Synthesis of the nonconserved dihydroorotase domain of the multifunctional hamster CAD protein in Escherichia coli

Ligand binding by recombinant domains from insect ecdysone receptors

The ligand binding domains (LBDs) from the EcR and USP proteins of four insect pests (Lucilia cuprina, Myzus persicae, Bemisia tabaci, Helicoverpa armigera) were purified as recombinant heterodimers. The K(d) values for [(3)H]-ponasterone A binding by LBD heterodimers that included the hinge regions (i.e., DE/F heterodimers) ranged 0.7-2.5 nM, with K(i) values for ecdysteroid and dibenzoylhydrazine ligands ranging from 0.1 nM to >448 microM. The K(d) and K(i) values for a recombinant H. armigera LBD heterodimer that lacked D-regions (i.e., an E/F heterodimer) were approximately 4 times higher than those for its DE/F counterpart. Rate constants were estimated for the L. cuprina LBD heterodimer. A fluorescein-inokosterone conjugate (K(i)~40 nM) was used to develop a novel binding assay based on fluorescence polarization. This assay, which ranked the affinity of competitor ecdysteroids in the same order as the [(3)H]-ponasterone A binding assay, is well suited to high-throughput screening. Ponasterone A had a higher affinity than muristerone A for the recombinant hemipteran LBD heterodimers, whereas the reverse was true for the recombinant dipteran one. The same preference was observed when these ligands were tested as inducers of ecdysone receptor-controlled gene expression in transfected mammalian cells. The binding data obtained in vitro using recombinant LBD heterodimers reflects the ability of agonists to induce transgene expression in recombinant mammalian cells, and can also reflect their efficacy as larvicides.

Aquifex aeolicus dihydroorotase: association with aspartate transcarbamoylase switches on catalytic activity

Dihydroorotase (DHOase) catalyzes the reversible condensation of carbamoyl aspartate to form dihydroorotate in de novo pyrimidine biosynthesis. The enzyme from Aquifex aeolicus, a hyperthermophilic organism of ancient lineage, was cloned and expressed in Escherichia coli. The purified protein was found to be a 45-kDa monomer containing a single zinc ion. Although there is no other DHOase gene in the A. aeolicus genome, the recombinant protein completely lacked catalytic activity at any temperature tested. However, DHOase formed an active complex with aspartate transcarbamoylase (ATCase) from the same organism. Whereas the k(cat) of 13.8 +/- 0.03 s(-1) was close to the value observed for the mammalian enzyme, the K (m)for dihydroorotate, 3.03 +/- 0.05 mM was 433-fold higher. Gel filtration and chemical cross-linking showed that the complex exists as a 240-kDa hexamer (DHO(3)-ATC(3)) and a 480-kDa duodecamer (DHO(6)-ATC(6)) probably in rapid equilibrium. Complex formation protects both DHOase and ATCase against thermal degradation at temperatures near 100 degrees C where the organism grows optimally. These results lead to the reclassification of both enzymes: ATCase, previously considered a Class C homotrimer, now falls into Class A, whereas the DHOase is a Class 1B enzyme. CD spectroscopy indicated that association with ATCase does not involve a significant perturbation of the DHOase secondary structure, but the visible absorption spectrum of a Co(2+)-substituted DHOase is appreciably altered upon complex formation suggesting a change in the electronic environment of the active site. The association of DHOase with ATCase probably serves as a molecular switch that ensures that free, uncomplexed DHOase in the cell remains inactive. At pH 7.4, the equilibrium ratio of carbamoyl aspartate to dihydroorotate is 17 and complex formation may drive the reaction in the biosynthetic direction.

Site-directed substitution of Ser1406 of hamster CAD with glutamic acid alters allosteric regulation of carbamyl phosphate synthetase II

Ser1406 of the allosteric region of the hamster CAD enzyme, carbamyl phosphate synthetase II (CPSase), is known to be phosphorylated in vitro by cAMP-dependent protein kinase (PKA). Metabolic labeling experiments described here demonstrate that CAD is phosphorylated in somatic cells in culture. Phosphorylation is stimulated by treating cells with 8-bromo-cAMP, a PKA activator. The stimulation is essentially prevented by pretreatment with H-89, a PKA specific inhibitor. Substitution of Ser1406 with alanine results in an enzyme with kinetics and allosteric regulation indistinguishable from unsubstituted CAD. However, substitution to glutamic acid increases CPSase activity by reducing the apparent Km (ATP). The UTP concentration required to give 50% inhibition is increased rendering this altered enzyme significantly less sensitive to feedback inhibition, but allosteric activation by PRPP is unaffected. While these data do not prove that Ser1406 is phosphorylated in vivo, they do indicate that a specific alteration at this residue can affect allosteric regulation.

UMP synthase activity expressed in deficient hamster cells by separate transferase and decarboxylase proteins or by linker-deleted bifunctional protein

Segments of the human UMP synthase cDNA coding for the orotate phosphoribosyl transferase (OPRT) and orotidylate decarboxylase (ODC) domains of the bifunctional protein UMP synthase were produced by polymerase chain reaction techniques and cloned into a eukaryotic expression vector. The separate OPRT and ODC vectors, along with a selectable marker, were cotransfected into UMP synthase-deficient hamster cells (Urd-C) that require exogeneous uridine for growth. Transfected Urd-C cells surviving selection in media without added uridine were isolated and designated transferase decarboxylase Urd-C (TDU). All of the selected colonies contained DNA corresponding to the OPRT and ODC expression vectors. Two cell lines (TDU3 and TDU5) integrated many more copies of the OPRT and ODC vectors into their genomes compared to the other TDU lines. A 28.6-kDa ODC protein band and a 24.4-kDa OPRT band were detected on western blots with UMP synthase-specific polyclonal antiserum. The OPRT activity of the TDU lines was up to 8.7 times the OPRT activity of control CHL cells, and the ODC activity was up to 12.5 times control levels. Both OPRT and ODC activities in the monofunctional proteins were less heat stable than in the bifunctional UMP synthase protein. The monofunctional OPRT protein was less stable than the ODC protein at 45 degrees C. Growth of transfected cells in 6-azauridine resulted in striking increases in activity and temperature stability for the monofunctional ODC protein. A UMP synthase bifunctional protein was constructed with a deletion of the suspected linker region joining the two catalytic domains. The linker-deleted UMP synthase showed no significant change in either OPRT or ODC activity or temperature stability. The increased stability of the bifunctional protein may be a factor in its evolutionary selection in mammalian cells.

The evolutionary history of the first three enzymes in pyrimidine biosynthesis

Some metabolic pathways are nearly ubiquitous among organisms: the genes encoding the enzymes for such pathways must therefore be ancient and essential. De novo pyrimidine biosynthesis is an example of one such metabolic pathway. In animals a single protein called CAD carries the first three steps of this pathway. The same three enzymes in prokaryotes are associated with separate proteins. The CAD gene appears to have evolved through a process of gene duplication and DNA rearrangement, leading to an in-frame gene fusion encoding a chimeric protein. A driving force for the creation of eukaryotic genes encoding multienzymatic proteins such as CAD may be the advantage of coordinate expression of enzymes catalyzing steps in a biosynthetic pathway. The analogous structure in bacteria is the operon. Differences in the translational mechanisms of eukaryotes and prokaryotes may have dictated the different strategies used by organisms to evolve coordinately regulated genes.

Expression of catalytic domains of human UMP synthase in uridine auxotrophic bacteria

Orotate phosphoribosyltransferase (OPRT) and orotidine-5'-monophosphate decarboxylase (ODC), which catalyze the last two steps in de novo UMP biosynthesis, are two distinct monofunctional proteins in bacteria and lower eukaryotes. In mammals, OPRT and ODC activities are contained in a single bifunctional protein labeled UMP synthase. The human UMP synthase cDNA was separated into the predicted OPRT and ODC domains using polymerase chain reaction techniques and the domains inserted into pUC19 expression vectors. Following transformation into OPRT- and ODC-deficient E. coli, the strains were able to grow on minimal media without uridine. The ODC-transformed bacteria expressed up to 24 times the level of activity found in a wild-type E. coli line. The OPRT-transformed E. coli contained only 4-9% of wild-type activity. Western blot analysis with antiserum to human UMP synthase demonstrates that OPRT and ODC domains are being produced in the deficient cells by the respective vectors. The level of the domain protein approximates the level of enzyme activity. The complementation of the OPRT and ODC activities in the transformed deficient E. coli strains demonstrates that human UMP synthase can be separated into active monofunctional domains that will function in the bacterial cell environment.

Complete hamster CAD protein and the carbamylphosphate synthetase domain of CAD complement mammalian cell mutants defective in de novo pyrimidine biosynthesis

The mammalian CAD gene codes for a 240-kDa multifunctional protein that catalyzes the first three steps of de novo pyrimidine biosynthesis. Previously, the longest cDNA construct available was missing approximately 500 bp of coding sequence at the 5' end, thereby lacking the sequence to encode the entire carbamylphosphate synthetase (CPSase) domain. Here, a complete CAD hamster cDNA is constructed, placed into a mammalian expression vector, and transfected into hamster cells deficient in CAD. Transfectants show coordinately restored levels of all three enzyme activities and the presence of full-length CAD protein. A derivative construct of the CAD cDNA was generated that should encode only the CPSase domain. When transfected into mammalian cells, a protein was synthesized that had significant CPSase activity both in vivo and in vitro. The two constructs generated in this study will facilitate the study of CAD structure, function, and allosteric regulation.

A protonated histidine residue in a phosphorylation site for cyclic AMP-dependent protein kinase. Comparison of a synthetic peptide with the exposed linking region in the multienzyme polypeptide CAD

The multienzyme polypeptide CAD is phosphorylated at two sites by cyclic AMP (cAMP)-dependent protein kinase. Site 2 has two interesting features: it is located in a 'linking region' between two discretely folded enzyme domains, and a histidine, instead of the more usual arginine, is found three positions N-terminal to the phosphorylated serine. A synthetic peptide corresponding to the sequence around site 2 has an extended or random structure in solution, and the proton n.m.r. chemical shift of the histidine residues can be titrated against pH in the range 6.0-8.0. The peptide is phosphorylated more rapidly by cAMP-dependent protein kinase at lower pH values, indicating that the protonated histidine side chain corresponds to the arginine in the consensus recognition sequence for the kinase. Kemptide, a specific synthetic substrate for the kinase, was phosphorylated with a higher affinity and at a similar rate at all pH values. CAD was a better substrate than the synthetic peptide, and labelling was not affected by the pH of the incubation conditions. The results indicate that the phosphorylation site in the interdomain linker is sufficiently exposed to the solvent to ensure accessibility to the kinase, but that secondary or tertiary structure in the intact protein allows the histidine residue to remain protonated at physiological pH and enhances recognition of the phosphorylatable serine residue.