Acetylcholinesterases from Musca domestica and Drosophila melanogaster brain are linked to membranes by a glycophospholipid anchor sensitive to an endogenous phospholipase

Purification and characterization of neuron-specific surface antigen defined by monoclonal antibody BM88

Monoclonal antibody BM88 recognizes a neurospecific surface antigen in the CNS and the PNS. In the present study, the antigen recognized by BM88 was immunopurified from pig brain and shown to be a 22-kDa polypeptide by reducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Under nonreducing conditions a protein of 40 kDa was obtained, a result indicating that the antigen is composed of two polypeptide chains of equal molecular weight linked by disulfide bridges. Gel filtration of the purified antigen in the presence of Emulphogene suggested that it may be either a monomeric or a dimeric protein. However, in the presence of Triton X-100 a monomeric structure was implied. N-Glycanase digestion indicated that the protein is probably not glycosylated. The purified antigen was characterized as an integral membrane protein by hydrophobic chromatography and phase-separation experiments with Triton X-114. The antigen, or at least the antibody binding region of the molecule, is very susceptible to protease attack, as judged by protease digestion experiments on brain membranes. By using very low concentrations of papain combined with short incubation times, the antigen was converted to a 16.3-kDa membrane-associated polypeptide as assessed by immunoblotting. This polypeptide contained the BM88 binding epitope. Soluble BM88 immunoreactive polypeptides were not obtained. Bacillus cereus phospholipase C was also unable to solubilize the antigen from the membrane. Our results suggest that the molecule, possessing at least one small extramembranous domain, is attached to the membrane via a polypeptide chain.

Rapid analysis of glycolipid anchors in amphiphilic dimers of acetylcholinesterases

1. We describe two simple procedures for the rapid identification of certain structural features of glycolipid anchors in acetylcholinesterases (AChEs). 2. Treatment with alkaline hydroxylamine (that cleaves ester-linked acyl chains but not ether-linked alkyl chains) converts molecules possessing a diacylglycerol, but not those with an alkylacylglycerol, into hydrophilic derivatives. AChEs in human and bovine erythrocytes possess an alkylacylglycerol (Roberts et al., J. Biol. Chem. 263:18766-18775, 1988; Biochem. Biophys. Res. Commun. 150:271-277, 1988) and are not converted to hydrophilic dimers by alkaline hydroxylamine. Amphiphilic dimers of AChE from Drosophila, from mouse erythrocytes, and from the human erythroleukaemia cell line K562 also resist the treatment with hydroxylamine and likely possess a terminal alkylacylglycerol. This indicates that the cellular pool of free glycolipids used as precursors of protein anchors is distinct from the pool of membrane phosphatidylinositols (which contain diacylglycerols). 3. Pretreatment with alkaline hydroxylamine is required to render the amphiphilic AChE from human erythrocytes susceptible to digestion by Bacillus thuringiensis phosphatidylinositol-specific phospholipase C (PI-PLC) (Toutant et al., Eur. J. Biochem. 180:503-508, 1989). We show here that this is also the case for the AChE from mouse erythrocytes, which therefore likely possesses an additional acyl chain in the anchor that prevents the action of PI-PLC. 4. In two sublines of K562 cells (48 and 243), we observed that AChE either was directly susceptible to PI-PLC (243) or required a prior deacylation by alkaline hydroxylamine (48). This suggests that glycolipid anchors in AChE of K562-48 cells, but not those in AChE of K562-243 cells, contain the additional acylation demonstrated in AChE from human erythrocytes. These observations illustrate the cell specificity (and the lack of species-specificity) of the structure of glycolipid anchors.

The acetylcholinesterase gene of Anopheles stephensi

1. The acetylcholinesterase (AChE) gene from the important malaria vector Anopheles stephensi has been isolated by homology to the Drosophila acetylcholinesterase gene. 2. The complete sequence and intron-exon organization has been determined. The encoded protein has 69% identity to Drosophila AChE and 38 and 36% identity to Torpedo AChE and human butyrylcholinesterase, respectively.

Amphiphilic, glycophosphatidylinositol-specific phospholipase C (PI-PLC)-insensitive monomers and dimers of acetylcholinesterase

1. In a recent study, we distinguished two classes of amphiphilic AChE3 dimers in Torpedo tissues: class I corresponds to glycolipid-anchored dimers and class II molecules are characterized by their lack of sensitivity to PI-PLC and PI-PLD, relatively small shift in sedimentation with detergent, and absence of aggregation without detergent. 2. In the present report, we analyze the amphiphlic or nonamphiphilic properties of globular AChE forms in T28 murine neural cells, rabbit muscle, and chicken muscle. The molecular forms were identified by sucrose gradient sedimentation in the presence and absence of detergent and analyzed by nondenaturing charge-shift electrophoresis. Some amphiphilic forms showed an abnormal electrophoretic migration in the absence of detergent, because of the retention of detergent micelles. 3. We show that the amphiphilic monomers (G1a) from these tissues, as well as the amphiphilic dimers (G2a) from chicken muscle, resemble the class II dimers of Torpedo AChE. We cannot exclude that these molecules possess a glycolipidic anchor but suggest that their hydrophobic domain may be of a different nature. We discuss their relationship with other cholinesterase molecular forms.

Effect of phospholipase C from Bacillus cereus on the release of membrane-bound choline-O-acetyltransferase from rat hippocampal tissue

Some of the enzyme choline-O-acetyltransferase (ChAT) associated with central cholinergic nerve terminals appears to be non-ionically associated with membranes. In the present study, we tested the possibility that some membrane-bound ChAT might be anchored to membranes by a phosphatidylinositol linkage by incubating rat hippocampal tissue with phospholipase C (PLC) from Bacillus cereus. The PLC selectively augmented the release of ChAT; also, the glycosylphosphatidylinositol-PLC inhibitor, zinc, blocked this increase in release. When control and PLC-treated hippocampal tissues were subjected to Triton X-114 phase separation, a procedure that separates amphiphilic from hydrophilic proteins, the detergent-soluble, membrane-bound fraction of tissue ChAT appeared to be the source of the ChAT released by PLC into the incubation medium. Zinc also blocked the temperature-dependent release of ChAT, but not lactic dehydrogenase, from hippocampal tissue. Extracellular membrane-bound ChAT appeared to be the source of the ChAT released by a low exogenous concentration of PLC, as well as that released by a temperature-dependent process during tissue incubation. Phosphatidylinositol-specific PLC from Bacillus thuringiensis released ChAT, but not lactic dehydrogenase, from a crude synaptosomal fraction prepared from rat hippocampal tissue. These results suggest that some of the membrane-bound ChAT in rat hippocampal tissue may be extracellular and anchored to the membrane by phosphatidylinositol, and also that an endogenous factor in hippocampal tissue may function to remove this extracellular ChAT from the membrane.

Molecular characterization of GP50: a neuron-specific, synaptic-enriched glycoprotein

The molecular properties of the neuron-specific, synaptic-enriched glycoprotein GP50 have been investigated with the aid of the monoclonal antibody MabSM-GP50. GP50 immunoreactivity was detected in the brains of the frog, trout, pigeon, snake, rabbit, mouse, cow, and human, although variation in quantity and electrophoretic mobility of the immunoreactive protein between species was apparent. Deglycosylation of synaptic membranes (SMs) with endoglycosidase H, peptide:N-glycosidase F, trifluoromethane-sulfonic acid, and alkaline sodium borohydride indicated that GP50 is associated primarily, if not exclusively, with high-mannose and/or hybrid-type oligosaccharides and lacks complex N-linked and O-linked sugar chains. GP50 remained associated with the membrane fraction following extraction of SMs with alkaline sodium carbonate, was partially (55%) present in the detergent phase following the phase partitioning of SMs in the presence of Triton X-114, and was resistant to proteolytic digestion with trypsin when present as a component of intact membranes. Taken together, these results indicate that GP50 is an integral component of the SM. Sucrose gradient centrifugation of Triton X-100 extracts of SMs or of forebrain and cerebellar homogenates resolved GP50 into two fractions with sedimentation coefficients of 3.6S and 7.3S, which accounted for 45 and 55% of the total, respectively. The 7.3S form occurred exclusively in the aqueous phase following partitioning with Triton X-114, whereas the 3.6S species was found in both the aqueous and detergent phases.(ABSTRACT TRUNCATED AT 250 WORDS)

Drosophila melanogaster acetylcholinesterase gene. Structure, evolution and mutations

Acetylcholinesterase is a key component of cholinergic neurotransmission. In Drosophila melanogaster, acetylcholinesterase is encoded by the Ace locus. We have determined the complete organization of the locus. The transcription unit is 34 kb (1 kb = 10(3) bases) long and encompasses ten exons. We have mapped the 5' end of the transcript, sequenced all the intron/exon boundaries, as well as the 3' end of the transcript. The deduced mature transcript is 4291 nucleotides long without poly(A). Sequencing of the promoter region reveals a potential TATA box and (GA)n motives. The Drosophila coding sequence is more split than its vertebrate counterparts, but the splicing sites of the two last exons are precisely conserved among Drosophila and vertebrate cholinesterases, and intriguingly also with the bovine thyroglobulin gene. Finally, a number of the mutations isolated in earlier genetic work are precisely placed on our molecular map in introns, exons and promoter regions. Among them, for example, a short deletion known to affect acetylcholinesterase level and tissue distribution removes promoter regions and the first non-coding exon.

Ectoenzymes of the kidney microvillar membrane. Isolation and characterization of the amphipathic form of renal dipeptidase and hydrolysis of its glycosyl-phosphatidylinositol anchor by an activity in plasma

Renal dipeptidase (EC 3.4.13.11) has been solubilized from pig kidney microvillar membranes with n-octyl-beta-D-glucopyranoside and then purified by affinity chromatography on cilastatin-Sepharose. The enzyme exists as a disulphide-linked dimer of two identical subunits of Mr 45,000 each. The purified dipeptidase partitioned into the detergent-rich phase upon phase separation in Triton X-114 and reconstituted into liposomes consistent with the presence of the glycosyl-phosphatidylinositol membrane anchor. The N-terminal amino acid sequence of the amphipathic, detergent-solubilized, form of renal dipeptidase was identical with that of the hydrophilic, phospholipase-solubilized, form, locating the membrane anchor at the C-terminus of the protein. The glycosyl-phosphatidylinositol anchor of both purified and microvillar membrane renal dipeptidase was a substrate for an activity in pig plasma which displayed properties similar to those of a previously described phospholipase D. The cross-reacting determinant of the glycosyl-phosphatidylinositol anchor was generated by incubation of purified renal dipeptidase with bacterial phosphatidylinositol-specific phospholipase c, whereas the anchor-degrading activity in plasma failed to generate this determinant.

Cholinesterases from flounder muscle. Purification and characterization of glycosyl-phosphatidylinositol-anchored and collagen-tailed forms differing in substrate specificity

Flounder (Platichthys flesus) muscle contains two types of cholinesterases, that differ in molecular form and in substrate specificity. Both enzymes were purified by affinity chromatography. About 8% of cholinesterase activity could be attributed to collagen-tailed asymmetric acetylcholinesterase sedimenting at 17S, 13S and 9S, which showed catalytic properties of a true acetylcholinesterase. 92% of cholinesterase activity corresponded to an amphiphilic dimeric enzyme sedimenting at 6S in the presence of Triton X-100. Treatment with phospholipase C yielded a hydrophilic form and uncovered an epitope called the cross-reacting determinant, which is found in the hydrophilic form of a number of glycosyl-phosphatidylinositol-anchored proteins. This enzyme showed catalytic properties intermediate to those of acetylcholinesterase and butyrylcholinesterase. It hydrolyzed acetylthiocholine, propionylthiocholine, butyrylthiocholine and benzoylthiocholine. The Km and the maximal velocity decreased with the length and hydrophobicity of the acyl chain. At high substrate concentrations the enzyme was inhibited. The p(IC50) values for BW284C51 and ethopropazine were between those found for acetylcholinesterase and butylcholinesterase. For purified detergent-soluble cholinesterase a specific activity of 8000 IU/mg protein, a turnover number of 2.8 x 10(7) h-1, and 1 active site/subunit were determined.

Conversion of human erythrocyte acetylcholinesterase from an amphiphilic to a hydrophilic form by phosphatidylinositol-specific phospholipase C and serum phospholipase D

Each catalytic subunit in the amphiphilic dimer of human erythrocyte acetylcholinesterase (AChE) is anchored in the plasma membrane exclusively by a glycoinositol phospholipid. In contrast to erythrocyte AChEs in other mammalian species, the human enzyme is resistant to direct cleavage by phosphatidylinositol-specific phospholipase C (PtdIns-specific PLC). The resistance is due to the existence of an additional fatty acyl chain on the inositol ring which blocks the action of PtdIns-specific PLC [Roberts et al. (1988) J. Biol. Chem. 263, 18766-18775]. In this report, nondenaturing polyacrylamide gel electrophoresis was applied to permit rapid and unambiguous distinction between amphiphilic AChE, in which each catalytic subunit binds one nonionic detergent micelle, and hydrophilic AChE, which does not interact with detergent. Deacylation of human erythrocyte AChE by an alkaline treatment with hydroxylamine rendered the amphiphilic AChE susceptible to PtdIns-specific PLC with the consequent release of hydrophilic AChE. Although serum anchor-specific phospholipase D (PLD) cleaves the intact human erythrocyte AChE anchor, this treatment, as judged by nondenaturing electrophoresis, did not release hydrophilic AChE. Hydroxylamine treatment before or after PLD digestion was necessary to achieve the conversion. These observations indicate that binding of a single detergent micelle was maintained when any of the three fatty acyl or alkyl groups in the human erythrocyte AChE anchor phospholipid were retained. For proteins that can be identified following nondenaturing gel electrophoresis, these procedures provide methods both for detecting glycoinositol phospholipid anchors resistant to PtdIns-specific PLC and for indicating fatty acyl and/or alkyl chains in these anchors.

Acetylcholinesterase from Apis mellifera head. Evidence for amphiphilic and hydrophilic forms characterized by Triton X-114 phase separation

The polymorphism of bee acetylcholinesterase was studied by sucrose-gradient-sedimentation analysis and non-denaturing electrophoretic analysis of fresh extracts. Lubrol-containing extracts exhibited only one form, which sedimented at 5 S when analysed on high-salt Lubrol-containing gradients and 6 S when analysed on low-salt Lubrol-containing gradients. The 5 S/6 S form aggregated upon removal of the detergent when sedimented on detergent-free gradients and was recovered in the detergent phase after Triton X-114 phase separation. Thus the 5 S/6 S enzyme corresponds to an amphiphilic acetylcholinesterase form. In detergent-free extracts three forms, whose apparent sedimentation coefficients are 14 S, 11 S and 7 S, were observed when sedimentations were performed on detergent-free gradients. Sedimentation analyses on detergent-containing gradients showed only a 5 S peak in high-salt detergent-free extracts and a 6 S peak, with a shoulder at about 7 S, in low-salt detergent-free extracts. Electrophoretic analysis in the presence of detergent demonstrated that the 14 S and 11 S peaks corresponded to aggregates of the 5 S/6 S form, whereas the 7 S peak corresponded to a hydrophilic acetylcholinesterase form which was recovered in the aqueous phase following Triton X-114 phase separation. The 5 S/6 S amphiphilic form could be converted into a 7.1 S hydrophilic form by phosphatidylinositol-specific phospholipase C digestion.

Acetylcholinesterase from Drosophila melanogaster. Identification of two subunits encoded by the same gene

Purified acetylcholinesterase from Drosophila melanogaster is composed of a 55 kDa and a 16 kDa noncovalently associated subunit. Cleavage of disulfide bonds reveals that two 55 kDa polypeptides are linked together in native dimeric AChE. Western blots with two antibodies directed against the N- and C-termini of the predicted AChE primary sequence show that the 55 and 16 kDa polypeptides originate from proteolysis of the same precursor encoded by the Ace locus.

Amphiphilic and nonamphiphilic forms of Torpedo cholinesterases: II. Electrophoretic variants and phosphatidylinositol phospholipase C-sensitive and -insensitive forms

We report an electrophoretic analysis of the hydrophobic properties of the globular forms of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) from various Torpedo tissues. In charge-shift electrophoresis, the rate of electrophoretic migration of globular amphiphilic forms (Ga) is increased at least twofold when the anionic detergent deoxycholate is added to Triton X-100, whereas that of globular nonamphiphilic forms (Gna) is not modified. The G2a forms of the first class, as defined by their aggregation properties, are converted to nonamphiphilic derivatives by phosphatidylinositol phospholipase C (PI-PLC) and human serum phospholipase D (PLD). AChE G2a forms from electric organs, nerves, skeletal muscle, and erythrocyte membranes correspond to this type, which also exists in very small quantities in detergent-solubilized extracts of electric lobes and spinal cord. They present different electrophoretic mobilities, so that each of these tissues contains a distinct "electromorph," or two in the case of electric organs. The G2a forms of the second class (AChE in plasma, BuChE in heart), as well as G4a forms of AChE and BuChE, are insensitive to PI-PLC and PLD but may be converted to nonamphiphilic derivatives by Pronase.

Drosophila acetylcholinesterase: demonstration of a glycoinositol phospholipid anchor and an endogenous proteolytic cleavage

The presence of a glycoinositol phospholipid anchor in Drosophila acetylcholinesterase (AChE) was shown by several criteria. Chemical analysis of highly purified Drosophila AChE demonstrated approximately one residue of inositol per enzyme subunit. Selective cleavage by Staphylococcus aureus phosphatidylinositol-specific phospholipase C (PI-PLC) was tested with Drosophila AChE radiolabeled by the photoactivatable affinity probe 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine [( 125I]TID), a reagent that specifically labels the lipid moiety of glycoinositol phospholipid-anchored proteins. Digestion with PI-PLC released 75% of this radiolabel from the protein. Gel electrophoresis of Drosophila AChE in sodium dodecyl sulfate indicated prominent 55- and 16-kDa bands and a faint 70-kDa band. The [125I]TID label was localized on the 55-kDa fragment, suggesting that this fragment is the C-terminal portion of the protein. In support of this conclusion, a sensitive microsequencing procedure that involved manual Edman degradation combined with radiomethylation was used to determine residues 2-5 of the 16-kDa fragment. Comparison with the Drosophila AChE cDNA sequence [Hall, L.M.C., & Spierer, P. (1986) EMBO J. 5, 2949-2954] confirmed that the 16-kDa fragment includes the N-terminus of AChE. Furthermore, the position of the N-terminal amino acid of the mature Drosophila AChE is closely homologous to that of Torpedo AChE. The presence of radiomethylatable ethanolamine in both 16- and 55-kDa fragments was also confirmed. Thus, Drosophila AChE may include a second posttranslational modification involving ethanolamine.