Molecular cloning and expression of adenosine kinase from Leishmania donovani: identification of unconventional P-loop motif.
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The unique catalytic characteristics of adenosine kinase (Adk) and its stage-specific differential activity pattern have made this enzyme a prospective target for chemotherapeutic manipulation in the purine-auxotrophic parasitic protozoan Leishmania donovani. However, nothing is known about the structure of the parasite Adk. We report here the cloning of its gene and the characterization of the gene product. The encoded protein, consisting of 345 amino acid residues with a calculated molecular mass of 37173 Da, shares limited but significant similarity with sugar kinases and inosine-guanosine kinase of microbial origin, supporting the notion that these enzymes might have the same ancestral origin. The identity of the parasite enzyme with the corresponding enzyme from two other sources so far described was only 40%. Furthermore, 5' RNA mapping studies indicated that the Adk gene transcript is matured post-transcriptionally with the trans-splicing of the mini-exon (spliced leader) occurring at nt -160 from the predicted translation initiation site. The biochemical properties of the recombinant enzyme were similar to those of the enzyme isolated from leishmanial cells. The intrinsic tryptophan fluorescence of the enzyme was substrate-sensitive. On the basis of a multiple protein-alignment sequence comparison and ATP-induced fluorescence quenching in the presence or the absence of KI and acrylamide, the docking site for ATP has been provisionally identified and shown to have marked divergence from the consensus P-loop motif reported for ATP- or GTP-binding proteins from other sources. (+info)
Antagonistic effect of ganglioside GM1 and GM3 on the activity and conformation of sarcoplasmic reticulum Ca(2+)-ATPase.
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It was found that rabbit skeletal muscle sarcoplasmic reticulum (SR) contained two main gangliosides: NeuNAc alpha 2-->3 Gal beta 1-->4 Glc beta 1-->1'ceramide (GM3) and Gal beta 1-->3 GalNAc beta 1-->4(NeuNAc alpha 2-->3) Gal beta 1-->4 Glc beta 1-->1'ceramide (GM1), and that the most abundant ganglioside GM3 could positively modulate the SR Ca(2+)-ATPase activity. In this paper, the effect of GM1 on Ca(2+)-ATPase was further investigated and compared with that of GM3. The study demonstrates that GM1 has an opposite effect with respect to GM3 on the activity of SR Ca(2+)-ATPase. Using assays, including intrinsic and time-resolved fluorescence and fluorescence quenching, the conformational changes of SR Ca(2+)-ATPase induced by GM1 and GM3 were compared. Obtained results indicate that GM1 could make the Ca(2+)-ATPase molecules less compact in the hydrophilic domain but more compact in the hydrophobic domain, while GM3 makes the enzyme more compact in both the hydrophilic and hydrophobic domain. Homogeneous GM1 and GM3 with the same ceramide moiety had similar effects on SR Ca(2+)-ATPase activities compared to their natural counterparts, suggesting that the carbohydrate chain may be the key moiety of the ganglioside molecule to be responsible for the difference of the effect on enzyme activity. (+info)
Subunit interactions in the clathrin-coated vesicle vacuolar (H(+))-ATPase complex.
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The vacuolar (H(+))-ATPases (or V-ATPases) are structurally related to the F(1)F(0) ATP synthases of mitochondria, chloroplasts and bacteria, being composed of a peripheral (V(1)) and an integral (V(0)) domain. To further investigate the arrangement of subunits in the V-ATPase complex, covalent cross-linking has been carried out on the V-ATPase from clathrin-coated vesicles using three different cross-linking reagents. Cross-linked products were identified by molecular weight and by Western blot analysis using polyclonal antibodies raised against individual V-ATPase subunits. In the intact V(1)V(0) complex, evidence for cross-linking of subunits C and E, D and F, as well as E and G by disuccinimidyl glutarate was obtained, while in the free V(1) domain, cross-linking of subunits H and E was also observed. Subunits C and E as well as D and E could be cross-linked by 1-ethyl-3-(dimethylaminopropyl)carbodiimide, while subunits a and E could be cross-linked by 4-(N-maleimido)benzophenone. It was further demonstrated that it is possible to treat the V-ATPase with potassium iodide and MgATP in such a way that while subunits A, B, and H are nearly quantitatively removed, significant amounts of subunits C, D, E, and F remain attached to the membrane, suggesting that one or more of these latter subunits are in contact with the V(0) domain. In addition, treatment of the V-ATPase with cystine, which modifies Cys-254 of the catalytic A subunit, results in dissociation of subunit H, suggesting communication between the catalytic nucleotide binding site and subunit H. Finally, the stoichiometry of subunits F, G, and H were determined by quantitative amino acid analysis. Based on these and previous observations, a new structural model of the V-ATPase from clathrin-coated vesicles is proposed. (+info)
Photoaffinity labeling of wild-type and mutant forms of the yeast V-ATPase A subunit by 2-azido-[(32)P]ADP.
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Molecular modeling studies have previously suggested the possible presence of four aromatic residues (Phe(452), Tyr(532), Tyr(535), and Phe(538)) near the adenine binding pocket of the catalytic site on the yeast V-ATPase A subunit (MacLeod, K. J., Vasilyeva, E., Baleja, J. D., and Forgac, M. (1998) J. Biol. Chem. 273, 150-156). To test the proximity of these aromatic residues to the adenine ring, the yeast V-ATPase containing wild-type and mutant forms of the A subunit was reacted with 2-azido-[(32)P]ADP, a photoaffinity analog that stably modifies tyrosine but not phenylalanine residues. Mutant forms of the A subunit were constructed in which the two endogenous tyrosine residues were replaced with phenylalanine and in which a single tyrosine was introduced at each of the four positions. Strong ATP-protectable labeling of the A subunit was observed for the wild-type and the mutant containing tyrosine at 532, significant ATP-protectable labeling was observed for the mutants containing tyrosine at positions 452 and 538, and only very weak labeling was observed for the mutants containing tyrosine at 535 or in which all four residues were phenylalanine. These results suggest that Tyr(532) and possibly Phe(452) and Tyr(538) are in close proximity to the adenine ring of ATP bound to the A subunit. In addition, the effects of mutations at Phe(452), Tyr(532), Tyr(535), and Glu(286) on dissociation of the peripheral V(1) and integral V(0) domains both in vivo and in vitro were examined. The results suggest that in vivo dissociation requires catalytic activity while in vitro dissociation requires nucleotide binding to the catalytic site. (+info)
The D-loop structure of human mtDNA is destabilized directly by 1-methyl-4-phenylpyridinium ion (MPP+), a parkinsonism-causing toxin.
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1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine has been reported to cause parkinsonism via its neurotoxic form, 1-methyl-4-phenylpyridinium ion (MPP+), which inhibits complex I of the mitochondrial respiratory chain. Its parkinsonism-causing mechanisms attract a great deal of interest as a model of the disease. Recently, we reported that MPP+ strongly decreases the amount of mtDNA independent of the inhibition of complex I. Maintenance of a proper amount of mtDNA is essential for the normal function of mitochondria as exemplified in many mitochondrial diseases. The most characteristic feature in vertebral mtDNA replication is that H-strand synthesis proceeds displacing the parental H-strand as a long single strand. It forms the D-loop, a triplex replication intermediate composed of the parental L-strand, nascent H-strand and displaced H-strand. Here we show that MPP+ does not inhibit DNA synthesis by DNA polymerase gamma, but rather releases the nascent H-strands from mtDNA both in organello and in vitro. This indicates that MPP+ directly destabilizes the D-loop structure, thereby inhibiting replication. This study raises a new mechanism, i.e. destabilization of replication intermediates, for depletion of mtDNA. (+info)
Sporotrichosis in Peru: description of an area of hyperendemicity.
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Sporotrichosis is a sporadic and rare mycotic infection in most of the developed world. In many parts of the developing world, sporotrichosis is much more commonly recognized, but epidemiological data are generally lacking from these regions. We report epidemiological, clinical, and treatment data from 238 cases of culture-proven sporotrichosis occurring in a relatively remote area of the south central highlands of Peru that were retrospectively collected during 1995-1997. Most cases (60%) occurred in children aged +info)
Dynamics at Lys-553 of the acto-myosin interface in the weakly and strongly bound states.
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Lys-553 of skeletal muscle myosin subfragment 1 (S1) was specifically labeled with the fluorescent probe FHS (6-[fluorescein-5(and 6)-carboxamido]hexanoic acid succinimidyl ester) and fluorescence quenching experiments were carried out to determine the accessibility of this probe at Lys-553 in both the strongly and weakly actin-bound states of the MgATPase cycle. Solvent quenchers of varying charge [nitromethane, (2,2,6, 6-tetramethyl-1-piperinyloxy) (TEMPO), iodide (I(-)), and thallium (Tl(+))] were used to assess both the steric and electrostatic accessibilities of the FHS probe at Lys-553. In the strongly bound rigor (nucleotide-free) and MgADP states, actin offered no protection from solvent quenching of FHS by nitromethane, TEMPO, or thallium, but did decrease the Stern-Volmer constant by almost a factor of two when iodide was used as the quencher. The protection from iodide quenching was almost fully reversed with the addition of 150 mM KCl, suggesting this effect is ionic in nature rather than steric. Conversely, actin offered no protection from iodide quenching at low ionic strength during steady-state ATP hydrolysis, even with a significant fraction of the myosin heads bound to actin. Thus, the lower 50 kD subdomain of myosin containing Lys-553 appears to interact differently with actin in the weakly and strongly bound states. (+info)
Detection of fluorescently labeled actin-bound cross-bridges in actively contracting myofibrils.
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Myosin subfragment 1 (S1) can be specifically modified at Lys-553 with the fluorescent probe FHS (6-[fluorescein-5(and 6)-carboxamido]hexanoic acid succinimidyl ester) (Bertrand, R., J. Derancourt, and R. Kassab. 1995. Biochemistry. 34:9500-9507), and solvent quenching of FHS-S1 with iodide has been shown to be sensitive to actin binding at low ionic strength (MacLean, Chrin, and Berger, 2000. Biophys. J. 000-000). In order to extend these results and examine the fraction of actin-bound myosin heads within the myofilament lattice during calcium activation, we have modified skeletal muscle myofibrils, mildly cross-linked with EDC (1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide) to prevent shortening, with FHS. The myosin heavy chain appears to be the predominant site of labeling, and the iodide quenching patterns are consistent with those obtained for myosin S1 in solution, suggesting that Lys-553 is indeed the primary site of FHS incorporation in skeletal muscle myofibrils. The iodide quenching results from calcium-activated FHS-myofibrils indicate that during isometric contraction 29% of the myosin heads are strongly bound to actin within the myofilament lattice at low ionic strength. These results suggest that myosin can be specifically modified with FHS in more complex and physiologically relevant preparations, allowing the real time examination of cross-bridge interactions with actin in in vitro motility assays and during isometric and isotonic contractions within single muscle fibers. (+info)