Isolation and characterization of velutin, a novel low-molecular-weight ribosome-inactivating protein from winter mushroom (Flammulina velutipes) fruiting bodies

From the fruiting bodies of the edible mushroom Flammulina velutipes a single-chained ribosome inactivating protein with a molecular weight of 13.8 kDa was isolated with a procedure involving ion exchange chromatography on DEAE-cellulose and SP-Sepharose and affinity chromatography on Affi-gel blue gel. The protein was novel in that it possessed a molecular weight lower than those of previously reported RIPs and that it was capable of inhibiting human immunodeficiency virus (HIV-1) reverse transcriptase, beta-glucosidase and beta-glucuronidase. Its N-terminal sequence exhibited a certain degree of similarity to those of plant ribosome inactivating proteins.

Inhibition of beta-glycosidases by acarbose analogues containing cellobiose and lactose structures

Acarbose analogues, containing cellobiose and lactose structures, were prepared by reaction of the two disaccharides with acarbose and Bacillus stearothermophilus maltogenic amylase. The kinetics for the inhibition by the two analogues was studied for beta-glucosidase, beta-galactosidase, cyclomaltodextrin glucanosyltransferase (CGTase), and alpha-glucosidase. Both analogues were potent competitive inhibitors for beta-glucosidase, with K(I) values in the range of 0.04-2.44 microM, and the lactose analogues were good uncompetitive inhibitors for beta-galactosidase, with K(I) values in the range of 159-415 microM, while acarbose was not an inhibitor for either enzyme at 10 and 5 mM, respectively. Both analogues were also potent mixed inhibitors for CGTase, with K(I) values in the range of 0.1-9.3 microM. The lactose analogue was a 6.4-fold better competitive inhibitor for alpha-glucosidase than was acarbose.

Synthesis of 1,2,4-triazino[5,6-b] indoles bearing 1,2,4-triazine moiety

Some new 3-(5,6-Diphenyl-1,2,4-triazin-3-yl)-5-substituted- 1,2,4-triazino[5,6-b]indole derivatives (6-12) have been obtained via treatment of both (4 and 5) with p-nitro-benzoyl chloride, ammonium thiocyante, formaldehyde-methanol, acrylonitrile and thiosemi-carbazide. The former structure of the new products was established by the help of elemental analyses, as well as spectral data. Some of their showed pronounced effect on the Cellobiase produced by Thermomyces lanuginosus and Chaetomium thermophilum.

Beta-glucosidase multiplicity from Aspergillus tubingensis CBS 643.92: purification and characterization of four beta-glucosidases and their differentiation with respect to substrate specificity, glucose inhibition and acid tolerance

From Aspergillus tubingensis CBS 643.92 four distinct beta-glucosidases (I-IV) were purified by a four-step purification procedure. SDS-PAGE revealed molecular masses of 131, 126, 54 and 54 kDa, respectively, and their isoelectric points were determined to be 4.2, 3.9, 3.7 and 3.6, respectively. The beta-glucosidases exhibited high diversity with respect to pH and temperature optima and stability, as well as to substrate specificity and glucose tolerance. The major beta-glucosidase (I) preferentially hydrolysed oligosaccharides. The acid-stable and heat-tolerant beta-glucosidase II hydrolysed aryl and terpenyl beta-D-glucosides as well as 1-O-trans-cinnamoyl beta-D-glucoside. In contrast to beta-glucosidases I and II, the minor beta-glucosidases III and IV were found to be glucose-tolerant; inhibition constants of 470 and 600 mM, respectively, were determined.

Evaluation of biological activity of crude extracts from plants used in Yucatecan traditional medicine part I. Antioxidant, antimicrobial and beta-glucosidase inhibition activities

Bioactivity of extracts from leaves, stems and roots of twelve plants commonly used in Yucatecan traditional medicine were evaluated in four bioassays. Crude extracts from ten plants showed significant activity in the inhibition of bleaching of beta-carotene assay, while thirteen extracts showed activity in the reduction of 2, 2- diphenyl-1-picrylhydrazyl radical (DPPH) assay. In the antimicrobial bioassay, the major activity was presented by the root extract of Jatropha gaumeri and in the beta-glucosidase inhibition activity assay the strongest activity was observed in the stem and root extracts of Solanum hirtum.

Beta-D-glycosylamidines: potent, selective, and easily accessible 1-glycosidase inhibitors

Beta-D-glycosylamidines, in which a glycon is connected via an N-glycoside linkage with a substituted amidine (aglycon), were synthesized in two steps from the corresponding sugars and served as stable and potent beta-glycosidase inhibitors with high selectivity according to the glycon- and alpha, beta-specificities of the enzymes.

Slow inhibition of almond beta-glucosidase by azasugars: determination of activation energies for slow binding

The thermodynamic and activation energies of the slow inhibition of almond beta-glucosidase with a series of azasugars were determined. The inhibitors studied were isofagomine ((3R,4R,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine, 1), isogalactofagomine ((3R,4S,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine, 2), (-)-1-azafagomine ((3R,4R,5R)-4,5-dihydroxy-3-hydroxymethylhexahydropyridazine, 3), 3-amino-3-deoxy-1-azafagomine (4) and 1-deoxynojirimycin (5). It was found that the binding of 1 to the enzyme has an activation enthalpy of 56.1 kJ/mol and an activation entropy of 25.8 J/molK. The dissociation of the enzyme-1 complex had an activation enthalpy of -2.5 kJ/mol and an activation entropy of -297 J/molK. It is suggested that the activation enthalpy of association is due to the breaking of bonds to water, while the large negative activation entropy of dissociation is due at least in part to the resolvation of the enzyme with water molecules. For the association of 1 DeltaH(0) is 58.6 kJ/mol and DeltaS(0) is 323.8 J/molK. Inhibitor 3 has an activation enthalpy of 39.3 kJ/mol and an activation entropy of -17.9 J/molK for binding to the enzyme, and an activation enthalpy of 40.8 kJ/mol and an activation entropy of -141.0 J/molK for dissociation of the enzyme-inhibitor complex. For the association of 3 DeltaH(0) is -1.5 kJ/mol and DeltaS(0) is 123.1 J/molK. Inhibitor 5 is not a slow inhibitor, but its DeltaH(0) and DeltaS(0) of association are -30 kJ/mol and -13.1 J/molK. The large difference in DeltaS(0) of association of the different inhibitors suggests that the anomeric nitrogen atom of inhibitors 1-4 is involved in an interaction that results in a large entropy increase.

Biochemical characterization and mechanism of action of a thermostable beta-glucosidase purified from Thermoascus aurantiacus

An extracellular beta-glucosidase from Thermoascus aurantiacus was purified to homogeneity by DEAE-Sepharose, Ultrogel AcA 44 and Mono-P column chromatography. The enzyme was a homotrimer, with a monomer molecular mass of 120 kDa; only the trimer was optimally active at 80 degrees C and at pH 4.5. At 90 degrees C, the enzyme showed 70% of its optimal activity. It was stable at pH 5.2 and at temperatures up to 70 degrees C for 48 h, but stability decreased above 70 degrees C and at pH values above and below 5.0. The enzyme hydrolysed aryl and alkyl beta-d-glucosides and cello-oligosaccharides, and was specific for substrates with a beta-glycosidic linkage. The hydroxy groups at positions 2, 4 and 6 of a glucose residue at the non-reducing end of a disaccharide appeared to be essential for catalysis. The enzyme had the lowest K(m) towards p-nitrophenyl beta-d-glucoside (0.1137 mM) and the highest k(cat) towards cellobiose and beta,beta-trehalose (17052 min(-1)). It released one glucose unit at a time from the non-reducing end of cello-oligosaccharides, and the rate of hydrolysis decreased with an increase in chain length. Glucose and d-delta-gluconolactone inhibited the beta-glucosidase competitively, with K(i) values of 0.29 mM and 8.3 nM respectively, while methanol, ethanol and propan-2-ol activated the enzyme. The enzyme catalysed the synthesis of methyl, ethyl and propyl beta-d-glucosides in the presence of methanol, ethanol and propan-2-ol respectively with either glucose or cellobiose, although cellobiose was preferred. An acidic pH favoured hydrolysis and transglycosylation, but high concentrations of alcohols favoured the latter reaction. The stereochemistry of cellobiose hydrolysis revealed that beta-glucosidase from T. aurantiacus is a retaining glycosidase, while N-terminal amino acid sequence alignment indicated that it is a member of glycoside hydrolase family 3.

New syntheses of 1D- and 1L-1,2-anhydro-myo-inositol and assessment of their glycosidase inhibitory activities

The 1D and 1L enantiomers of 1,2-anhydro-myo-inositol (conduritol B epoxide) were synthesised from 1D-pinitol and 1L-quebrachitol, respectively, and their activities were compared in selected glycosidase inhibition assays. The 1D enantiomer was found to be the active isomer, functioning as an irreversible inhibitor of sweet almond beta-D-glucosidase. Neither isomer was active against the alpha-D-glucosidase from Bacillus stearothermophilus or the beta-D-galactosidase from Aspergillus oryzae.

First demonstration of an inhibitory activity of milk proteins against human immunodeficiency virus-1 reverse transcriptase and the effect of succinylation

A variety of milk proteins including lactoferrin, angiogenin-1, alpha-lactalbumin, beta-lactoglobulin, lactoperoxidase, casein and the novel whey proteins lactogenin and glycolactin were tested for inhibitory activity toward human immunodeficiency virus-1 reverse transcriptase (HIV-1 RT), alpha-glucosidase, beta-glucosidase and beta-glucuronidase. Lactoferrin exerted the most potent inhibitory action with an IC50 of about 6 microM. Lactoperoxidase, lactogenin, angiogenin-1 and glycolactin inhibited HIV-1 RT activity with decreasing potencies. Beta-lactoglobulin, alpha-lactalbumin and casein displayed little or no inhibitory effect. Succinylation with succinic anhydride augmented the inhibitory effect of glycolactin, beta-lactoglobulin, alpha-lactalbumin, casein and human lactoferrin. The inhibitory effect of the various milk proteins on the activities of alpha-glucosidase, beta-glucosidase and beta-glucuronidase was meager. Succinylation tended to increase the alpha-glucosidase-inhibitory effect of milk proteins but neither their beta-glucosidase-inhibitory nor beta-glucuronidase-inhibitory effect was affected.

Direct NMR-spectroscopic determination of active-enzyme concentration by titration with a labeled inhibitor: determination of the k(cat) value of almond beta-glucosidase

A new method for the determination of active-enzyme concentration of a glucosidase by using (13)C NMR spectroscopy is reported. The method consists of quantifying the binding between a (13)C-labelled, strong competitive inhibitor, [5-(13)C]-1-azafagomine (1), and the enzyme. The concentration of free inhibitor 1 is measured in a series of binding experiments from the intensity of its NMR signal relative to that of a reference. From a plot of the concentrations of bound vs. free inhibitor 1, the amount of specifically bound 1, that is, the amount of active sites, is determined. From this value, active-enzyme concentration and k(cat) value can be calculated.

Detailed structural analysis of glycosidase/inhibitor interactions: complexes of Cex from Cellulomonas fimi with xylobiose-derived aza-sugars

Detailed insights into the mode of binding of a series of tight-binding aza-sugar glycosidase inhibitors of two fundamentally different classes are described through X-ray crystallographic studies of complexes with the retaining family 10 xylanase Cex from Cellulomonas fimi. Complexes with xylobiose-derived aza-sugar inhibitors of the substituted "amidine" class (xylobio-imidazole, K(i) = 150 nM; xylobio-lactam oxime, K(i) = 370 nM) reveal lateral interaction of the "glycosidic" nitrogen with the acid/base catalyst (Glu127) and hydrogen bonding of the sugar 2-hydroxyl with the catalytic nucleophile (Glu233), as expected. Tight binding of xylobio-isofagomine (K(i) = 130 nM) appears to be a consequence of strong interactions of the ring nitrogen with the catalytic nucleophile while, surprisingly, no direct protein contacts are made with the ring nitrogen of the xylobio-deoxynojirimycin analogue (K(i) = 5800 nM). Instead the nitrogen interacts with two ordered water molecules, thereby accounting for its relatively weaker binding, though it still binds some 1200-fold more tightly than does xylobiose, presumably as a consequence of electrostatic interactions at the active site. Dramatically weaker binding of these same inhibitors to the family 11 xylanase Bcx from Bacillus circulans (K(i) from 0.5 to 1.5 mM) is rationalized for the substituted amidines on the basis that this enzyme utilizes a syn protonation trajectory and likely hydrolyzes via a (2,5)B boat transition state. Weaker binding of the deoxynojirimycin and isofagomine analogues likely reflects the energetic penalty for distortion of these analogues to a (2,5)B conformation, possibly coupled with destabilizing interactions with Tyr69, a conserved, catalytically essential active site residue.

Studies on the constituents of Broussonetia species. VII. Four new pyrrolidine alkaloids, broussonetines M, O, P, and Q, as inhibitors of glycosidase, from Broussonetia kazinoki SIEB

Four new pyrrolidine alkaloids, broussonetines M, O, P, and Q, were isolated from the branches of Broussonetia kazinoki SIEB, (Moraceae). Broussonetines M, O, P, and Q were formulated as (2R,3R,4R,5R)-2-hydroxymethyl-3,4-dihydroxy-5-[(10S)-10,13-dihydroxy-tri decyl]pyrrolidine (1), (2R,3R,4R,5R)-2-hydroxymethyl-3,4-dihydroxy-5-[(E)9-oxo-13-hydroxy-3- tridecenyl]pyrrolidine (2), (2R,3R,4R,5R)-2-hydroxymethyl-3,4-dihydroxy-5-[(E)10-oxo-13-hydroxy-3-++ +tridecenyl]pyrrolidine (3), and (2R,3S,4R,5R)-2-hydroxymethyl-3-hydroxy-4-(beta-D-glucopyranosyloxy++ +)-5-[10-oxo-13-(beta-D-glucopyranosyloxy)tridecyl]pyrrolidine (4) respectively, by spectroscopic and chemical methods. 1-4 inhibited beta-glucosidase, beta-galactosidase and beta-mannosidase.

All eight stereoisomeric D-glyconic-delta-lactams: synthesis, conformational analysis, and evaluation as glycosidase inhibitors

An efficient and general synthetic route to all eight stereoisomeric D-glycono-delta-lactams has been developed. The strategy involves, as a key step, a stereodivergent delta-lactam formation with configurational retention or inversion at C-4 of a starting gamma-lactone to lead to two epimers of delta-lactam from one parent gamma-lactone. Conformations of eight glycono-delta-lactams were examined by X-ray crystallographic analysis and molecular modeling. Analyses of conformation and glycosidase-inhibition provide useful information for the design of new glycosidase inhibitors.

Development of beta-1,3-glucanase activity in germinated tomato seeds

Laminarin-hydrolysing activity developed in the endosperm of tomato (Lycopersicon esculentum) seeds following germination. The enzyme was basic (pI>10) and the apparent molecular mass was estimated to be 35 kDa by SDS-PAGE. It was specific for linear beta-1,3-glucan substrates. Laminarin was hydrolysed by the enzyme to yield a mixture of oligoglucosides, indicating that the enzyme had an endo-action pattern. Thus, the enzyme was identified as beta-1,3- endoglucanase (EC 3.2.1.39). The activity of the enzyme developed in the endosperm after radicle protrusion (germination) had occurred and the enzyme activity was localized exclusively in the micropylar region of the endosperm where the radicle had penetrated. When the lateral endosperm region, where no induction of the enzyme occurred, was wounded (cut or punctured), there was a marked enhancement of beta-1,3-glucanase activity. Thus the post-germinative beta-1, 3-glucanase activity in the micropylar endosperm portion might be brought about by wounding resulting from endosperm rupture by radicle penetration.

Mungin, a novel cyclophilin-like antifungal protein from the mung bean

A protein designated mungin, isolated from mung bean (Phaseolus mungo) seeds, possessed activity against the fungi Rhizoctonia solani, Coprinus comatus, Mycosphaerella arachidicola, Botrytis cinerea, and Fusarium oxysporum. The 18-kDa protein also possessed a novel N-terminal sequence with similarity to cyclophilins. It exerts an inhibitory action against alpha- and beta-glucosidases suppresses [(3)H]thymidine in corporation by mouse splenocytes.

Biologically active alkaloids and a free radical scavenger from Prosopis species

The biological activity of extracts from the aerial parts of five Argentinian Prosopis species and the exudate of P. flexuosa were assessed for DNA binding, beta-glucosidase inhibition and free radical scavenging effect using the DPPH decoloration assay. DNA binding effect was found mainly in the basic fraction. The alkaloids tryptamine as well as piperidine and phenethylamine derivatives were isolated from the basic extracts. At 0.50 mg/ml, DNA binding activities ranged from 28% for tryptamine to 0-27% for the phenethylamine and 47-54% for the piperidine derivatives. Tryptamine and 2-beta-methyl-3-beta-hydroxy-6-beta-piperidinedodecanol showed a moderate inhibition (27-32%) of the enzyme beta-glucosidase at 100 microg/ml. The exudate of P. flexuosa displayed a strong free radical scavenger effect in the DPPH decoloration assay. The main active constituent was identified as catechin.

Investigation of the slow inhibition of almond beta-glucosidase and yeast isomaltase by 1-azasugar inhibitors: evidence for the 'direct binding' model

(-)-1-Azafagomine [(3R,4R,5R)-4,5-dihydroxy-3-hydroxymethylhexahydropyridazine; inhibitor 1] is a potent glycosidase inhibitor designed to mimic the transition state of a substrate undergoing glycoside cleavage. The inhibition of glycosidases by inhbitor 1 and analogues has been found to be a relatively slow process. This 'slow inhibition' process was investigated in the inhibition of almond beta-glucosidase and yeast isomaltase by inhibitor 1 and analogues. Progress-curve experiments established that the time-dependent inhibition of both enzymes by inhibitor 1 was a consequence of relatively slow dissociation and association of the inhibitor from and to the enzyme, and not a result of slow interchanges between protein conformations. A number of hydrazine-containing analogues of inhibitor 1 also inhibited beta-glucosidase and isomaltase slowly, while the amine isofagomine [(3R,4R,5R)-3,4-dihydroxy-5-hydroxymethylpiperidine; inhibitor 5] only inhibited beta-glucosidase slowly. Inhibitor 1 and related inhibitors were found to leave almond beta-glucosidase with almost identical rate constants, so that the difference in K(i) values depended almost entirely on changes in the binding rate constant, k(on). The same trend was observed for the inhibition of yeast isomaltase by inhibitor 1 and a related inhibitor. The values of the rate constants were obtained at 25 degrees C and at pH 6.8.

Kinetics of inhibition of beta-glucosidase from Ampullarium crossean by bromoacetic acid

The kinetics of inhibition of beta-glucosidase from Ampullarium crossean by bromoacetic acid (BrAc) has been studied. The results show that the enzyme can be irreversibly and completely inactivated at high BrAc concentration, while at low BrAc concentration, inhibition of the enzyme is a slow, reversible reaction. The microscopic rate constants for the reactions of BrAc with the enzyme were determined. The presence of the substrate offers obvious protection of the enzyme against inhibition by BrAc. The above results suggest that the histidine residue is essential for activity and is situated at or near the active site of the enzyme.

Improving low-temperature catalysis in the hyperthermostable Pyrococcus furiosus beta-glucosidase CelB by directed evolution

The beta-glucosidase from the hyperthermophilic archaeon Pyrococcus furiosus (CelB) is the most thermostable and thermoactive family 1 glycosylhydrolase described to date. To obtain more insight in the molecular determinants of adaptations to high temperatures and study the possibility of optimizing low-temperature activity of a hyperthermostable enzyme, we generated a library of random CelB mutants in Escherichia coli. This library was screened for increased activity on p-nitrophenyl-beta-D-glucopyranoside at room temperature. Multiple CelB variants were identified with up to 3-fold increased rates of hydrolysis of this aryl glucoside, and 10 of them were characterized in detail. Amino acid substitutions were identified in the active-site region, at subunit interfaces, at the enzyme surface, and buried in the interior of the monomers. Characterization of the mutants revealed that the increase in low-temperature activity was achieved in different ways, including altered substrate specificity and increased flexibility by an apparent overall destabilization of the enzyme. Kinetic characterization of the active-site mutants showed that in all cases the catalytic efficiency at 20 degrees C on p-nitrophenyl-beta-D-glucose, as well as on the disaccharide cellobiose, was increased up to 2-fold. In most cases, this was achieved at the expense of beta-galactosidase activity at 20 degrees C and total catalytic efficiency at 90 degrees C. Substrate specificity was found to be affected by many of the observed amino acid substitutions, of which only some are located in the vicinity of the active site. The largest effect on substrate specificity was observed with the CelB variant N415S that showed a 7.5-fold increase in the ratio of p-nitrophenyl-beta-D-glucopyranoside/p-nitrophenyl-beta-D-galactopyra noside hydrolysis. This asparagine at position 415 is predicted to interact with active-site residues that stabilize the hydroxyl group at the C4 position of the substrate, the conformation of which is equatorial in glucose-containing substrates and axial in galactose-containing substrates.

Synthesis and evaluation of aminocyclopentitol inhibitors of beta-glucosidases

[reaction: see text] (1R,2S,3S,4R,5R)-4-Amino-5-(hydroxymethyl)cyclopentane-1,2,3-triol 1, prepared from D-glucose, inhibits beta-glucosidases from Caldocellum saccharolyticum (Ki = 1.8 x 10(-7) M) and from almonds (Ki = 3.4 x 10(-6) M). Inhibition is not influenced by N-ethylation (--> 15) but is strongly reduced upon N-acetylation (--> 12). Inversion of stereochemistry at C(5) (--> 14) has little effect on inhibition of beta-glucosidases. These experiments suggest that 1 acts as an analogue of a protonated beta-glucoside.

Detection of small-molecule enzyme inhibitors with peptides isolated from phage-displayed combinatorial peptide libraries

BACKGROUND

The rapidly expanding list of pharmacologically important targets has highlighted the need for ways to discover new inhibitors that are independent of functional assays. We have utilized peptides to detect inhibitors of protein function. We hypothesized that most peptide ligands identified by phage display would bind to regions of biological interaction in target proteins and that these peptides could be used as sensitive probes for detecting low molecular weight inhibitors that bind to these sites.

RESULTS

We selected a broad range of enzymes as targets for phage display and isolated a series of peptides that bound specifically to each target. Peptide ligands for each target contained similar amino acid sequences and competition analysis indicated that they bound one or two sites per target. Of 17 peptides tested, 13 were found to be specific inhibitors of enzyme function. Finally, we used two peptides specific for Haemophilus influenzae tyrosyl-tRNA synthetase to show that a simple binding assay can be used to detect small-molecule inhibitors with potencies in the micromolar to nanomolar range.

CONCLUSIONS

Peptidic surrogate ligands identified using phage display are preferentially targeted to a limited number of sites that inhibit enzyme function. These peptides can be utilized in a binding assay as a rapid and sensitive method to detect small-molecule inhibitors of target protein function. The binding assay can be used with a variety of detection systems and is readily adaptable to automation, making this platform ideal for high-throughput screening of compound libraries for drug discovery.

Purification of cytosolic beta-glucosidase from pig liver and its reactivity towards flavonoid glycosides

Flavonoid glycosides are common dietary components which may have health-promoting activities. The metabolism of these compounds is thought to influence their bioactivity and uptake from the small intestine. It has been suggested that the enzyme cytosolic beta-glucosidase could deglycosylate certain flavonoid glycosides. To test this hypothesis, the enzyme was purified to homogeneity from pig liver for the first time. It was found to have a molecular weight (55 kDa) and specific activity (with p-nitrophenol glucoside) consistent with other mammalian cytosolic beta-glucosidases. The pure enzyme was indeed found to deglycosylate various flavonoid glycosides. Genistein 7-glucoside, daidzein 7-glucoside, apigenin 7-glucoside and naringenin 7-glucoside all acted as substrates, but we were unable to detect activity with naringenin 7-rhamnoglucoside. Quercetin 4'-glucoside was a substrate, but neither quercetin 3, 4'-diglucoside, quercetin 3-glucoside nor quercetin 3-rhamnoglucoside were deglycosylated. Estimates of K(m) ranged from 25 to 90 microM while those for V(max) were about 10% of that found with the standard artificial substrate p-nitrophenol glucoside. The non-substrate quercetin 3-glucoside was found to partially inhibit deglycosylation of quercetin 4'-glucoside, but it had no effect upon activity with p-nitrophenol glucoside. This study confirms that mammalian cytosolic beta-glucosidase can deglycosylate some, but not all, common dietary flavonoid glycosides. This enzyme may, therefore, be important in the metabolism of these compounds.

Analysis of the early biogenesis of CD1b: involvement of the chaperones calnexin and calreticulin, the proteasome and beta(2)-microglobulin

beta(2)-Microglobulin (beta(2)m)-associated human CD1b proteins present lipid and glycolipid antigens, which are loaded on CD1b in endosomal compartments. In contrast, the related MHC class I molecules acquire antigenic peptides in the endoplasmic reticulum. Here, we investigated the biogenesis of CD1b before beta(2)m binding in comparison to MHC class I. In beta(2)m-deficient FO-1 cells, we found CD1b heavy chains (HC) complexed with the chaperones calnexin and calreticulin, while MHC class I HC associated only with calnexin. Despite this difference, both CD1b HC and MHC class I HC were degraded when the chaperone interactions were prevented by the glucosidase inhibitor castanospermine. The degradation of both molecules included the proteasome and mannosidases. Chaperone-unassociated CD1b could be rescued from degradation by supplementing FO-1 cells with beta(2)m. Finally, prevention of chaperone interaction significantly reduced neoexpression of CD1b upon differentiation of monocytes to dendritic cells, underlining the importance of chaperones for proper expression of CD1b under physiological conditions.