Functional and Structural Characterization of OXA-935, a Novel OXA-10-Family β-Lactamase from Pseudomonas aeruginosa

Resistance to antipseudomonal penicillins and cephalosporins is often driven by the overproduction of the intrinsic β-lactamase AmpC. However, OXA-10-family β-lactamases are a rich source of resistance in Pseudomonas aeruginosa. OXA β-lactamases have a propensity for mutation that leads to extended spectrum cephalosporinase and carbapenemase activity. In this study, we identified isolates from a subclade of the multidrug-resistant (MDR) high risk P. aeruginosa clonal complex CC446 with a resistance to ceftazidime. A genomic analysis revealed that these isolates harbored a plasmid containing a novel allele of blaOXA-10, named blaOXA-935, which was predicted to produce an OXA-10 variant with two amino acid substitutions: an aspartic acid instead of a glycine at position 157 and a serine instead of a phenylalanine at position 153. The G157D mutation, present in OXA-14, is associated with the resistance of P. aeruginosa to ceftazidime. Compared to OXA-14, OXA-935 showed increased catalytic efficiency for ceftazidime. The deletion of blaOXA-935 restored the sensitivity to ceftazidime, and susceptibility profiling of P. aeruginosa laboratory strains expressing blaOXA-935 revealed that OXA-935 conferred ceftazidime resistance. To better understand the impacts of the variant amino acids, we determined the crystal structures of OXA-14 and OXA-935. Compared to OXA-14, the F153S mutation in OXA-935 conferred increased flexibility in the omega (Ω) loop. Amino acid changes that confer extended spectrum cephalosporinase activity to OXA-10-family β-lactamases are concerning, given the rising reliance on novel β-lactam/β-lactamase inhibitor combinations, such as ceftolozane-tazobactam and ceftazidime-avibactam, to treat MDR P. aeruginosa infections.

CSGID Solves Structures and Identifies Phenotypes for Five Enzymes in Toxoplasma gondii

Toxoplasma gondii, an Apicomplexan parasite, causes significant morbidity and mortality, including severe disease in immunocompromised hosts and devastating congenital disease, with no effective treatment for the bradyzoite stage. To address this, we used the Tropical Disease Research database, crystallography, molecular modeling, and antisense to identify and characterize a range of potential therapeutic targets for toxoplasmosis. Phosphoglycerate mutase II (PGMII), nucleoside diphosphate kinase (NDK), ribulose phosphate 3-epimerase (RPE), ribose-5-phosphate isomerase (RPI), and ornithine aminotransferase (OAT) were structurally characterized. Crystallography revealed insights into the overall structure, protein oligomeric states and molecular details of active sites important for ligand recognition. Literature and molecular modeling suggested potential inhibitors and druggability. The targets were further studied with vivoPMO to interrupt enzyme synthesis, identifying the targets as potentially important to parasitic replication and, therefore, of therapeutic interest. Targeted vivoPMO resulted in statistically significant perturbation of parasite replication without concomitant host cell toxicity, consistent with a previous CRISPR/Cas9 screen showing PGM, RPE, and RPI contribute to parasite fitness. PGM, RPE, and RPI have the greatest promise for affecting replication in tachyzoites. These targets are shared between other medically important parasites and may have wider therapeutic potential.

Structural and molecular mechanism for autoprocessing of MARTX toxin of Vibrio cholerae at multiple sites

The multifunctional autoprocessing repeats-in-toxin (MARTX) toxin of Vibrio cholerae causes destruction of the actin cytoskeleton by covalent cross-linking of actin and inactivation of Rho GTPases. The effector domains responsible for these activities are here shown to be independent proteins released from the large toxin by autoproteolysis catalyzed by an embedded cysteine protease domain (CPD). The CPD is activated upon binding inositol hexakisphosphate (InsP(6)). In this study, we demonstrated that InsP(6) is not simply an allosteric cofactor, but rather binding of InsP(6) stabilized the CPD structure, facilitating formation of the enzyme-substrate complex. The 1.95-A crystal structure of this InsP(6)-bound unprocessed form of CPD was determined and revealed the scissile bond Leu(3428)-Ala(3429) captured in the catalytic site. Upon processing at this site, CPD was converted to a form with 500-fold reduced affinity for InsP(6), but was reactivated for high affinity binding of InsP(6) by cooperative binding of both a new substrate and InsP(6). Reactivation of CPD allowed cleavage of the MARTX toxin at other sites, specifically at leucine residues between the effector domains. Processed CPD also cleaved other proteins in trans, including the leucine-rich protein YopM, demonstrating that it is a promiscuous leucine-specific protease.

A hypothalamic activator of calmodulin-dependent enzymes is thymosin beta 4 (1-39)

A new class of stimulators of basal activity of a number of calmodulin-dependent enzymes have been previously isolated from bovine hypothalamus. One of these stimulators, denoted as C3, has been purified to homogeneity by reverse phase HPLC and tentatively identified as thymosin beta 4 (1-39) by mass spectrometry and Edman microsequence analysis. The stimulating effect of C3 on rabbit skeletal muscle MLCK basal activity was compared with that of thymosin alpha 1 and thymosin beta 4 (16-38). Evidence is presented that all the indicated compounds are Ca(2+)-independent high-affinity MLCK stimulators. The potency of the stimulators in activating the enzyme was: C3 greater than beta 4 greater than (CaM+Ca2+) greater than alpha 1.

[Changes in the structural state of myosin filaments in glycerol-treated fibers of rabbit skeletal muscles induced by phosphorylation of myosin light chains]

Using the polarization microfluorimetry method, it was demonstrated that the increase in the degree of phosphorylation of myosin light chains (LC2) in extended single glycerinated fibers from rabbit psoas muscle changes the anisotropy of polarized fluorescence both tryptophan residue in the rod parts of the myosin molecule and the fluorescent label-N (iodoacetyl-aminoethyl)-5-naphthylamine-1-sulfonate (1,5-IAEDANS) bound to the SH1-group in myosin molecule heads. The changes in fluorescence anisotropy during LC2 phosphorylation were observed, when the measurements were performed only in the presence of 5 mM MgCl2. It was suggested that in the presence of MgCl2 the phosphorylation of LC2 associated with myosin heads changes their orientation and causes conformational shifts in the myosin filament core.

The effect of myosin light chain phosphorylation on the actin-stimulated ATPase activity of myosin minifilaments

It has been shown that in the absence of KCl, the actin-stimulated Mg2+-ATPase activity of rabbit skeletal myosin minifilaments with phosphorylated regulatory lights chains (LC2) exceeds 3-4-fold that of myosin minifilaments with dephosphorylated LC2. Addition of KCl leads to a decrease in the difference between the two ATPase activities. LC2 phosphorylation considerably increases the rate of ATPase reaction and only slightly decreases the affinity of myosin minifilaments for F-actin. It is suggested that the unusual effect of LC2 phosphorylation on the kinetic parameters of the actin-stimulated ATPase reaction of myosin minifilaments can be accounted for by its influence on the interaction between myosin heads which results in the ordered self-assembly of minifilaments.

Ordered assemblies of myosin minifilaments

Electron microscopic observations of negatively stained myosin minifilaments in a solution containing 10 mM-citrate buffer (pH 8.0), 4 mM-MgCl2 reveal ordered assemblies. They reveal bundles of parallel minifilaments of about 330 nm in length, aggregated into periodic linear and hexagonal structures. These structures are formed by means of interaction between myosin heads, arranged at the ends of minifilament bundles. The addition of 1 mM-ATP to the above solution causes dissociation of minifilament bundles into individual minifilaments without preventing the latter from association into linear "end-to-end" assemblies. Phosphorylation of myosin light chains does not exert any noticeable effect on the structure of the ordered minifilament assemblies but significantly increases their quantity. The interaction between minifilaments observed by us should be taken into consideration in studies on the properties of myosin which make use of minifilament systems.

[Production of glycerinated models of muscle fibers with phosphorylated myosin light chains]

A simple method for obtaining glycerinated muscle fibres of m. psoas of rabbit containing regulatory myosin light chains (LC2) of different levels of phosphorylation. The glycerination conditions stimulated endogenic kinase LC2 or phosphatase LC2. Glycerinated muscle fibres contained phosphorylated and dephosphorylated (levels of phosphorylation are 95 +/- 5%, and 5 +/- 5%, respectively) LC2 of myosin. To determine the level of phosphorylation the method of polyacrylamide gel electrophoresis in 8 M urea was modified.

[Effect of phosphorylation of myosin light chains on the interaction of myosin minifilaments with F-actin]

The effect of phosphorylation of light chains-2 (LC2) of rabbit skeletal muscle myosin on the interaction of myosin minifilaments with F-actin as well as on the actin-stimulated Mg2+-ATPase of minifilaments was studied. It was shown that in the absence of KCl the degree of F-actin-induced stimulation of myosin minifilament Mg2+-ATPase with phosphorylated LC2 exceeds 2-4-fold that with unphosphorylated LC2. Phosphorylation of LC2 considerably increases the rate of actin-stimulated Mg2+-ATPase reaction of myosin minifilaments but exerts only a very weak influence on the affinity of minifilaments for F-actin. After addition of KCl the differences in the actin-stimulated Mg2+-ATPase activity disappear in a great degree; in the presence of 50 mM KCl they do not exceed 50%. It was assumed that the observed specific influence of LC2 phosphorylation on the kinetic parameters of actin-stimulated Mg2+-ATPase reaction of myosin minifilaments is due to unique properties of the minifilaments (e.g., their ability to ordered self-assembly as a result of interaction between the heads of myosin molecules) which reflect their structural peculiarities.