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Lipopeptides are antifungal agents that inhibit cell wall β-(1, 3)-glucan biosynthesis in fungal organisms. A mutant resistant to lipopeptides was generated by UV mutagenesis and characterized. The Candida albicans mutant (LP3-1) was stable and showed resistance specificity to a broad range of lipopeptides and certain glycolipid inhibitors. Other antifungal agents with diverse modes of action had a normal minimum inhibitory concentration profile for LP3-1 compared with the wild-type strain (CCH 442). In the in vitro β-(1, 3)-glucan synthase assay, both the lipopeptides and papulacandin-related agents had considerably higher 50% inhibitory concentration values in the LP3-1 strain than in the wild-type strain. In reconstitution assays, the resistance factor was associated with the integral membrane pellet rather than the peripheral GTP-binding protein. The LP3-1 strain had a membrane lipid profile similar to that of the parent strain and was virulent in a murine model of systemic candidiasis. Taken together, these results indicate that the resistance factor is associated with the integral membrane component of β-(1, 3)-glucan synthase. Lipopeptides are common antifungal agents encountered during screening of natural products. The LP3-1 strain was resistant to natural product extracts known to contain various lipopeptides. Thus, LP3-1 can be used in a dereplication assay.Key words: Candida albicans, β-(1, 3)-glucan synthase, lipopeptides, drug resistance.

The fuscandins, antifungal agents of the papulacandin class, are produced by a strain of Fusarium sambucinum. Fermentation yielded 60 mg/liter of fusacandin A and minor amounts of fusacandin B. As expected, the fusacandins inhibit (1,3)-beta-glucan synthesis. Fusacandin A is slightly less active than papulacandin B against Candida albicans and, like papulacandin, loses activity in the presence of serum.

Fungal (1,3)-β-glucan synthases are sensitive to a wide range of lipophilic inhibitors and it has been proposed that enzyme activity is highly sensitive to perturbations of the membrane environment. Yeast membranes were exposed to phospholipases and various lipophilic compounds, and the resultant effects on glucan synthase activity were ascertained. Glucan synthase from Saccharomyces cerevisiae was rapidly inactivated by phospholipase A 2 (PLA 2 ), and to a lesser extent by phospholipase C. Inactivation was time and dose-dependent and was protected against by EDTA and fatty-acid binding proteins (bovine and human serum albumins). Albumins also partially protected against inhibition by papulacandin B. PLA 2 reaction products were structurally characterized and it was shown that fatty acids and lysophospholipids were the inhibitory moieties, with no novel inhibitory compounds apparent. Glucan synthase was inhibited by a range of fatty acids, monoglycerides and lysophospholipids. Inhibition by fatty acids was non-competitive, and progressive binding of [ 14 C]oleic acid correlated with activity loss. Fluorescence anisotropy studies using diphenylhexatriene (DPH) confirm that fatty acids increase membrane fluidity. These results are consistent with proposais suggesting that glucan synthase inhibition is due in part to non-specific detergent-like disruption of the membrane environment, in addition to direct interactions of lipophilic inhibitors with specific target sites on the enzyme complex.

Plasma membranes were purified from celery ( Apium graveolens L.) by two-phase partitioning and the UDP-glucose (1,3)-β-glucan (callose) synthase was solubilized with the detergent CHAPS. The techniques of moderate pressure anion exchange chromatography (FPLC), gel filtration chromatography, affinity chromatography on UDP-agarose and product-entrapment were then investigated as means for further purification. Enriched fractions were obtained by anion exchange chromatography in 0.1% CHAPS and 50 mM Tris-HCl (pH 7.5, spec. act., 568 nmol min −1 (mg prot.) −1 ), by affinity chromatography using UDP-agarose (1405 nmol min −1 (mg prot.) −1 ) and by product entrapment (914 nmol min −1 (mg prot.) −1 ). Fractions purified by anion exchange chromatography contained numerous polypeptides, however callose synthase purified by the affinity step yielded a silver-stained profile of six prominent polypeptides of 26.8, 30.5, 35, 41, 56 and 72 kDa. In the product-entrapped sample, the polypeptides of 41, 56 and 72 kDa incorporated [ 32 P]5N 3 UDP-Glc in a Ca 2+ -dependent manner, implying that these could play a role in the binding of UDP-Glc to the enzyme. The polypeptides at 26.8, 30.5, 35 and 72 kDa have also been observed in purified preparations from various other sources and could represent other components of the callose synthase complex.

Rapid enrichment of CHAPS-solubilized UDP-glucose:(1,3)-β-glucan (callose) synthase from storage tissue of red beet (Beta vulgaris L.) is obtained when the preparation is incubated with an enzyme assay mixture, then centrifuged and the enzyme released from the callose pellet with a buffer containing EDTA and CHAPS (20-fold purification relative to microsomes). When centrifuged at high speed (80,000g), the enzyme can also be pelleted in the absence of substrate (UDP-Glc) or synthesis of callose, due to nonspecific aggregation of proteins caused by excess cations and insufficient detergent in the assay buffer. True time-dependent and substrate-dependent product-entrapment of callose synthase is obtained by low-speed centrifugation (7,000-11,000g) of enzyme incubated in reaction mixtures containing low levels of cations (0.5 millimolar Mg2+, 1 millimolar Ca2+) and sufficient detergent (0.02% digitonin, 0.12% CHAPS), together with cellobiose, buffer, and UDP-Glc. Entrapment conditions, therefore, are a compromise between preventing nonspecific precipitation of proteins and permitting sufficient enzyme activity for callose synthesis. Further enrichment of the enzyme released from the callose pellet was not obtained by rate-zonal glycerol gradient centrifugation, although its sedimentation rate was greatly enhanced by inclusion of divalent cations in the gradient. Preparations were markedly cleaner when product-entrapment was conducted on enzyme solubilized from plasma membranes isolated by aqueous two-phase partitioning rather than by gradient centrifugation. Product-entrapped preparations consistently contained polypeptides or groups of closely-migrating polypeptides at molecular masses of 92, 83, 70, 57, 43, 35, 31/29, and 27 kilodaltons. This polypeptide profile is in accordance with the findings of other callose synthase enrichment studies using a variety of tissue sources, and is consistent with the existence of a multi-subunit enzyme complex.

The effects of the lysine-reactive chemical modification reagents, uridine 5’ diphospho (UDP)-pyridoxal and formaldehyde (HCHO), on the activity of membrane-bound and solubilized UDP-Glc: (1,3)-β-D-glucan synthase (callose synthase) from red beet (Beta vulgaris L.) storage tissue were compared. Exposure to micromolar levels of UDP-pyridoxal, or millimolar levels of HCHO in the presence of NaCNBH3, resulted in complete enzyme inactivation. Conditions for inhibition of membrane-bound enzyme activity by the two reagents were markedly similar; divalent cations were required for inactivation, and complete protection of activity was obtained with EDTA or EGTA. The substrate, UDP-Glc, protected membrane-bound callose synthase against inactivation by UDP-pyridoxal or HCHO, but protected the solubilized enzyme only against inhibition by UDP-pyridoxal, suggesting that the lysine residue modified by both these reagents is at the enzyme active site, and that the site is more open or has a certain conformational flexibility in the solubilized enzyme. Potential UDP-Glc-binding polypeptides of callose synthase were identified by a two-step labeling procedure. First, nonessential lysine residues were blocked by irreversible modification reaction with HCHO or UDP-pyridoxal in the presence of UDP-Glc to protect lysines at UDP-Glc-binding sites. In the second step, proteins were recovered, reacted with [14C]-HCHO in the absence of UDP-Glc, and polypeptide labeling patterns analyzed by SDS-polyacrylamide gel electrophoresis and fluorography. This procedure reduced incorporation of label by 5- to 8-fold compared to a procedure omitting the preblocking step, and with enzyme partially purified by solubilization in CHAPS followed by product entrapment, labeling was limited to a small set of polypeptides. Taken together with the results of other studies, the data suggest that one or more polypeptides migrating in the 54–57 kDa region are good candidates for the UDP-Glc-binding components of callose synthase.

The photoaffinity probe 5-azidouridine 5'-[beta-32P]diphosphate glucose (5N3[32P]UDP-Glc) was used to identify a 57-kDa polypeptide as a strong candidate for the UDP-Glc-binding polypeptide of UDP-glucose: (1,3)-beta-glucan (callose) synthase from red beet (Beta vulgaris L.) storage tissue. Unlabeled 5N3UDP-Glc was a competitive inhibitor of callose synthase with a Ki of 310 microM. Callose synthase was purified from plasma membranes by a two-step solubilization with 3-[(3-cholamidopropyl)dimethylammonio]-1-propane-sulfonate, followed by product entrapment, and photoincorporation of radioactivity from 5N3[32P]UDP-Glc was used to identify UDP-Glc-binding polypeptides that copurified with callose synthase activity. Photoinsertion into the 57-kDa band was closely correlated with all catalytic properties examined. Photolabeling of the 57-kDa polypeptide was enriched upon purification of callose synthase by product entrapment, was abolished with increasing levels of unlabeled UDP-Glc, was dependent upon the presence of divalent cations, and the pH dependence of photolabeling correlated with the pH activity profile of callose synthase. In addition, photolabeling of the 57-kDa band did not occur after phospholipase treatment, which destroys enzyme activity. The extent of labeling of this polypeptide thus correlates closely with the activity of callose synthase under a wide variety of conditions. These results imply that the polypeptide at 57 kDa represents the substrate-binding and cation-regulated component of the callose synthase complex of higher plants.

A whole-cell C. albicans screen was designed to identify novel inhibitors interacting with the synthesis, assembly and regulation of the fungal cell wall. C. albicans was grown in a paired broth assay in 96-well plates with natural product extracts or pure chemical compounds in the presence and absence of the osmotic stabilizer, sorbitol. Growth was visually examined over a 7-day period and scored into different growth categories. Positives from the sorbitol rescue were then examined under the microscope for morphological alterations and grouped into several morphological classes. Sorbitol protection and cell morphology were indicators of novel antifungal agents from natural product extracts and pure compounds.

(1,3)beta-Glucan synthase activity from the filamentous Ascomycete Neurospora crassa was purified 1300-fold to a specific activity of 14,000 nmol glucose incorporated/min per mg protein. Hyphae were disrupted and crude membrane fractions obtained by high-speed centrifugation. Membrane fractions were extracted with Tergitol NP-40 and a second high-speed particulate fraction was obtained. Enzyme activity was solubilized with (3-((3-cholamidopropyl)dimethylammonio)-1-propanesulfonate and octyl-beta-D-glucoside from Tergitol-extracted membrane preparations. Solubilized enzyme activity was purified by product entrapment and recovered by low-speed centrifugation through a layer of sucrose. SDS-PAGE analysis revealed 2 proteins of 165 and 100 kDa as likely candidates for subunits of the (1,3)beta-glucan synthase enzyme complex. 5-Azido-[32P]UDP-glucose was photo-crosslinked to UDP-glucose-binding proteins in each fraction of the purification procedure. Autoradiograms of SDS-PAGE gels revealed a single protein of 165 kDa enriching with enzyme activity and labeling with the substrate analog. Photoincorporation of 5-azido-[32P]UDP-glucose by the 165 kDa protein was competed by 0.25 mM UDP-glucose (80%) and TDP-glucose (65%) while ADP-glucose (27%), CDP-glucose (36%), and GDP-glucose (8%) where less effective. These results were similar to in vitro inhibition of enzyme activity by the same compounds. These data strongly suggest that the 165 kDa protein is a substrate-binding subunit of (1,3)beta-glucan synthase.