Inversion of allosteric effect of arginine on N-acetylglutamate synthase, a molecular marker for evolution of tetrapods.

BACKGROUND: The efficient conversion of ammonia, a potent neurotoxin, into non-toxic metabolites was an essential adaptation that allowed animals to move from the aquatic to terrestrial biosphere. The urea cycle converts ammonia into urea in mammals, amphibians, turtles, snails, worms and many aquatic animals and requires N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthetase I (CPSI) in mammals and amphibians, and carbamylphosphate synthetase III (CPSIII) in fish and invertebrates. NAG-dependent CPSI and CPSIII catalyze the formation of carbamylphosphate in the first and rate limiting step of ureagenesis. NAG is produced enzymatically by N-acetylglutamate synthase (NAGS), which is also found in bacteria and plants as the first enzyme of arginine biosynthesis. Arginine is an allosteric inhibitor of microbial and plant NAGS, and allosteric activator of mammalian NAGS. RESULTS: Information from mutagenesis studies of E. coli and P. aeruginosa NAGS was combined with structural information from the related bacterial N-acetylglutamate kinases to identify four residues in mammalian NAGS that interact with arginine. Substitutions of these four residues were engineered in mouse NAGS and into the vertebrate-like N-acetylglutamate synthase-kinase (NAGS-K) of Xanthomonas campestris, which is inhibited by arginine. All mutations resulted in arginine losing the ability to activate mouse NAGS, and inhibit X. campestris NAGS-K. To examine at what point in evolution inversion of arginine effect on NAGS occur, we cloned NAGS from fish and frogs and examined the arginine response of their corresponding proteins. Fish NAGS were partially inhibited by arginine and frog NAGS were activated by arginine. CONCLUSION: Difference in arginine effect on bacterial and mammalian NAGS most likely stems from the difference in the type of conformational change triggered by arginine binding to these proteins. The change from arginine inhibition of NAGS to activation was gradual, from complete inhibition of bacterial NAGS, to partial inhibition of fish NAGS, to activation of frog and mammalian NAGS. This change also coincided with the conquest of land by amphibians and mammals.

Late onset N-acetylglutamate synthase deficiency caused by hypomorphic alleles.

N-acetylglutamate (NAG) is a unique cofactor that is essential for the conversion of ammonia to urea in the liver. N-acetylglutamate synthase (NAGS) catalyzes the formation of NAG. Deficiency of NAGS causes a block in ureagenesis resulting in hyperammonemia. Although a number of mutations have been identified in the NAGS gene, their effects on NAGS enzymatic activity have not been examined. We describe here three mutations in two families with NAGS deficiency. Studies of the purified recombinant mutant proteins revealed deleterious effects on NAGS affinity for substrates, and on the rate of catalysis. These studies provide a better understanding of the function of NAGS, and the mechanisms for deleterious effect of mutations causing inherited NAGS deficiency.

Transcriptional regulation of N-acetylglutamate synthase.

The urea cycle converts toxic ammonia to urea within the liver of mammals. At least 6 enzymes are required for ureagenesis, which correlates with dietary protein intake. The transcription of urea cycle genes is, at least in part, regulated by glucocorticoid and glucagon hormone signaling pathways. N-acetylglutamate synthase (NAGS) produces a unique cofactor, N-acetylglutamate (NAG), that is essential for the catalytic function of the first and rate-limiting enzyme of ureagenesis, carbamyl phosphate synthetase 1 (CPS1). However, despite the important role of NAGS in ammonia removal, little is known about the mechanisms of its regulation. We identified two regions of high conservation upstream of the translation start of the NAGS gene. Reporter assays confirmed that these regions represent promoter and enhancer and that the enhancer is tissue specific. Within the promoter, we identified multiple transcription start sites that differed between liver and small intestine. Several transcription factor binding motifs were conserved within the promoter and enhancer regions while a TATA-box motif was absent. DNA-protein pull-down assays and chromatin immunoprecipitation confirmed binding of Sp1 and CREB, but not C/EBP in the promoter and HNF-1 and NF-Y, but not SMAD3 or AP-2 in the enhancer. The functional importance of these motifs was demonstrated by decreased transcription of reporter constructs following mutagenesis of each motif. The presented data strongly suggest that Sp1, CREB, HNF-1, and NF-Y, that are known to be responsive to hormones and diet, regulate NAGS transcription. This provides molecular mechanism of regulation of ureagenesis in response to hormonal and dietary changes.

Biochemical properties of recombinant human and mouse N-acetylglutamate synthase.

N-Acetylglutamate synthase (NAGS, EC 2.3.1.1) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate (NAG) from glutamate and acetylcoenzyme A. NAG is an obligatory activator of carbamylphosphate I (CPSI), the first and a rate limiting enzyme of ureagenesis. The enzymatic activity of NAGS increases in the presence of arginine. Since the level of NAGS activity depends on the concentrations of two amino acids, glutamate and arginine, and it supplies the essential cofactor for CPSI, NAGS may play an important role in the regulation of ureagenesis. The amino acid sequences of human and mouse NAGS consist of three regions with different degrees of conservation: the mitochondrial targeting signal (MTS), the variable domain, and the conserved domain. Removal of the MTS results in mature NAGS (NAGS-M) while removal of the MTS and the variable domain results in conserved NAGS (NAGS-C). The biochemical properties of purified recombinant human and mouse NAGS-M and NAGS-C were determined in this study with the goal of better understanding the role of the variable domain in NAGS function. The activity of all four proteins doubled in the presence of arginine, while the affinities for substrates changed less than two fold. The turnover numbers of NAGS-C are double those of NAGS-M proteins. Processing of NAGS-M to form NAGS-C results in an enzyme with higher catalytic activity and could play a role in the regulation of NAG production, CPSI function, and urea synthesis.

Null mutations in the N-acetylglutamate synthase gene associated with acute neonatal disease and hyperammonemia.

N-acetylglutamate synthase (NAGS) is a mitochondrial enzyme that catalyzes the formation of N-acetylglutamate, an essential allosteric activator of carbamyl phosphate synthetase I, the first enzyme of the urea cycle. Liver NAGS deficiency has previously been found in a small number of patients with hyperammonemia. The mouse and human NAGS genes have recently been cloned and expressed in our laboratory. We searched for mutations in the NAGS gene of two families with presumed NAGS deficiency. The exons and exon/intron boundaries of the NAGS gene were sequenced from genomic DNA obtained from the parents of an infant from the Faroe Islands who died in the neonatal period and from two Hispanic sisters who presented with acute neonatal hyperammonemia. Both parents of the first patient were found to be heterozygous for a null mutation in exon 4 (TGG-->TAG, Trp324Ter). Both sisters from the second family were homozygous for a single base deletion in exon 4 (1025delG) causing a frameshift and premature termination of translation. The finding of deleterious mutations in the NAGS gene confirms the genetic origin of NAGS deficiency. This disorder can now be diagnosed by DNA testing allowing for carrier detection and prenatal diagnosis.

An ordered collection of expressed sequences from Cryphonectria parasitica and evidence of genomic microsynteny with Neurospora crassa and Magnaporthe grisea.

Cryphonectria parasitica, the causative agent of chestnut blight, has proven to be a tractable experimental system for studying fungal pathogenesis. Moreover, the development of infectious cDNA clones of C. parasitica hypoviruses, capable of attenuating fungal virulence, has provided the opportunity to examine molecular aspects of fungal plant pathogenesis in the context of biological control. In order to establish a genomic base for future studies of C. parasitica, the authors have analysed a collection of expressed sequences. A mixed cDNA library was prepared from RNA isolated from wild-type (virus-free) and hypovirus-infected C. parasitica strains. Plasmid DNA was recovered from individual transformants and sequenced from the 5' end of the insert. Contig analysis of the collected sequences revealed that they represented approximately 2200 individual ORFs. An assessment of functional diversity present in this collection was achieved by using the BLAST software utilities and the NCBI protein database. Candidate genes were identified with significant potential relevance to C. parasitica growth, development, pathogenesis and vegetative incompatibility. Additional investigations of a 12.9 kbp genomic region revealed microsynteny between C. parasitica and both Neurospora crassa and Magnaporthe grisea, two closely related fungi. These data represent the largest collection of sequence information currently available for C. parasitica and are now forming the basis of further studies using microarray analyses to determine global changes in transcription that occur in response to hypovirus infection.

Cloning and expression of the human N-acetylglutamate synthase gene.

N-acetylglutamate synthase (NAGS, E.C. 2.3.1.1) is a mitochondrial enzyme catalyzing the formation of N-acetylglutamate (NAG), an essential allosteric activator of carbamylphosphate synthase I (CPSI), the first enzyme of the urea cycle. Patients with NAGS deficiency develop hyperammonemia because CPSI is inactive without NAG. The human NAGS cDNA was isolated from a liver library based on its similarity to mouse NAGS. The deduced amino acid sequence contains an N-terminal putative mitochondrial targeting signal of 49 amino acids (63% identity with mouse NAGS) followed by a "variable domain" of 45 amino acids (35% identity) and a "conserved domain" of 440 amino acids (92% identity). A cDNA sequence containing the "conserved domain" complements an NAGS-deficient Escherichia coli strain and the recombinant protein has arginine-responsive NAGS catalytic activity. The NAGS gene is expressed in the liver and small intestine; the intestinal transcript is smaller in size than liver transcript.