Membrane-enclosed crystals in Dictyostelium discoideum cells, consisting of developmentally regulated proteins with sequence similarities to known esterases

A recombinant antibody toolbox for Dictyostelium discoideum

Objective

The amoeba Dictyostelium discoideum has been a valuable model organism to study numerous facets of eukaryotic cell biology, such as cell motility, cell adhesion, macropinocytosis and phagocytosis, host–pathogen interactions and multicellular development. However, the relative small size of the Dictyostelium community hampers the production and distribution of reagents and tools, such as antibodies, by commercial vendors.

Results

For the past 5 years, our laboratory has worked to promote an increased use of recombinant antibodies (rAbs) by academic laboratories. Here we report our efforts to ensure that Dictyostelium researchers have access to rAbs. Using hybridoma sequencing and phage display techniques, we generated a panel of recombinant antibodies against D. discoideum antigens, providing a useful and reliable set of reagents for labelling and characterization of proteins and subcellular compartments in D. discoideum, accessible to the entire Dictyostelium community.

The Dictyostelium discoideum GPHR ortholog is an endoplasmic reticulum and Golgi protein with roles during development

Dictyostelium discoideum GPHR (Golgi pH regulator)/Gpr89 is a developmentally regulated transmembrane protein present on the endoplasmic reticulum (ER) and the Golgi apparatus. Transcript levels are low during growth and vary during development, reaching high levels during the aggregation and late developmental stages. The Arabidopsis ortholog was described as a G protein-coupled receptor (GPCR) for abscisic acid present at the plasma membrane, whereas the mammalian ortholog is a Golgi apparatus-associated anion channel functioning as a Golgi apparatus pH regulator. To probe its role in D. discoideum, we generated a strain lacking GPHR. The mutant had different growth characteristics than the AX2 parent strain, exhibited changes during late development, and formed abnormally shaped small slugs and fruiting bodies. An analysis of development-specific markers revealed that their expression was disturbed. The distributions of the endoplasmic reticulum and the Golgi apparatus were unaltered at the immunofluorescence level. Likewise, their functions did not appear to be impaired, since membrane proteins were properly processed and glycosylated. Also, changes in the external pH were sensed by the ER, as indicated by a pH-sensitive ER probe, as in the wild type.

Dynamics of the Dictyostelium discoideum mitochondrial proteome during vegetative growth, starvation and early stages of development

In this study, a quantitative comparative proteomics approach has been used to analyze the Dictyostelium discoideum mitochondrial proteome variations during vegetative growth, starvation and the early stages of development. Application of 2-D DIGE technology allowed the detection of around 2000 protein spots on each 2-D gel with 180 proteins exhibiting significant changes in their expression level. In total, 96 proteins (51 unique and 45 redundant) were unambiguously identified. We show that the D. discoideum mitochondrial proteome adaptations mainly affect energy metabolism enzymes (the Krebs cycle, anaplerotic pathways, the oxidative phosphorylation system and energy dissipation), proteins involved in developmental and signaling processes as well as in protein biosynthesis and fate. The most striking observations were the opposite regulation of expression of citrate synthase and aconitase and the very large variation in the expression of the alternative oxidase that highlighted the importance of citrate and alternative oxidase in the physiology of the development of D. discoideum. Mitochondrial energy states measured in vivo with MitoTracker Orange CM Ros showed an increase in mitochondrial membrane polarization during D. discoideum starvation and starvation-induced development.

Vacuolin, a flotillin/reggie-related protein from Dictyostelium oligomerizes for endosome association

We have analysed the domain structure of vacuolin, a Dictyostelium protein binding to the cytoplasmic surface of late endosomes. Localisation studies using GFP fusions together with a yeast two-hybrid analysis and co-immunoprecipitation experiments reveal that a region close to the C-terminus mediates oligomer formation of the protein through a coiled-coil mechanism which in turn is a prerequisite for the efficient binding to endosomal membranes via a prohibitin (PHB) domain in the middle of the molecule. Overexpression of the coiled-coil domain strongly competes with endogenous vacuolin in the oligomers and reduces the efficiency of membrane targeting. The domain arrangement of vacuolin is most similar to flotillin/reggie, a protein found on late endosomes of mammalian cells.

A Dictyostelium mutant with reduced lysozyme levels compensates by increased phagocytic activity

Lysozymes are bacteria-degrading enzymes and play a major role in the immune defense of animals. In free-living protozoa, lysozyme-like proteins are involved in the digestion of phagocytosed bacteria. Here, we purified a protein with lysozyme activity from Dictyostelium amoebae, which constitutes the founding member, a novel class of lysozymes. By tagging the protein with green fluorescent protein or the Myc epitope, a new type of lysozyme-containing vesicle was identified that was devoid of other known lysosomal enzymes. The most highly expressed isoform, encoded by the alyA gene, was knocked out by homologous recombination. The mutant cells had greatly reduced enzymatic activity and grew inefficiently when bacteria were the sole food source. Over time the mutant gained the ability to internalize bacteria more efficiently, so that the defect in digestion was compensated by increased uptake of food particles.

A Dictyostelium long chain fatty acyl coenzyme A-synthetase mediates fatty acid retrieval from endosomes

We have identified a subset of Dictyostelium endosomes that carry a long chain fatty acyl coenzyme A-synthetase (LC-FACS 1) on their cytosolic surface. Immunofluorescence studies and observations using GFP-fusion proteins collectively suggest that LC-FACS 1 associates with endosomes a few minutes after their formation, remains bound through the acidic phase of endocytic maturation and dissociates early in the phase where the endosomal content is neutralised prior to exocytosis. Mutants in the fcsA gene, encoding the LC-FACS 1 protein, were constructed by homologous recombination. These cells show a strong defect in the intracellular accumulation of fatty acids, either taken up together with the liquid medium or bound to the surface of particles. Because the mutant cells are otherwise fully competent for macropinocytosis and phagocytosis, we conclude that the LC-FACS 1 protein mediates the retrieval of fatty acids from the lumen of endosomes into the cytoplasm.

Proteomics opens doors to the mechanisms of developmentally regulated secretion

The program of multicellular development in Dictyostelium discoideum culminates with the assembly of a rugged, environmentally resistant spore coat around each spore cell. After synthesis, the proteins that will constitute the coat are stored in prespore vesicles (PSVs) until an unknown developmental signal triggers the PSVs to move to the cell surface where they fuse with the plasma membrane and secrete their cargo by exocytosis. These events occur synchronously in 80% of the cells in each developing multicellular aggregate, and thus the system offers a unique opportunity to study the developmental regulation of protein secretion in situ. Proteomic analysis of purified PSVs identified many of the constituent proteins, which in turn has lead to novel hypotheses and new experimental avenues regarding the molecular mechanisms regulating secretion from the PSVs.

Conserved features of endocytosis in Dictyostelium

Endocytosis in protozoa is often regarded as largely different from the pathways operating in mammalian cells. Experiments in the amoeba Dictyostelium, one of the genetically tractable single-celled organisms, have allowed us to manipulate the flow through endocytic compartments and to study the dynamic distribution of molecules by means of green fluorescent protein fusions. This review attempts to compile the molecular data available from Dictyostelium and assign them to specific steps of internalization by phagocytosis or macropinocytosis and to subsequent stages of the endocytic pathway. Parallels to phagocytes of the mammalian immune system are emphasized. The major distinctive feature between mammalian phagocytes and free-living cells is the need for osmoregulation. Therefore Dictyostelium cells possess a contractile vacuole that has occasionally obscured analysis of endocytosis but is now found to be entirely separate from endocytic organelles. In conclusion, the potential of Dictyostelium amoebas to provide a model system of mammalian phagocytes is ever increasing.

Role of esterase gp70 and its influence on growth and development of Dictyostelium discoideum

Gp70 is an esterase originally called crystal protein because of its presence in crystalline structures in aggregation-competent Dictyostelium discoideum cells. Although postulated to break down spore coats, the function of gp70 in vivo was incompletely investigated. Our immunolocalization and biochemical studies of vegetative D. discoideum amoebae show that gp70 was recruited to phagosomes and found in lysosomes. Purified gp70 was effective at hydrolyzing naphthyl substrates with acyl chains typical of lipids and lipopolysaccharides, indicating that the gp70 was involved in digesting endocytosed molecules. The activity of purified gp70 was inhibited by reductants that retarded its electrophoretic mobility and verified the presence of intramolecular disulfide bonds predicted by its amino acid sequence. Compared to wild-type cells, cells overexpressing gp70 were more phagocytically active, had shorter generation times, and produced more fruiting bodies per unit area, while cells lacking gp70 were phagocytically less active with longer doubling times, developed more slowly, and had significantly fewer fruiting bodies per unit area. Consistent with the phenotype of a disrupted metabolism, one-third of the gp70-minus cells were large and multinucleated. Together, these results indicated that despite its crystalline appearance, gp70 was an active esterase involved in both the growth and the development of D. discoideum.

estA, a gene coding for a cell-bound esterase from Paenibacillus sp. BP-23, is a new member of the bacterial subclass of type B carboxylesterases

Screening of a gene library from Paenibacillus sp. BP-23 generated in Escherichia coli led to identification of a clone that directed the production of lipolytic activity. From the sequencing data, we found an open reading frame encoding a protein of 485 amino acids with an estimated molecular mass of 53 kDa and a pI of 5.1. Absence of a signal peptide indicated that it was a cell-bound protein. Sequence analysis showed that the protein contained the signature G-XI-S-X2-G included in most serine-esterases and lipases. The cloned protein showed high homology with enzymes belonging to the bacterial subclass of type B carboxylesterases. The enzyme had a significant preference for esters of short-chain fatty acids and showed the kinetics behaviour of a true esterase. Maximum activity was found at pH 7.5 and 37 degrees C, although the enzyme was active in the pH range 6.0- 9.0 and at temperatures up to 45 degrees C. As expected for a serine-esterase, activity was inhibited by phenylmethylsulphonyl fluoride.

Carboxyl/cholinesterases: a case study of the evolution of a successful multigene family

The evolution of organismal diversity among the Metazoa is dependent on the proliferation of genes and diversification of functions in multigene families. Here we analyse these processes for one highly successful family, the carboxyl/cholinesterases. One key to the expansion of the functional niche of this group of enzymes is associated with versatile substrate binding and catalytic machinery. Qualitatively new functions can be obtained by substitution of one or a very few amino acids. This crudely adapted new functionality is then refined rapidly by a pulse of change elsewhere in the molecule; in one case about 13% amino acid divergence occurred in 5-10 million years. Furthermore, we postulate that the versatility of the substrate binding motifs underpins the recruitment of several family members to additional noncatalytic signal transduction functions.

Nonspecific interactions alter lipopolysaccharide patterns and protein mobility on sodium dodecyl sulfate polyacrylamide gels

In testing whether bacterial lipopolysaccharide (LPS) was a natural substrate for an esterase from the soil amebae Dictyostelium discoideum, we observed altered banding patterns of the LPS and changed protein mobility on sodium dodecyl sulfate (SDS) polyacrylamide gels after incubation of LPS with the enzyme. The initial interpretation of these results was that the enzyme had removed ester-linked acyl chains from the LPS, leading to a change in its migration on gels. However, esterase inactivated by treatment with either dithiothreitol (DTT), heat, or SDS generated the same mobility shifts. Bovine serum albumin (BSA) also induced the same change in the electrophoretic pattern. We conclude that the altered LPS patterns and protein mobility on SDS gels were caused by nonspecific interactions between LPS and protein.

Cytoskeletal association of an esterase in Dictyostelium discoideum

A 70-kDa glycoprotein, gp70, was found enriched in the detergent-insoluble cytoskeletal fraction of axenically grown Dictyostelium discoideum cells. Its N-terminal amino acid sequence identified it as 'crystal protein' (L. Bomblies et al., 1990, J. Cell Biol. 110, 669-679). This finding was corroborated when antibody to crystal protein cross-reacted with gp70 and its deglycosylated form. The postulated esterase activity of gp70/crystal protein was verified through comparative enzyme assays of extracts derived from cells that either overexpressed or lacked gp70. Gp70 cosedimented with cytoskeletons on sucrose gradients, suggesting an interaction with the cytoskeleton. Coisolation of gp70 with detergent-extracted cells, observed by immunofluorescence microscopy, also implied a gp70-cytoskeletal association. These data supported the idea that the localization or secretion of gp70, or both, was cytoskeletally mediated. Although axenically grown cells contained high levels of gp70, the same cell lines had reduced levels of gp70 when grown in bacterial suspension or in nutrient media containing bacteria. Bacterially grown cells, compared to axenically grown cells, had lower fluid-phase uptake rates even when nutrient media was present, indicating that phagocytosis was a preferred mode of feeding. Thus, bacteria inhibited gp70 expression, which suggested a role for prestarvation factor, in regulating its synthesis.

Targeted gene disruption reveals a role for vacuolin B in the late endocytic pathway and exocytosis

Cells of Dictyostelium discoideum take up fluid by macropinocytosis. The contents of macropinosomes are acidified and digested by lysosomal enzymes. Thereafter, an endocytic marker progresses in an F-actin dependent mechanism from the acidic lysosomal phase to a neutral post-lysosomal phase. From the post-lysosomal compartment indigestible remnants are released by exocytosis. This compartment is characterised by two isoforms of vacuolin, A and B, which are encoded by different genes. Fusions of the vacuolin isoforms to the green fluorescent protein associate with the cytoplasmic side of post-lysosomal vacuoles in vivo. Vacuolin isoforms also localise to patches at the plasma membrane. Since vacuolins have no homologies to known proteins and do not contain domains of obvious function, we investigated their role by knocking out the genes separately. Although the sequences of vacuolins A and B are about 80% identical, only deletion of the vacuolin B gene results in a defect in the endocytic pathway; the vacuolin A knock-out appeared to be phenotypically normal. In vacuolin B- mutants endocytosis is normal, but the progression of fluid-phase marker from acidic to neutral pH is impaired. Furthermore, in the mutants post-lysosomal vacuoles are dramatically increased in size and accumulate endocytic marker, suggesting a role for vacuolin B in targeting the vacuole for exocytosis.

A novel fusidic acid resistance gene from Streptomyces lividans 66 encodes a highly specific esterase

Resistance to fusidic acid in Streptomyces lividans is due to secretion of an extracellular enzyme (FusH) that converts the steroid antibiotic into an inactive derivative. NH2-terminal and several internal amino acid sequences were prepared from the purified enzyme. Using one of the deduced oligonucleotides to probe a subgenomic DNA library, the fusH gene was cloned and sequenced. Sequence analysis located an ORF which, owing to the presence of two putative start codons, indicates a predicted protein with 520 or 509 amino acids. A signal peptide was identified by aligning the deduced amino acids with the N-terminal sequence determined for the mature enzyme. The C-terminal part of the deduced FusH contains a region of three tandemly repeated stretches of 50 amino acids, which is preceded and followed by amino acids showing high homology with the repeats. FusH was found to share a GDS motif with some deduced esterases. S. lividans transformants carrying fusH on a multicopy vector synthesized high levels of FusH. Purified FusH cleaved equally well an acetyl, a thioacetyl or a formyl group from the 16 beta-position of fusidic acid and its derivatives. However, a propionyl group at the 16 beta-position was attacked with difficulty and a 16 alpha-acetyl group was not hydrolysed at all. These data indicate that FusH is a highly specific esterase. The fusH gene is widely distributed among streptomycetes that modify fusidic acid to its inactive lactone derivative.

Directed evolution of a para-nitrobenzyl esterase for aqueous-organic solvents

Through sequential generations of random mutagenesis and screening, we have directed the evolution of an esterase for deprotection of an antibiotic p-nitrobenzyl ester in aqueous-organic solvents. Because rapid screening directly on the desired antibiotic (loracarbef) nucleus p-nitrobenzyl ester was not feasible, the p-nitrophenyl ester was employed. Catalytic performance on the screening substrate was shown to reasonably mimic enzyme activity toward the desired ester. One p-nitrobenzyl esterase variant performs as well in 30% dimethylformamide as the wildtype enzyme in water, reflecting a 16-fold increase in esterase activity. Random pairwise gene recombination of two positive variants led to a further two-fold improvement in activity. Considering also the increased expression level achieved during these experiments, the net result of four sequential generations of random mutagenesis and the one recombination step is a 50-60-fold increase in total activity. Although the contributions of individual effective amino acid substitutions to enhanced activity are small (< 2-fold increases), the accumulation of multiple mutations by directed evolution allows significant improvement of the biocatalyst for reactions on substrates and under conditions not already optimized in nature. The positions of the effective amino acid substitutions have been identified in a pNB esterase structural model developed based on its homology to acetylcholinesterase and triacylglycerol lipase. None appear to interact directly with the antibiotic substrate, further underscoring the difficulty of predicting their effects in a 'rational' design effort.

Structure, expression and gene sequence of a juvenile hormone esterase-related protein from metamorphosing larvae of Trichoplusia ni

A carboxylesterase with an encoded molecular size of 61 kDa and a high sequence similarity to juvenile hormone esterase (JHE) has been cloned from cDNA prepared from final instar larvae of Trichoplusia ni. The absence of a recognizable encoded signal peptide suggests that the enzyme, JHER (for JHE-related) may not be secreted, in contrast to JHE. When the amino acid sequence of JHE, JHER and other esterases were mapped onto the secondary and tertiary structure determined crystallographically for acetylcholinesterase, certain structural features for the substrate binding/catalytic site were identified as common only to JHE and JHER. However, several differences between JHE and JHER were identified in residues at the binding/catalytic site, suggesting that although the two enzymes prefer similar natural substrates, these substrates are not identical. JHER is present as a single-copy gene, transcribed during the feeding stage of the final stage of the final larval stadium, but not after metamorphic commitment to the pupal developmental programme. The gene transcribes a single-size message of 2.0 kb. The genes for JHER and JHE appear to be physically juxtaposed in the T. ni genome. The 5' flanking sequence to the JHER gene possesses some sequences in common with the JHE gene, but is also missing some regulatory elements previously identified in the JHE gene. Sequences conserved between the promoters for the two genes were identified that were different from previously reported regulatory elements of eukaryotic transcription factors.

Calcium-sequestering organelles of Dictyostelium discoideum: changes in element content during early development as measured by electron probe X-ray microanalysis

Starving Dictyostelium discoideum amoebae aggregate within a few hours by chemotaxis towards the attractant cAMP to form a multicellular organism. The differentiating cells possess rapid and efficient calcium buffering and sequestration systems which enable them to restrict changes in the cytosolic free calcium concentration temporally and spatially during their chemotactic reaction and allow the continuous accumulation of Ca2+ during development. In order to identify and to characterize calcium storage compartments, we analyzed the element content of amoebae at three consecutive stages of differentiation. Determination of the element distribution was done using energy-dispersive X-ray microanalysis of freeze-dried cryosections of rapid-frozen cells. Amoebae were frozen in the vegetative and aggregation-competent state and after formation of aggregates. Aggregation-competent as well as aggregated cells contained mass dense granules with large amounts of calcium together with phosphorous and either potassium or magnesium: in aggregation-competent cells calcium was colocalized with potassium, whereas in aggregated cells the mass dense granules contained calcium and magnesium. Although mass dense granules were also present in undifferentiated, vegetative cells, they contained only low amounts of phosphorous and potassium together with little Ca and Mg. We conclude that during their differentiation D. discoideum cells use an intracellular storage compartment to sequester Ca and other cations constantly throughout development.

Hormone-sensitive lipase is closely related to several bacterial proteins, and distantly related to acetylcholinesterase and lipoprotein lipase: identification of a superfamily of esterases and lipases

We have sequenced a gene from Bacillus acidocaldarius which encodes an open reading frame (ORF3) of 310 amino acids. The ORF3 was found to be related to the mammalian hormone-sensitive lipase (HSL). Searching the protein data base revealed five other bacterial proteins related to the HSL. Upon further sequence comparisons this HSL-group was found to be related to the family of carboxylesterases, and to a family of lipases (lipoprotein, hepatic and pancreatic lipases). The evolutionary relationship of these serine-dependent hydrolytic enzymes has not been studied previously, and it has not been known that these proteins belong to the same superfamily. Finally, the alignment of the HSL with the bacterial proteins allowed us to infer the location of the hormone-sensitive regulatory domain of the HSL-protein.

Relationship between sequence conservation and three-dimensional structure in a large family of esterases, lipases, and related proteins

Based on the recently determined X-ray structures of Torpedo californica acetylcholinesterase and Geotrichum candidum lipase and on their three-dimensional superposition, an improved alignment of a collection of 32 related amino acid sequences of other esterases, lipases, and related proteins was obtained. On the basis of this alignment, 24 residues are found to be invariant in 29 sequences of hydrolytic enzymes, and an additional 49 are well conserved. The conservation in the three remaining sequences is somewhat lower. The conserved residues include the active site, disulfide bridges, salt bridges, and residues in the core of the proteins. Most invariant residues are located at the edges of secondary structural elements. A clear structural basis for the preservation of many of these residues can be determined from comparison of the two X-ray structures.

Dictyostelium mutants lacking the cytoskeletal protein coronin are defective in cytokinesis and cell motility

Coronin is an actin-binding protein in Dictyostelium discoideum that is enriched at the leading edge of the cells and in projections of the cell surface called crowns. The polypeptide sequence of coronin is distinguished by its similarities to the beta-subunits of trimeric G proteins (E. L. de Hostos, B. Bradtke, F. Lottspeich, R. Guggenheim, and G. Gerisch, 1991. EMBO (Eur. Mol. Biol. Organ.) J. 10:4097-4104). To elucidate the in vivo function of coronin, null mutants have been generated by gene replacement. The mutant cells lacking coronin grow and migrate more slowly than wild-type cells. When these cor- cells grow in liquid medium they become multinucleate, indicating a role of coronin in cytokinesis. To explore this role, coronin has been localized in mitotic wild-type cells by immunofluorescence labeling. During separation of the daughter cells, coronin is strongly accumulated at their distal portions including the leading edges. This contrasts with the localization of myosin II in the cleavage furrow and suggests that coronin functions independently of the conventional myosin in facilitating cytokinesis.

Analysis of the catalytic mechanism of juvenile hormone esterase by site-directed mutagenesis

1. Juvenile hormone esterase (JHE) is a serine hydrolase selective for hydrolysis of the conjugated methyl esters of insect juvenile hormones. 2. We have investigated the mechanism of catalytic action of this enzyme by site-directed mutagenesis of the cloned enzyme and expression of the mutants in a baculovirus system. 3. A series of individual mutations of JHE were made to residues possibly involved in catalysis of juvenile hormones, and which are highly conserved in both esterases and lipases. 4. Mutation of the serine residue at position 201 to glycine (S201G), or aspartate 173 to asparagine (D173N), or histidine 446 to lysine (H446K), removed all detectable activity and these mutagenized enzymes were determined to be at least 10(6)-fold less active than wild type JHE. 5. Mutation of arginine 47 to histidine (R47H) decreased but did not abolish activity, with Km essentially unchanged at 66 nM for R47H compared to 34 nM for wild type JHE. 6. The kcat for R47H was decreased from 103 min-1 for wild type JHE to 1.9 min-1. 7. In addition, glutamate residue 332 was altered to glutamine (E332Q) and expressed in an Escherichia coli system. 8. This mutation was also found to remove all detectable activity. 9. From the results presented in this study and by comparison of JHE to other serine esterases and lipases, we predict that JHE possesses a Ser201-His446-Glu332 catalytic triad. 10. In addition, aspartate 173 and arginine 47 are essential for the efficient functioning of JHE.

Characterization and functional expression of a cDNA encoding egasyn (esterase-22): the endoplasmic reticulum-targeting protein of beta-glucuronidase

Egasyn (esterase-22), a member of the nonspecific carboxylesterase multigene family (E.C. 3.1.1.1), is the endoplasmic reticulum (ER)-targeting protein of beta-glucuronidase. We utilized the polymerase chain reaction (PCR) in the eventual isolation of murine egasyn cDNAs. PCR primers were based upon: (1) partial amino acid sequences derived from egasyn peptides and (2) a conserved active site region shared by carboxylesterases. The amino acid sequence deduced from the PCR product matched that obtained from egasyn protein. This product was utilized as a probe to screen a cDNA library. Two cDNAs whose composite sequence encoded an open reading frame of 562 amino acids were isolated. A message size of 1700-2000 bp was revealed by RNA blot hybridization analysis. S1 nuclease protection analyses detected mRNA in liver, kidney, lung, and submandibular gland, but not in spleen, brain, and testes. Genetic mapping confirmed the location of an egasyn cDNA fragment in cluster 1 of the esterase region on chromosome 8. Transfection of COS cells with the 2022-bp cDNA resulted in the expression of esterase activity, which comigrated on native gels with liver esterase-22. The features of the deduced amino acid sequence of the egasyn cDNA are compared with previously characterized carboxylesterases and with other lumenal ER proteins.

Substrate-binding sites in acetylcholinesterase

Acetylcholinesterase is among the most efficient enzymes known. In order to provide an explanation for its catalytic and regulatory mechanisms, including the high turnover rate, the specific amino acid residues involved in substrate binding and hydrolysis need to be identified. In this article, Ferdinand Hucho, Jaak Järv and Christoph Weise describe the topography of the enzyme as deduced from protein chemistry studies. One result of this approach is the finding that the binding pocket for the substrate's cationic cholinium group appears to be hydrophobic rather than anionic.

Cholinesterase-like domains in enzymes and structural proteins: functional and evolutionary relationships and identification of a catalytically essential aspartic acid

Primary sequences of cholinesterases and related proteins have been systematically compared. The cholinesterase-like domain of these proteins, about 500 amino acids, may fulfill a catalytic and a structural function. We identified an aspartic acid residue that is conserved among esterases and lipases (Asp-397 in Torpedo acetylcholinesterase) but that had not been considered to be involved in the catalytic mechanism. Site-directed mutagenesis demonstrated that this residue is necessary for activity. Analysis of evolutionary relationships shows that the noncatalytic members of the family do not constitute a separate subgroup, suggesting that loss of catalytic activity occurred independently on several occasions, probably from bifunctional molecules. Cholinesterases may thus be involved in cell-cell interactions in addition to the hydrolysis of acetylcholine. This would explain their specific expression in well-defined territories during embryogenesis before the formation of cholinergic synapses and their presence in noncholinergic tissues.

Anionic subsites of the catalytic center of acetylcholinesterase from Torpedo and from cobra venom

A peptide of acetylcholinesterase (AcChoEase; acetylcholine acetylhydrolase, EC 3.1.1.7) from the venom of the cobra Naja naja oxiana labeled by the affinity reagent N,N-dimethyl-2-phenylaziridinium (DPA) has been identified. The sequence is Gly-Ala-Glu-Met-Trp-Asn-Pro-Asn. In AcChoEase from Torpedo californica, a homologous peptide was labeled and isolated. Its sequence is Ser-Gly-Ser-Glu-Met-Trp-Asn-Pro-Asn, representing positions 79 through 87. In both cases labeling can be prevented by 0.1 mM edrophonium, indicating that the respective peptides form part of the anionic subsite of the catalytic center. The modified residue was tryptophan (Trp-84 in Torpedo AcChoEase) in both enzymes. In contrast to AcChoEase from Torpedo, the enzyme from cobra venom does not contain a peripheral anionic binding site.

Mutagenesis of essential functional residues in acetylcholinesterase

The cholinesterases are serine hydrolases that show no global similarities in sequence with either the trypsin or the subtilisin family of serine proteases. The cholinesterase superfamily includes several esterases with distinct functions and other proteins devoid of the catalytic serine and known esterase activity. To identify the residues involved in catalysis and conferring specificity on the enzyme, we have expressed wild-type Torpedo acetylcholinesterase (EC 3.1.1.7) and several site-directed mutants in a heterologous system. Mutation of serine-200 to cysteine results in diminished activity, while its mutation to valine abolishes detectable activity. Two conserved histidines can be identified at positions 425 and 440 in the cholinesterase family; glutamine replacement at position 440 eliminates activity whereas the mutation at 425 reduces activity only slightly. The assignment of the catalytic histidine to position 440 defines a rank ordering of catalytic residues in cholinesterases distinct from trypsin and subtilisin and suggests a convergence of a catalytic triad to form a third, distinct family of serine hydrolases. Mutation of glutamate-199 to glutamine yields an enzyme with a higher Km and without the substrate-inhibition behavior characteristic of acetylcholinesterase. Hence, modification of the acidic amino acid adjacent to the serine influences substrate association and the capacity of a second substrate molecule to affect catalysis.