermediates during the catalytic cycle of the Na+/K+-ATPase provide novel insights into the mechanism of Na+ transport

The Na⁺/K⁺-ATPase is an integral plasma membrane glycoprotein of all animal cells that couples the exchange of intracellular Na⁺ for extracellular K⁺ to the hydrolysis of ATP. The asymmetric distribution of Na⁺ and K⁺ is essential for cellular life and constitutes the physical basis of a series of fundamental biological phenomena. The pumping mechanism is explained by the Albers–Post model. It involves the presence of gates alternatively exposing Na⁺/K⁺-ATPase transport sites to the intracellular and extracellular sides and includes occluded states in which both gates are simultaneously closed. Unlike for K⁺, information is lacking about Na⁺-occluded intermediates, as occluded Na⁺ was only detected in states incapable of performing a catalytic cycle, including two Na⁺-containing crystallographic structures. The current knowledge is that intracellular Na⁺ must bind to the transport sites and become occluded upon phosphorylation by ATP to be transported to the extracellular medium. Here, taking advantage of epigallocatechin-3-gallate to instantaneously stabilize native Na⁺-occluded intermediates, we isolated species with tightly bound Na⁺ in an enzyme able to perform a catalytic cycle, consistent with a genuine occluded state. We found that Na⁺ becomes spontaneously occluded in the E1 dephosphorylated form of the Na⁺/K⁺-ATPase, exhibiting positive interactions between binding sites. In fact, the addition of ATP does not produce an increase in Na⁺ occlusion as it would have been expected; on the contrary, occluded Na⁺ transiently decreases, whereas ATP lasts. These results reveal new properties of E1 intermediates of the Albers–Post model for explaining the Na⁺ transport pathway.     

Pre-steady-state Kinetic and Structural Analysis of Interaction of Methionine γ-Lyase from Citrobacter freundii with Inhibitors

Methionine γ-lyase (MGL) catalyzes the γ-elimination of l-methionine and its derivatives as well as the β-elimination of l-cysteine and its analogs. These reactions yield α-keto acids and thiols. The mechanism of chemical conversion of amino acids includes numerous reaction intermediates. The detailed analysis of MGL interaction with glycine, l-alanine, l-norvaline, and l-cycloserine was performed by pre-steady-state stopped-flow kinetics. The structure of side chains of the amino acids is important both for their binding with enzyme and for the stability of the external aldimine and ketimine intermediates. X-ray structure of the MGL·l-cycloserine complex has been solved at 1.6 Å resolution. The structure models the ketimine intermediate of physiological reaction. The results elucidate the mechanisms of the intermediate interconversion at the stages of external aldimine and ketimine formation.     

Conventional protein kinase C-α (PKC-α) and PKC-β negatively regulate RIG-I antiviral signal transduction

Retinoic acid-inducible gene I (RIG-I) is a key sensor for viral RNA in the cytosol, and it initiates a signaling cascade that leads to the establishment of an interferon (IFN)-mediated antiviral state. Because of its integral role in immune signaling, RIG-I activity must be precisely controlled. Recent studies have shown that RIG-I CARD-dependent signaling function is regulated by the dynamic balance between phosphorylation and TRIM25-induced K₆₃-linked ubiquitination. While ubiquitination of RIG-I is critical for RIG-I''s ability to induce an antiviral IFN response, phosphorylation of RIG-I at S₈ or T₁₇₀ suppresses RIG-I signal-transducing activity under normal conditions. Here, we not only further define the roles of S₈ and T₁₇₀ phosphorylation for controlling RIG-I activity but also identify conventional protein kinase C-α (PKC-α) and PKC-β as important negative regulators of the RIG-I signaling pathway. Mutational analysis indicated that while the phosphorylation of S₈ or T₁₇₀ potently inhibits RIG-I downstream signaling, the dephosphorylation of RIG-I at both residues is necessary for optimal TRIM25 binding and ubiquitination-mediated RIG-I activation. Furthermore, exogenous expression, gene silencing, and specific inhibitor treatment demonstrated that PKC-α/β are the primary kinases responsible for RIG-I S₈ and T₁₇₀ phosphorylation. Coimmunoprecipitation showed that PKC-α/β interact with RIG-I under normal conditions, leading to its phosphorylation, which suppresses TRIM25 binding, RIG-I CARD ubiquitination, and thereby RIG-I-mediated IFN induction. PKC-α/β double-knockdown cells exhibited markedly decreased S₈/T₁₇₀ phosphorylation levels of RIG-I and resistance to infection by vesicular stomatitis virus. Thus, these findings demonstrate that PKC-α/β-induced RIG-I phosphorylation is a critical regulatory mechanism for controlling RIG-I antiviral signal transduction under normal conditions.     

Retroviruses pseudotyped with the severe acute respiratory syndrome coronavirus spike protein efficiently infect cells expressing angiotensin-converting enzyme 2

Infection of receptor-bearing cells by coronaviruses is mediated by their spike (S) proteins. The coronavirus (SARS-CoV) that causes severe acute respiratory syndrome (SARS) infects cells expressing the receptor angiotensin-converting enzyme 2 (ACE2). Here we show that codon optimization of the SARS-CoV S-protein gene substantially enhanced S-protein expression. We also found that two retroviruses, simian immunodeficiency virus (SIV) and murine leukemia virus, both expressing green fluorescent protein and pseudotyped with SARS-CoV S protein or S-protein variants, efficiently infected HEK293T cells stably expressing ACE2. Infection mediated by an S-protein variant whose cytoplasmic domain had been truncated and altered to include a fragment of the cytoplasmic tail of the human immunodeficiency virus type 1 envelope glycoprotein was, in both cases, substantially more efficient than that mediated by wild-type S protein. Using S-protein-pseudotyped SIV, we found that the enzymatic activity of ACE2 made no contribution to S-protein-mediated infection. Finally, we show that a soluble and catalytically inactive form of ACE2 potently blocked infection by S-protein-pseudotyped retrovirus and by SARS-CoV. These results permit studies of SARS-CoV entry inhibitors without the use of live virus and suggest a candidate therapy for SARS.     

Examination of the 1,4-disubstituted azetidinone ring system as a template for combretastatin A-4 conformationally restricted analogue design

A series of novel 1,4-diaryl-2-azetidinones was prepared by stereospecific Staudinger reaction as conformationally restricted analogues of combretastatin A-4 because molecular modeling studies suggested close geometric similarities. They were evaluated for cytotoxicity against a number of human tumor and normal cell lines. Strong potencies were observed, with the best compounds exhibiting IC(50)'s of 25-74 nM against human neuroblastoma IMR 32 cell growth and a variety of other cell lines. Compounds inhibited tubulin polymerization with potencies commensurate with their cytotoxic activity and a more soluble anilino-containing analogue was very effective in inhibiting the growth of AR42J rat pancreatic tumors transplanted into in nude mice. Further studies on this interesting group of compounds as anti-cancer agents appear warranted.     

Nucleotide Binding to Na,K-ATPase: The Role of Electrostatic Interactions.

The contribution of electrostatic forces to the interaction of Na,K-ATPase with adenine nucleotides was investigated by studying the effect of ionic strength on nucleotide binding. At pH 7.0 and 20 degrees C, there was a qualitative correlation between the equilibrium dissociation constant (K(d)) values for ATP, ADP, and MgADP and their total charges. All K(d) values increased with increasing ionic strength. According to the Debye-Hückel theory, this suggests that the nucleotide binding site and its ligands have "effective" charges of opposite signs. However, quantitative analysis of the dependence on ionic strength shows that the product of the effective electrostatic charges on the ligand and the binding site is the same for all nucleotides, and is therefore independent of the total charge of the nucleotide. The data suggest that association of nucleotides with Na,K-ATPase is governed by a partial charge rather than the total charge of the nucleotide. This charge, interacting with positive charges on the protein, is probably the one corresponding to the alpha-phosphate of the nucleotide. Dissociation rate constants measured in complementary transient kinetic experiments were 13 s(-1) for ATP and 27 s(-1) for ADP, independent of the ionic strength in the range 0.1-0.5 M. This implies similar association rate constants for the two nucleotides (about 40 x 10(6) M(-1) s(-1) at I = 0.1 M). The results suggest that long-range Coulombic forces, affecting association rates, are not the main contributors to the observed differences in affinities, and that local interactions, affecting dissociation rates, may play an even greater role.     

If space is provided,bulky modification on the rim of azurin's beta-barrel results in folded protein

Pseudomonas aeruginosa azurin is a blue-copper protein with a beta-barrel fold. Here we report that, at conditions where thermal unfolding of apo-azurin is reversible, the reaction occurs in a single step with a transition midpoint (T(m)) of 69 degrees C (pH 7). The active-site mutation His117Gly creates a cavity in the beta-barrel near the surface but does not perturb the overall fold (T(m) of 64 degrees C, pH 7). Oxidation of the active-site cysteine (Cysteine-112) in wild-type azurin, which occurs readily at higher temperatures, results in a modified protein that cannot adopt a native-like structure. In sharp contrast, Cysteine-112 oxidation in His117Gly azurin yields a modified apo-azurin that appears folded and displays cooperative, reversible unfolding (T(m) approximately 55 degrees C, pH 7). We conclude that azurin's beta-barrel is a rigid structural element that constrains the structure of its surface; a bulky modification can only be accommodated if complementary space is provided.     

The role of post-translational modifications of the CXCR4 amino terminus in stromal-derived factor 1 alpha association and HIV-1 entry

The chemokine receptor CXCR4 plays critical roles in development, immune function, and human immunodeficiency virus type 1 (HIV-1) entry. Here we demonstrate that, like the CC-chemokine receptors CCR5 and CCR2b, CXCR4 is posttranslationally modified by sulfation of its amino-terminal tyrosines. The sulfate group at tyrosine 21 contributes substantially to the ability of CXCR4 to bind its ligand, stromal derived factor 1 alpha. Tyrosine sulfation plays a less significant role in CXCR4-dependent HIV-1 entry than in CCR5-dependent HIV-1 entry. In some cell lines, CXCR4 is efficiently modified by a chondroitin sulfate chain at serine 18, but neither HIV-1 entry nor stromal derived factor 1 alpha binding was affected by loss of this glycosaminoglycan. These data demonstrate a functional role for tyrosine sulfate in the CXC-chemokine receptor family and underscore a general difference in HIV-1 utilization of CCR5 and CXCR4.