Production, crystallization, and preliminary X-ray analysis of rabbit skeletal muscle troponin complex consisting of troponin C and fragment (1-47) of troponin I.

Troponin is a ternary protein complex consisting of subunits TnC. TnI, and TnT, and plays a key role in calcium regulation of the skeletal and cardiac muscle contraction. In the present study, a partial complex (CI47) was prepared from Escherichia coli-expressed rabbit skeletal muscle TnC and fragment 1-47 of TnI, which is obtained by chemical cleavage of an E. coli-expressed mutant of rabbit skeletal muscle TnI. Within the ternary troponin complex, CI47 is thought to form a core that is resistant to proteolytic digestion, and the interaction within CI47 likely maintains the integrity of the troponin complex. Complex CI47 was crystallized in the presence of sodium citrate. The addition of trehalose improved the diffraction pattern of the crystals substantially. The crystal lattice belongs to the space group P3(1)(2)21, with unit cell dimensions a = b = 48.2 A, c = 162 A. The asymmetric unit presumably contains one CI47 complex. Soaking with p-chloromercuribenzenesulfonate (PCMBS) resulted in loss of isomorphism, but enhanced the quality of the crystals. The crystals diffracted up to 2.3 A resolution, with completeness of 91% and R(merge) = 6.4%. The crystals of PCMBS-derivative should be suitable for X-ray studies using the multiple-wavelength anomalous diffraction technique. This is the first step for elucidating the structure of the full troponin complex.     

Molecular cloning of a human serotonin receptor (S12) with a pharmacological profile resembling that of the 5-HT1D subtype.

We report the molecular cloning of a fragment of human genomic DNA called S12, containing an open reading frame of 1170 nucleotides, which encodes a receptor for serotonin of 390 amino acids. The receptor function of the S12 protein was demonstrated by functional expression in mouse LS12 cells obtained by stable transfection of Ltk- cells, and LM5S12 cells, derived from LM5 cells (Ltk- cells previously transfected with the M5 muscarinic acetylcholine receptor). Adenylyl cyclase studies showed that the S12 receptor is able to mediate inhibition of adenylyl cyclase in response to serotonin in both types of cells. As studied in LM5S12 cells, the S12 receptor did not promote Ca2+ mobilization from internal stores, nor did it significantly modulate the sustained increase in [Ca2+]i elicited by stimulation of the phospholipase C stimulating M5 acetylcholine receptor. The pharmacologic profile of S12 as seen in adenylyl cyclase assays is as follows: (EC50 in nM): serotonin, full agonist (37 nM), 5-carboxamidotryptamine, full agonist (10 nM), sumatriptan, full agonist (50 nM), metergoline, partial agonist (10 nM), methysergide, partial agonist (40 nM), yohimbine, partial agonist (150 nM), metitepin, antagonist (KB = 0.7 to 1.2 nM). We propose that the human S12 serotonin receptor is a receptor of the 5-hydroxytryptamine1D subtype.     

Intermolecular triplex formation distorts the DNA duplex in the regulatory region of human papillomavirus type-11.

A conformational distortion in the DNA duplex at the regulatory region of human papillomavirus type-11 next to an intermolecular triplex, formed with a synthetic oligonucleotide, was investigated with several chemical probes. The sequence targeted for triplex formation borders on the binding sites for the regulatory proteins encoded by the viral E2 open reading frame. Dimethyl sulfate, diethyl pyrocarbonate, and OsO4 all react to a greater extent with nucleotides in the duplex that are immediately adjacent to the triplex as compared to other bases throughout the duplex. This hypermodification was observed on both the polypurine and polypyrimidine strands of the duplex DNA. Similar hyperreactivity of bases flanking a triplex also was seen when the contiguous target polypurine tract was effectively extended by mutating interrupting pyrimidines in the human papillomavirus type-11 sequence to purines. We propose that this hyperreactivity is due to a structural distortion caused by the junction between the triplex and the duplex tracts.     

The toxicity of methanol

Methanol toxicity in humans and monkeys is characterized by a latent period of many hours followed by a metabolic acidosis and ocular toxicity. This is not observed in most lower animals. The metabolic acidosis and blindness is apparently due to formic acid accumulation in humans and monkeys, a feature not seen in lower animals. The accumulation of formate is due to a deficiency in formate metabolism which is, in turn, related, in part, to low hepatic tetrahydrofolate (H4 folate). An excellent correlation between hepatic H4 folate and formate oxidation rates has been shown within and across species. Thus, humans and monkeys possess low hepatic H4 folate levels, low rates of formate oxidation and accumulation of formate after methanol. Formate, itself, produces blindness in monkeys in the absence of metabolic acidosis. In addition to low hepatic H4 folate concentrations, monkeys and humans also have low hepatic 10-formyl H4 folate dehydrogenase levels, the enzyme which is the ultimate catalyst for conversion of formate to carbon dioxide. This review presents the basis for the role of folic acid-dependent reactions in the regulation of methanol toxicity.     

Magnetic circular dichroism and electron paramagnetic resonance studies of hydrogenases I and II from Clostridium pasteurianum

The two iron-only hydrogenases (I and II) from Clostridium pasteurianum have been investigated by variable temperature magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopies. Samples were studied both reduced with dithionite under an atmosphere of H2 and after oxidation with thionine. The results are consistent with four and two [4Fe-4S]1+,2+ (F)-clusters in hydrogenases I and II, respectively. All four F-clusters are reduced and paramagnetic in reduced hydrogenase I, with up to one exhibiting an S = 3/2 ground state and the remainder having conventional S = 1/2 ground states. Both F-clusters have S = 1/2 ground states in reduced hydrogenase II; however, one appears to be only partially reduced under the conditions used for reduction. MCD studies of the oxidized enzymes show no temperature-dependent features in the visible region which can be attributed to the EPR-active S = 1/2 hydrogen-activating cluster, suggesting predominantly oxygen and nitrogen coordination for the iron atoms of this center. However, temperature-dependent MCD transitions arising from a hitherto undetected S greater than 1/2 Fe-S clusters are apparent in both oxidized hydrogenases. Detailed EPR studies of oxidized hydrogenase I revealed resonances from an S = 3/2 species, however, spin quantitation reveals this to be a trace component that is unlikely to be responsible for the observed low temperature MCD spectrum. The nature and origin of these S greater than 1/2 Fe-S clusters are discussed in light of the available spectroscopic data for these and other iron-only hydrogenases.     

Kinetics of deactivation for thermostable DNA polymerase enzymes

Summary The kinetics of thermal deactivation for thermostable DNA polymerase enzymes were investigated by using the experimental data published elsewhere (Nielson et al. 1996. Strategies. 9, 7–8). The order of deactivation (a) and the deactivation rate constants (k _d) were determined for different Taq DNA polymerase enzymes and were found to be of first order.     

The simian virus 40 T antigen double hexamer assembles around the DNA at the replication origin.

An initial step in the replication of simian virus (SV40) DNA is the ATP-dependent formation of a double hexamer of the SV40 large tumor (T) antigen at the SV40 DNA replication origin. In the absence of DNA, T antigen assembled into hexamers in the presence of magnesium and ATP. Hexameric T antigen was stable and could be isolated by glycerol gradient centrifugation. The ATPase activities of hexameric and monomeric T antigen isolated from parallel glycerol gradients were identical. However, while monomeric T antigen was active in the ATP-dependent binding, untwisting, unwinding, and replication of SV40 origin-containing DNA, hexameric T antigen was inactive in these reactions. Isolated hexamers incubated at 37 degrees C in the presence of ATP remained intact, but dissociated into monomers when incubated at 37 degrees C in the absence of ATP. This dissociation restored the activity of these preparations in the DNA replication reaction, indicating that hexameric T antigen is not permanently inactivated but merely assembled into a nonproductive structure. We propose that the two hexamers of T antigen at the SV40 origin assemble around the DNA from monomer T antigen in solution. This complex untwists the DNA at the origin, melting specific DNA sequences. The resulting single-stranded regions may be utilized by the T antigen helicase activity to initiate DNA unwinding bidirectionally from the origin.