The acid phosphatase positive organelles of the Giardia lamblia trophozoite contain a membrane bound cathepsin C activity

We found a dipeptidyl aminopeptidase activity in the parasitic protozoan Giardia lamblia with properties similar to the lysosomal cathepsin C of rat-liver lysosomes. Subcellular fractionation of this parasite indicated that the cathepsin C activity is located in organelles not distinguishable from the ones containing acid phosphatase, a known marker enzyme of Giardia lysosome-like peripheral vesicles. Contrary to the rat lysosomal enzyme, Giardia cathepsin C behaved like a membrane protein. Moreover, the enzyme was not solubilized by Triton X-100 or Triton X-100/SDS at 0 degrees C but could be substantially solubilized by octylglucoside, Triton X-100 at 37 degrees C or by a pretreatment with the cholesterol complexing agent beta-cyclodextrin before the Triton/SDS treatment carried out at 0 degrees C. These observations suggest that binding/anchorage of this enzyme to membranes occurs in cholesterol-rich microdomains.     

Acetyl coenzyme A: alpha-glucosaminide N-acetyltransferase. Evidence for a transmembrane acetylation mechanism

The lysosomal membrane enzyme acetyl-CoA: alpha-glucosaminide N-acetyltransferase catalyzes the transfer of an acetyl group from acetyl-CoA to terminal alpha-linked glucosamine residues of heparan sulfate. The reaction mechanism was examined using highly purified lysosomal membranes from rat liver. The reaction was followed by measuring the acetylation of a monosaccharide acetyl acceptor, glucosamine. The enzyme reaction was optimal above pH 5.5, and a 2-3-fold stimulation of activity was observed when the membranes were assayed in the presence of 0.1% taurodeoxycholate. Double reciprocal analysis and product inhibition studies indicated that the enzyme works by a Di-Iso Ping Pong Bi Bi mechanism. Further evidence to support this mechanism was provided by characterization of the enzyme half-reactions. Membranes incubated with acetyl-CoA and [3H]CoA were found to produce acetyl-[3H]CoA. This exchange was optimal at pH values above 7.0. Treating membranes with [3H] acetyl-CoA resulted in the formation of an acetyl-enzyme intermediate. The acetyl group could then be transferred to glucosamine, forming [3H]N-acetylglucosamine. The transfer of the acetyl group from the enzyme to glucosamine was optimal between pH 4 and 5. The results suggest that acetyl-CoA does not cross the lysosomal membrane. Instead, the enzyme is acetylated on the cytoplasmic side of the lysosome and the acetyl group is then transferred to the inside where it is used to acetylate heparan sulfate.     

The plasma membrane ganglioside sialidase cofractionates with markers of lipid rafts

Gangliosides of the plasma membrane are important modulators of cellular functions. Recent reports have shown their enrichment in glycosphingolipid-containing membrane microdomains, called glycosphingolipid-signaling domain or rafts, which can be isolated due to their insolubility in Triton X-100 and flotation through a sucrose gradient. In previous work on neuroblastoma cells we had found that a ganglioside-specific sialidase activity of the plasma membrane controlled proliferation and differentiation through selective ganglioside desialylation. Assuming the ganglioside sialidase to be close to its substrates in the membrane, we investigated its association with detergent-insoluble microdomains in the neuroblastoma cell line SK-N-MC. The results show that the ganglioside sialidase codistributes with the raft markers ganglioside GM1, flotillin, src family kinases, and glycosylphosphatidylinositol-anchored proteins in a fraction containing about 2% of cellular protein. The association of the ganglioside sialidase with glycosphingolipid-enriched membrane fractions therefore is in support of a role of this glycosidase in ganglioside-dependent signaling processes.     

Isolation of the major chondroitin sulfate/dermatan sulfate and heparan sulfate proteoglycans from embryonic chicken retina

A technique is presented for the preparation of three major proteoglycans from 14-day embryonic chicken retinas following their culture overnight with [35S]sulfate and either [3H]glucosamine or [3H]serine. Homogenization of the tissue in saline permitted extraction of heterogeneous soluble proteoglycans separately from most of the heparan sulfate proteoglycans. The latter were extracted from the 140,000g pellet with 0.5% Triton X-100 in 8 M urea. The medium plus the saline and urea-detergent extracts were separated from low-molecular-weight contaminants, and fractionated into two peaks of radioactivity on Sephacryl S-300 in saline with 3 M urea and 0.5% Triton X-100. The proteoglycans were isolated directly from these fractions on DEAE-Sephacel, and subjected to ultrafiltration concentration and then further purification on cesium chloride density gradient centrifugation in 4 M guanidine hydrochloride. A further step involving cetylpyridinium chloride precipitation was examined, but it resulted in essentially no further purification. The fractionations separated a large chondroitin sulfate/dermatan sulfate proteoglycan from the culture medium that was excluded from S-300 and of low buoyant density; a large heparan sulfate proteoglycan from the urea-detergent extract that was also excluded from S-300 and of low buoyant density; and two smaller and possibly related heparan sulfate proteoglycans. One was found in the medium and showed low to intermediate buoyant density; the other was isolated from the urea-detergent extract and showed a significantly higher buoyant density, associated with a lower protein content. The saline extract contained both of the two larger proteoglycans and only minor amounts of the smaller molecules.     

Enzyme systems involved in phosphorylation and dephosphorylation of two endogenous phosphoproteins in neuronal membranes.

Adenosine 3′:5′-monophosphate-dependent protein kinase and phosphoprotein phosphatases were solubilized by Triton X-100, from a particulate fraction of bovine cerebral cortex enriched in synaptic membranes, and partially purified. The properties of these partially purified enzymes were studied using two substrates, Protein I and Protein II, prepared from the synaptic membrane fraction, as well as the substrates protamine and histone. The results suggest that the phosphorylation of Protein I and Protein II, as well as protamine and histone, are catalyzed by a single species of cAMP-deperident protein kinase. Thus, a single peak of protein kinase activity was observed, upon DEAE-cellulose hromatography of the Triton X-100 extract of the synaptic membrane preparation, which catalyzed the phosphorylation of all four substrate proteins. Moreover, the activity of this partially purified protein kinase toward the various substrate proteins was altered in a parallel fashion, either when the protein kinase preparation was subjected to heat inactivation or pH inactivation, or when the enzyme was assayed in the presence of various concentrations of cyclic nucleotides or of a protein kinase modulator. The individual protein substrates acted as competitive inhibitors with respect to one another. Upon sucrose density gradient centrifugation, the protein kinase activity toward the various substrates sedimented as a single peak. Finally, the relative specific activities toward the various substrates did not change significantly during a 2000-fold purification of the enzyme. In contrast to these observations with protein kinase, two peaks of protein phosphatase activity, with markedly different specificities toward Protein I and Protein II, were found upon DEAE-cellulose and Bio-Gel P-200 column chromatography of the Triton X-100 extract of the synaptic membrane fractions. One peak catalyzed the dephosphorylation of Phosphoprotein I but not of Phosphoprotein II, whereas the other peak catalyzed the dephosphorylation of Phosphoprotein II but not of Phosphoprotein I. The dephosphorylation of Phosphoprotein I by Phosphoprotein I phosphatase was not affected by adenosine 3':5'-monophosphate, whereas the dephosphorylation of Phosphoprotein II by Phosphoprotein II phosphatase required the presence of this nucleotide. Moreover, the two phosphatases differed from one another with respect to Stokes' radius as well as sedimentation coefficient.     

Extraction and partial purification of mouse uterine alkaline phosphatase.

Alkaline phosphatase in uterine homogenates from day 7 pregnant mice was solubilized using 0.2% (v/v) Triton X-100 and extracted wtih 20% (v/v) n-butanol. The procedure, which resulted in 182-fold purification, included ammonium sulfate precipitation, DEAE-cellulose anion exchange chromatography and Sephadex G200 gel filtration. Solubilization with Triton X-100 was an important step in the procedure since extraction with n-butanol alone only partially solubilized the enzyme and gave low extraction yields, much of the enzyme activity remaining in association with negatively charged residues. However, butanol extraction of Triton X-100-treated homogenates gave high yields of enzyme and eliminated p-nitrophenyl phosphatases which displayed activity in the pH range 3.0--7.5, together with a large proportion of inactive protein. The activity of the purified enzyme preparations was electrophoretically homogeneous on cellulose acetate membranes, suggesting that the alkaline phosphatase in the mouse uterus exists in a single isozymic form. Polyacrylamide-gel electrophoresis revealed that the purified preparations contained at least one protein as an impurity. Attempts to further purify the alkaline phosphatase by isoelectric focusing were unsuccessful since the enzyme was found to have an isoelectric point of about 5.0 and at this pH it was rapidly inactivated.     

Acetyl-coenzyme A:alpha-glucosaminide N-acetyltransferase. Evidence for an active site histidine residue

The lysosomal membrane enzyme acetyl-CoA:alpha-glucosaminide N-acetyltransferase catalyzes the transfer of the acetyl group from acetyl-CoA to terminal alpha-linked glucosamine residues of heparan sulfate. The reaction appears to be a transmembrane process: the enzyme is acetylated on the outside of the lysosome, and the acetyl group is transferred across the membrane to the inside of the lysosome where it is used to acetylate glucosamine. To determine the reactive site residues involved in the acetylation reaction, lysosomal membranes were treated with various amino acid modification reagents and assayed for enzyme activity. Although four thiol modification reagents were examined, only one, p-chloromercuribenzoate inactivated the N-acetyltransferase. Thiol modification by p-chloromercuribenzoate did not appear to occur at the active site since inactivation was still observed in the presence of the substrate acetyl-CoA. N-Acetyltransferase could be inactivated by N-bromosuccinimide, even after pretreatment with reagents specific for tyrosine and tryptophan, suggesting that the modified residue is a histidine. Diethyl pyrocarbonate, another histidine modification reagent, could also inactivate the enzyme; this inactivation could be reversed by incubation with hydroxylamine. N-Bromosuccinimide and diethyl pyrocarbonate modifications appear to be at the active site of the enzyme since co-incubation with acetyl-CoA protects the N-acetyltransferase from inactivation. This protection is lost if glucosamine is also present. Pre-acetylated lysosomal membranes are also able to provide protection from N-bromosuccinimide inactivation, providing further evidence for a histidine moiety at the active site and for the existence of an acetyl-enzyme intermediate.     

Sphingolipid and cholesterol dependence of alphavirus membrane fusion. Lack of correlation with lipid raft formation in target liposomes

Semliki Forest virus (SFV) and Sindbis virus (SIN) are enveloped viruses that infect their host cells by receptor-mediated endocytosis and subsequent fusion from within acidic endosomes. Fusion of the viral envelope requires the presence of both cholesterol and sphingolipids in the target membrane. This is suggestive of a possible involvement of sphingolipid-cholesterol microdomains, or "lipid rafts," in the membrane fusion and cell entry process of the virus. In this study, large unilamellar vesicles (LUVs) were prepared from synthetic sphingolipids and sterols that vary with respect to their capacity to promote microdomain formation, as assessed by gradient flotation analysis in the presence of Triton X-100. SFV and SIN fused with LUVs irrespective of the presence or absence of Triton X-100-insoluble microdomains. These results suggest that SFV and SIN do not require the presence of lipid rafts for fusion with target membranes. Furthermore, it is not necessary for sphingolipids to reside in a detergent-insoluble complex with cholesterol to promote SFV or SIN fusion.     

Isolation of a synaptic junction-enriched fraction from the forebrain of day-old chickens

Synaptosomal plasma membrane (SPM) and other subcellular fractions were isolated from the forebrain of 1-day-old chickens by a procedure based on that of Davis and Bloom (16) and Cotman and Taylor (13). The procedure involves the centrifugation through a discontinuous sucrose gradient of a crude synaptosomal-mitochondrial fraction which has been lysed and weighted with iodonitrotetrazolium. SPM isolated by this method contains only small amounts of lysosomal or mitochondrial membranes and is practically devoid of contaminating microsomal membranes, as estimated by enzyme marker assays. The purity of chick-brain SPM prepared by this method is compared to the purity of chickbrain fractions obtained by two other laboratories, using different methods (4, 59). The SPM were extracted with Triton X-100 and all fractions solubilized in sodium dodecyl sulfate (SDS). The delipidated proteins of all fractions were subjected to SDS-polyacrylamide electrophoresis on slab gels and stained for protein. A distinct difference was observed between the patterns given by the Triton-soluble and-insoluble fractions. Electron microscopy of the synaptic junction fraction showed numerous junctional complexes.     

Distinct Lipid Rafts in Subdomains from Human Placental Apical Syncytiotrophoblast Membranes

We report on the characteristics of raft domains in the apical membrane from human placental syncytiotrophoblast (hSTB), an epithelium responsible for maternal-fetal exchange. Previously, we described two isolated fractions of the hSTB apical membrane: a classical microvillous membrane (MVM) and a light microvillous membrane (LMVM). Detergent-resistant microdomains (DRMs) from MVM and LMVM were prepared with Triton X-100 followed by flotation in a sucrose gradient and tested by Western and dot blot with raft markers (placental alkaline phosphatase, lipid ganglioside, annexin 2) and transferrin receptor as a nonraft marker. DRMs from both fractions showed a consistent peak for these markers, except that the DRMs from MVM had no annexin 2 mark. Cholesterol depletion modified the segregation in both groups of DRMs. Our results show two distinguishable lipid raft subsets from MVM and LMVM. Additionally, we found significant differences between MVM and LMVM in cholesterol content and in expression of cytoskeletal proteins. MVM is enriched in ezrin and beta-actin; in contrast, cholesterol and cytokeratin-7 are more abundant in LMVM. These differences may explain the distinct properties of the lipid raft subtypes.     

Release and activation of a particulate bound acid phosphatase from Tetrahymena pyriformis

A sedimentable form of acid phosphatase (EC 3.1.3.2) from Tetrahymena pyriformis was found to be solubilized by Triton X-100. The total enzyme activity in the insoluble cell fraction increased almost 200% upon solubilization with Triton X-100 or Nonidet P-40.Removal of membrane lipids and Triton X-100 from the particulate wash solution with a chloroform extraction resulted in non-specific enzyme protein aggregation which was reversible upon addition of Triton X-100. The results indicate that this acid phosphatase is an integral membrane protein.The pH optima for this particulate bound acid phosphatase was 3.5 with o-carboxyphenyl phosphate and 4.0 with p-nitrophenyl phosphate as substrates. The K_m values of each substrate were 3.1 and 0.031 mM, respectively.     

ErbB-4 and TNF-alpha converting enzyme localization to membrane microdomains

Sequential proteolytic processing of ErbB-4 occurs in response to ligand addition. Here, we assess the localization of cleavable and non-cleavable ErbB-4 isoforms to membrane microdomains using three methodologies: (1) Triton X-100-insolubility, (2) Brij98-insolubility, and (3) detergent-free density gradient centrifugation. Whereas ErbB-4 translocated to a Triton X-100-insoluble fraction upon treatment of T47D cells with heregulin, it constitutively associated with a Brij98-insoluble fraction and a lipid raft fraction isolated using detergent-free methodology. Comparison of cleavable and non-cleavable isoforms of ErbB-4 revealed that both ErbB-4 isoforms are constitutively localized to either a Triton X-100-soluble or Brij98-insoluble fraction. In contrast, addition of heregulin resulted in translocation of the cleavable isoform to a detergent-free lipid raft. Tumor necrosis factor-alpha converting enzyme (TACE), the ectodomain secretase for ErbB-4, was present predominantly in its mature active form in most microdomains analyzed. These data suggest the assembly of ErbB-4 ectodomain cleavage apparatus in a membrane microdomain.     

Acid Sphingomyelinase of Human Brain: Purification to Homogeneity

Abstract: Acid sphingomyelinase (sphingomyelin phosphodiesterase, EC 3.1.4.12) was purified from human brain by extraction with 0.1% Triton X-100, followed by sequential chromatography on Concanavalin A-Sepharose, octyl-Sepharose, hydroxylapatite, DEAE-cellulose, red A-agarose, Sephadex G-200, and DEAE-cellulose with ampholyte elution. Sphingomyelinase activity was purified more than 20,000-fold from the starting homogenate with a 1% yield. Specific activity of up to 800 μmol/h/mg protein could be achieved. Gel electrophoresis with 6% polyacrylamide containing sodium dodecyl sulfate gave a single protein band with a molecular weight of 70,000, in good agreement with the molecular weight previously estimated from sucrose density gradient centrifugation in 0.1% Triton X-100. Triton X-100 could be readily removed from the enzyme by sucrose density gradient centrifugation. The Triton-free enzyme showed the same K _m and pH optimum. Heat stability of the enzyme was reversibly affected by Triton X-100, in that removal of the detergent made the enzyme more heat labile. The K _m of purified enzyme for sphingomyelin was 36 μ M . It was unaffected by sulfhydryl reagents, but was inhibited by dithiothreitol at high concentrations. The preparation was free of all lysosomal hydrolase activities tested, including galactosylceramidase and α-mannosidase, which tended to copurify in our previous procedure. The enzyme was inactive toward sphingosylphosphorylcholine. It was active with bis[ p -nitrophenyll- and bis[4-methylumbelliferyl]phosphate and the chromogenic and fluorogenic sphingomyelin analogues.     

Heparan sulphate-degrading endoglycosidase in liver plasma membranes.

An endoglycosidase is described in isolated liver plasma membranes that brings about a rapid and selective degradation of membrane-associated heparan sulphate, pre-labelled biosynthetically with Na2(35)SO4. The enzyme attacked mainly the polysaccharide chains of a hydrophobic membrane proteoglycan and it had little effect on a proteoglycan that could be displaced from the membranes with 1.0 M-NaCl. The highest activity was measured in the pH range 7.5-8.0, and the enzyme was almost completely inhibited below pH 5.5. Breakdown of susceptible polysaccharide chains was fast, being complete in 20-30 min. The major oligosaccharide fraction (Mr approx. 6000) produced by the enzyme was considerably smaller than the intact heparan sulphate chains. Enzyme activity was retained in membranes solubilized in 1% (v/v) Triton X-100. The high pH optimum and plasma-membrane association distinguish this enzyme from other heparan sulphate-degrading endoglycosidases that have acid pH optima and may be of lysosomal origin. A plasma-membrane endoglycosidase could modulate cellular interactions mediated by heparan sulphate, and/or release biologically active fragments of the polysaccharide from the cell periphery.     

Effect of cations on structure-linked sedimentability of lysosomal hydrolases

1. A partially purified lysosomal preparation was obtained from mouse liver sucrose homogenates by differential and discontinuous gradient centrifugation. 2. Triton X-100 or repeated freezing and thawing of the lysosomal suspension (subfraction B) allowed comparison of free and activated values for acid phosphohydrolase, beta-glucuronidase and N-acetylglucosaminidase in the presence and absence of ascorbate. 3. The distribution of hydrolase activities between supernatant and pellet after high-speed centrifugation was measured and the percentages of total enzyme found in the supernatant were: acid phosphohydrolase, 40.7; beta-glucuronidase, 51; N-acetylglucosaminidase, 39.4. 4. Differential rates of elution of the three hydrolases from the membrane fraction occurred with increasing Na(+) and K(+) concentrations, whereas complex biphasic elution curves were obtained as a function of bivalent cation concentration with Ca(2+) and Mg(2+). 5. Sucrose-density-gradient centrifugation of frozen-and-thawed subfraction B demonstrated highly significant changes in the protein gradient profile in the presence of a low concentration of bivalent cation, indicating membrane aggregation and enzyme-membrane association. 6. The data provide further evidence for the nature of lysosomal enzyme binding and indicate the presence of different enzyme-membrane bonds conferring structure-linked latency upon individual lysosomal enzymes.     

Isoelectric-focusing behavior of acid hydrolases in rat kidney lysosomes. Effects of the pH gradient, autolysis and neuraminidase.

Isoelectric focusing was used to study the multiple forms of acid phosphatase, arylsulfatase, beta-glucuronidase and beta-N-acetylhexosaminidase in lysosomes isolated from rat kidney. The isoelectric points of the main protein and hydrolase peaks were 1-1.5 units lower when electrofocusing was done in a pH 3-10 gradient than in a pH 10-3 gradient, apparently because the lysosomal constituents aggregated strongly at their isoelectric points and tended to settle somewhat in the gradient due to gravity. In the extended pH gradient the acidic form of each hydrolase occurred as asingle, relatively discrete peak. However, when pooled acidic fractions were refocused in a restricted pH gradient (pH 6-3 or 3-5) multiple acidic enzyme and protein components were resolved with isoelectric points between 2.7 and 5.1. When autolysis was minimized by extracting lysosomal fractions at alkaline pH (0.2% Triton X-100, 0.1%p-nitrophenyloxamic acid, 0.1 M glycine buffer, pH9) and including 0.1%p-NITROPHENYLOXAMIC ACID, AN INHIBITOR OF LYSOSOMAL NEURAMINIDASE AND CATHEPSIN D, in the pH gradient, arylsulfatase, beta-glucuronidase and beta-N-acetylhexosaminidase occurred in two forms, an acidic form with an isoelectric point of about 4.4, and a basic form with an isoelectric point close to 6.2, 6.7 and 8.0, respectively. Acid phosphatase occurred in three forms with isoelectric points of 4.1, 5.6 and 7.4. When some autolytic digestion was permitted by extracting lysosomal fractions in an acidic medium (0.2% Triton X-100, 0.1 M sodium acetate buffer, pH 5.2) AT 0-4DEGREES C and omitting p-nitrophenyloxamic acid from the gradient, the acidic form of beta-glucuronidase and the intermediate form of acid phosphatase were lost, the isoelectric points of the acidic forms of acid phosphatase, arylsulfatase and beta-N-acetylhexosaminidase were increased 0.6-1.2 units, and the isoelectric point of the basic forms of acid phosphatase, arylsulfatase and beta-glucuronidase was increased 0.5 unit. When lysosomal extracts were incubated with bacterial neuraminidase before electrofocusing, the acidic forms of acid phosphatase, arylsulfatase and beta-glucuronidase were largely lost, the isoelectric point of the acidic form of beta-N-acetylhexosaminidase was increased from 4.5 to 6.4, and the isoelectric points of the basic forms of all four hydrolases were increased 0.5-1.5 units. Autoincubation of lysosomal extracts in vitro at pH 5.2 PRODUCED SIMILAR, THOUGH LESS MARKED, effects. cont'd     

A phospholamban protein phosphatase activity associated with cardiac sarcoplasmic reticulum

Canine cardiac sarcoplasmic reticulum vesicles contain intrinsic phospholamban protein phosphatase activity, which is also effective in dephosphorylating phosphorylase a. The phosphatase associated with sarcoplasmic reticulum membranes was solubilized with Triton X-100 and subjected to chromatography on Mono Q HR 5/5 and polylysine-agarose. A single peak of phosphatase activity was eluted from each column and it was coincident for both phospholamban and phosphorylase a, used as substrates. Thermal denaturation of the enzyme resulted in progressive and coincident loss of both phospholamban and phosphorylase a phosphatase activities. Enzymic activity was partially inhibited by protein phosphatase inhibitor 1. Migration of the enzyme during sucrose density gradient ultracentrifugation corresponded to a globular protein with an apparent Mr of 46,000. This enzyme preparation could dephosphorylate both the calcium-calmodulin-dependent as well as the cAMP-dependent sites on phospholamban. Thus, dephosphorylation of phospholamban by this sarcoplasmic reticulum-associated phosphatase may participate in modulating sarcoplasmic reticulum function in cardiac muscle.     

Human immunodeficiency virus type 1 assembly and lipid rafts: Pr55(gag) associates with membrane domains that are largely resistant to Brij98 but sensitive to Triton X-100

The assembly and budding of human immunodeficiency virus type 1 (HIV-1) at the plasma membrane are directed by the viral core protein Pr55(gag). We have analyzed whether Pr55(gag) has intrinsic affinity for sphingolipid- and cholesterol-enriched raft microdomains at the plasma membrane. Pr55(gag) has previously been reported to associate with Triton X-100-resistant rafts, since both intracellular membranes and virus-like Pr55(gag) particles (VLPs) yield buoyant Pr55(gag) complexes upon Triton X-100 extraction at cold temperatures, a phenotype that is usually considered to indicate association of a protein with rafts. However, we show here that the buoyant density of Triton X-100-treated Pr55(gag) complexes cannot be taken as a proof for raft association of Pr55(gag), since lipid analyses of Triton X-100-treated VLPs demonstrated that the detergent readily solubilizes the bulk of membrane lipids from Pr55(gag). However, Pr55(gag) might nevertheless be a raft-associated protein, since confocal fluorescence microscopy indicated that coalescence of GM1-positive rafts at the cell surface led to copatching of membrane-bound Pr55(gag). Furthermore, extraction of intracellular membranes or VLPs with Brij98 yielded buoyant Pr55(gag) complexes of low density. Lipid analyses of Brij98-treated VLPs suggested that a large fraction of the envelope cholesterol and phospholipids was resistant to Brij98. Collectively, these results suggest that Pr55(gag) localizes to membrane microdomains that are largely resistant to Brij98 but sensitive to Triton X-100, and these membrane domains provide the platform for assembly and budding of Pr55(gag) VLPs.     

Immunochemical Analysis of Triton X-100-Insoluble Residues from Micrococcus lysodeikticus Membranes

Triton X-100-insoluble residues from Micrococcus lysodeikticus membranes were analyzed by crossed immunoelectrophoresis after dispersal of the residues in sodium dodecyl sulfate (SDS). Conditions which produce no obvious distortion of the immunoprecipitate profile and which allow qualitative and quantitative analyses of the antigens present in the extracts are described. Two main antigens were detected; these were identified as succinate dehydrogenase (EC 1.3.99.1) and adenosine triphosphatase (EC 3.6.1.3). As determined by peak area estimations, the maximal release of succinate dehydrogenase and of adenosine triphosphatase from Triton X-100-insoluble membrane residues occurred at protein/SDS ratios of about 4.3:1 (0.2% SDS) and 6.8:1 (0.13% SDS), respectively. A comparison of enzyme activities of SDS extracts with those of untreated, control Triton X-100-insoluble membrane residues indicated that both the succinate dehydrogenase and the adenosine triphosphatase antigens were released with a full (or enhanced) catalytic potential at or below concentrations of SDS required to effect maximal solubilization of the enzyme in question. Evidence is also presented to suggest that the more acidic of the two components detected by crossed immunoelectrophoresis for the heterogeneous adenosine triphosphatase antigen is more sensitive to SDS than is the other. Both succinate dehydrogenase and adenosine triphosphatase lost catalytic activity and were denatured at protein/SDS ratios lower than 3.4:1.     

Release and activation of a particulate bound acid phosphatase from Tetrahymena pyriformis.

A sedimentable form of acid phosphatase (EC 3.1.3.2) from Tetrahymena pyriformis was found to be solubilized by Triton X-100. The total enzyme activity in the insoluble cell fraction increased almost 200% upon solubilization with Triton X-100 or Nonidet P-40. Removal of membrane lipids and Triton X-100 from the particulate wash solution with a chloroform extraction resulted in non-specific enzyme-protein aggregation which was reversible upon addition of Triton X-100. The results indicate that this acid phosphatase is an integral membrane protein. The pH optima for this particulate bound acid phosphatase was 3.5 with o-carboxyphenyl phosphate and 4.0 with p-nitrophenyl phosphate as substrates. The Km values of each substrate were 3.1 and 0.031 mM, respectively.