Expression, purification, crystallization and preliminary X-ray analysis of human argininosuccinic acid lyase

Recovery of Argininosuccinate Lyase Activity in Duck δ1 Crystallin

Delta-crystallin, the major soluble protein component in the avian eye lens, is homologous to argininosuccinate lyase (ASL). Two delta-crystallin isoforms exist in ducks, delta1- and delta2-crystallin, which are 94% identical in amino acid sequence. While duck delta2-crystallin (ddeltac2) has maintained ASL activity, evolution has rendered duck delta1-crystallin (ddeltac1) enzymatically inactive. Previous attempts to regenerate ASL activity in ddeltac1 by mutating the residues in the 20s (residues 22-31) and 70s (residues 74-89) loops to those found in ddeltac2 resulted in a double loop mutant (DLM) which was enzymatically inactive (Tsai, M. et al. (2004) Biochemistry 43, 11672-82). This result suggested that one or more of the remaining five amino acid substitutions in domain 1 of the DLM contributes to the loss of ASL activity in ddeltac1. In the current study, residues Met-9, Val-14, Ala-41, Ile-43, and Glu-115 were targeted for mutagenesis, either alone or in combination, to the residues found in ddeltac2. ASL activity was recovered in the DLM by changing Met-9 to Trp, and this activity is further potentiated in the DLM-M9W mutant when Glu-115 is changed to Asp. The roles of Trp-9 and Asp-115 were further investigated by site-directed mutagenesis in wild-type ddeltac2. Changing the identity of either Trp-9 or Asp-115 in ddeltac2 resulted in a dramatic drop in enzymatic activity. The loss of activity in Trp-9 mutants indicates a preference for an aromatic residue at this position. Truncation mutants of ddeltac2 in which the first 8, 9, or 14 N-terminal residues were removed displayed either decreased or no ASL activity, suggesting residues 1-14 are crucial for enzymatic activity in ddeltac2. Our kinetic studies combined with available structural data suggest that the N-terminal arm in ASL/delta2-crystallin is involved in stabilizing regions of the protein involved in substrate binding and catalysis, and in completely sequestering the substrate from the solvent.

Mechanisms for intragenic complementation at the human argininosuccinate lyase locus

Argininosuccinate lyase (ASL) is a homotetrameric enzyme that catalyzes the reversible cleavage of argininosuccinate to arginine and fumarate. Deficiencies in the enzyme result in the autosomal, recessive disorder argininosuccinic aciduria. Considerable clinical and genetic heterogeneity is associated with this disorder, which is thought to be a consequence of the extensive intragenic complementation identified in patient strains. Our ability to predict genotype-phenotype relationships is hampered by the current lack of understanding of the mechanisms by which complementation can occur. The 3-dimensional structure of wild-type ASL has enabled us to propose that the complementation between two ASL active site mutant subunits, Q286R and D87G, occurs through a regeneration of functional active sites in the heteromutant protein. We have reconstructed this complementation event, both in vivo and in vitro, using recombinant proteins and have confirmed this hypothesis. The complementation events between Q286R and two nonactive site mutants, M360T and A398D, have also been characterized. The M360T and A398D substitutions have adverse effects on the thermodynamic stability of the protein. Complementation between either the M360T or the A398D mutant and the stable Q286R mutant occurs through the formation of a more stable heteromeric protein with partial recovery of catalytic activity. The detection and characterization of a novel complementation event between the A398D and D87G mutants has shown how complementation in patients with argininosuccinic aciduria may correlate with the clinical phenotype.

Three-dimensional structure of the argininosuccinate lyase frequently complementing allele Q286R

Argininosuccinate lyase (ASL) catalyzes the reversible breakdown of argininosuccinate to arginine and fumarate, a reaction involved in the biosynthesis of arginine in all species and in the production of urea in ureotelic species. In humans, mutations in the enzyme result in the autosomal recessive disorder argininosuccinic aciduria. Intragenic complementation has been demonstrated to occur at the ASL locus, with two distinct classes of ASL-deficient strains having been identified, the frequent and high-activity complementers. The frequent complementers participate in the majority of the complementation events observed and were found to be either homozygous or heterozygous for a glutamine to arginine mutation at residue 286. The three-dimensional structure of the frequently complementing allele Q286R has been determined at 2.65 A resolution. This is the first high-resolution structure of human ASL. Comparison of this structure with the structures of wild-type and mutant duck delta1 and delta2 crystallins suggests that the Q286R mutation may sterically and/or electrostatically hinder a conformational change in the 280's loop (residues 270-290) and domain 3 that is thought to be necessary for catalysis to occur. The comparison also suggests that residues other than R33, F333, and D337 play a role in maintaining the structural integrity of domain 1 and reinforces the suggestion that residues 74-89 require a particular conformation for catalysis. The electron density has enabled the structure of residues 6-18 to be modeled for the first time. Residues 7-9 and 15-18 are in type IV beta-turns and are connected by a loop. The conformation observed is stabilized, in part, by a salt bridge between the side chains of R12 and D18. Although the disease causing mutation R12Q would disrupt this salt bridge, it is unclear why this mutation has such a significant effect on the catalytic activity as residues 1-18 are disordered in all other delta-crystallin structures determined to date.

Human argininosuccinate lyase: a structural basis for intragenic complementation

Intragenic complementation has been observed at the argininosuccinate lyase (ASL) locus. Intragenic complementation is a phenomenon that occurs when a multimeric protein is formed from subunits produced by different mutant alleles of a gene. The resulting hybrid protein exhibits enzymatic activity that is greater than that found in the oligomeric proteins produced by each mutant allele alone. The mutations involved in the most successful complementation event observed in ASL deficiency were found to be an aspartate to glycine mutation at codon 87 of one allele (D87G) coupled with a glutamine to arginine mutation at codon 286 of the other (Q286R). To understand the structural basis of the Q286R:D87G intragenic complementation event at the ASL locus, we have determined the x-ray crystal structure of recombinant human ASL at 4. 0 A resolution. The structure has been refined to an R factor of 18. 8%. Two monomers related by a noncrystallographic 2-fold axis comprise the asymmetric unit, and a crystallographic 2-fold axis of space group P3121 completes the tetramer. Each of the four active sites is composed of residues from three monomers. Structural mapping of the Q286R and D87G mutations indicate that both are near the active site and each is contributed by a different monomer. Thus when mutant monomers combine randomly such that one active site contains both mutations, it is required by molecular symmetry that another active site exists with no mutations. These "native" active sites give rise to the observed partial recovery of enzymatic activity.

Characterization and enzyme activity of argininosuccinate lyase/delta-crystallin of the embryonic duck lens

Argininosuccinate lyase (ASL)/delta-crystallin, a major soluble protein of the transparent eye lens of birds and reptiles, is a mixture of tetramers comprising all possible combinations of two similar polypeptides (delta 1 and delta 2). Only the delta 2 polypeptide has ASL activity. In the present investigation we have purified each of the 5 major isoforms (delta A to delta E, pI 5.2 to 5.8) of delta-crystallin tetramers from the embryonic duck lens by isoelectric focussing and established by peptide sequencing that the delta 1 and delta 2 polypeptides are encoded in the previously identified, linked delta 1 and delta 2 genes, respectively. The relative amounts of the different tetramers in the 14-day-old embryonic lens were consistent with equal expression of the 2 delta-crystallin genes and no preference for assembly of the 2 delta polypeptides. The relative amount of ASL activity of the tetramers was a linear function of the relative amount of their delta 2 polypeptides, with delta A (only delta 1) lacking enzymatic activity altogether. delta B (3 delta 1:1 delta 2), delta C (2 delta 1:2 delta 2), delta D (1 delta 1:3 delta 2) and delta E (4 delta 2) all gave normal Michaelis-Menten kinetics for fumarate production from argininosuccinate at 40 degrees C and had a similar Km (average Km for mixture was 0.15 mM). delta E had a Km of 0.187 mM and a Vmax of 9 mumol/min per mg protein. Unlike bovine and like human ASL, both reported previously, embryonic duck ASL/delta-crystallin showed no evidence of cooperativity or activation by GTP. Each isoform had a similar far ultraviolet circular dichroism spectrum and thermal stability between 20 degrees C and 60 degrees C, with denaturation occurring at 65 degrees C. Our data suggest that gene duplication, structural modifications leading to greater thermal stability of the delta 1 and delta 2 polypeptides, and selective loss of ASL activity in the delta 1 polypeptide all occurred during the recruitment of ASL for a refractive role in the duck lens, resulting in the generation of ASL isoenzymes.