Sterols and acylated proteins in mycoplasmas

Catalytic function, mechanism, and application of plant acyltransferases

Acyltransferases (ATs) are important tailoring enzymes that contribute to the diversity of natural products. They catalyze the transfer of acyl groups to the skeleton, which improves the lipid solubility, stability, and pharmacological activity of natural compounds. In recent years, a number of ATs have been isolated from plants. In this review, we have summarized 141 biochemically characterized ATs during the period July 1997 to October 2020, including their function, heterologous expression systems, and catalytic mechanisms. Their catalytic performance and application potential has been further discussed.

Identification and functional analysis of a galactosyltransferase capable of cholesterol glycolipid formation in the Lyme disease spirochete Borrelia burgdorferi

Borrelia burgdorferi (Bb), the etiological agent of Lyme disease, produces a series of simple glycolipids where diacylglycerol and cholesterol serve as the precursor. The cholesterol-based glycolipids, cholesteryl 6-O-acyl-β-D-galactopyranoside (ACGal) and cholesteryl-β-D-galactopyranoside (CGal) are immunogenic and proposed to contribute to the pathogenesis of Lyme disease. Detailed studies of CGal and ACGal in Bb have been hampered by a lack of knowledge of their underlying biosynthetic processes. The genome of Bb encodes four putative glycosyltransferases, and only one of these, BB0572, was predicted to be an inverting family 2 glycosyltransferase (GT2 enzyme) capable of using UDP-galactose as a substrate and forming a β-glycosidic bond. Comparison of the 42 kDa BB0572 amino acid sequence from Bb with other Borrelia spp demonstrates that this protein is highly conserved. To establish BB0572 as the galactosyltransferase capable of cholesterol glycolipid formation in Bb, the protein was produced as a recombinant product in Escherichia coli and tested in a cell-free assay with 14C-cholesterol and UDP-galactose as the substrates. This experiment resulted in a radiolabeled lipid that migrated with the cholesterol glycolipid standard of CGal when evaluated by thin layer chromatography. Additionally, mutation in the predicted active site of BB0572 resulted in a recombinant protein that was unable to catalyze the formation of the cholesterol glycolipid. These data characterize BB0572 as a putative cholesterol galactosyltransferase. This provides the first step in understanding how Bb cholesterol glycolipids are formed and will allow investigations into their involvement in pathogen transmission and disease development.

Novel synthetic lipopeptides derived from Mycoplasma hyorhinis upregulate calpastatin in SH-SY5Y neuroblastoma cells and induce a neuroprotective effect against amyloid-β-peptide toxicity

Previously, we showed that contamination of SH-SY5Y neuroblastoma cells by Mycoplasma hyorhinis strains NDMh and MCLD leads to increased levels of calpastatin (the endogenous, specific inhibitor of the Ca2+-dependent protease calpain), resulting in inhibition of calpain activation. We have found that the increased calpastatin level is promoted by the lipoprotein fraction (MhLpp) of the mycoplasmal membrane. Here, we present MhLpp-based novel synthetic lipopeptides that induce upregulation of calpastatin in SH-SY5Y neuroblastoma cells, leading to protection of the treated cells against Ca2+/amyloid-β-peptide toxicity. These lipopeptides present a new class of promising agents against calpain-induced cell toxicity.

The phospholipid profile of mycoplasmas

The de novo synthesized polar lipids of Mycoplasma species are rather simple, comprising primarily of the acidic glycerophospholipids PG and CL. In addition, when grown in a medium containing serum, significant amounts of PC and SPM are incorporated into the mycoplasma cell membrane although these lipids are very uncommon in wall-covered bacteria. The exogenous lipids are either incorporated unchanged or the PC incorporated is modified by a deacylation-acylation enzymatic cycle to form disaturated PC. Although their small genome, in some Mycoplasma species, other genes involved in lipid biosynthesis were detected, resulting in the synthesis of a variety of glycolipis, phosphoglycolipids and ether lipids. We suggest that analyses and comparisons of mycoplasma polar lipids may serve as a novel and useful tool for classification. Nonetheless, to evaluate the importance of polar lipids in mycoplasma, further systematic and extensive studies on more Mycoplasma species are needed. While studies are needed to elucidate the role of lipids in the mechanisms governing the interaction of mycoplasmas with host eukaryotic cells, the finding that a terminal phosphocholine containing glycolipids of M. fermentans serves both as a major immune determinants and as a trigger of the inflammatory responses, and the findings that the fusogenicity of M. fermentans with host cells is markedly stimulated by lyso-ether lipids, are important steps toward understanding the molecular mechanisms of M. fermentans pathogenicity.

Complete genome and proteome of Acholeplasma laidlawii

We present the complete genome sequence and proteogenomic map for Acholeplasma laidlawii PG-8A (class Mollicutes, order Acholeplasmatales, family Acholeplasmataceae). The genome of A. laidlawii is represented by a single 1,496,992-bp circular chromosome with an average G+C content of 31 mol%. This is the longest genome among the Mollicutes with a known nucleotide sequence. It contains genes of polymerase type I, SOS response, and signal transduction systems, as well as RNA regulatory elements, riboswitches, and T boxes. This demonstrates a significant capability for the regulation of gene expression and mutagenic response to stress. Acholeplasma laidlawii and phytoplasmas are the only Mollicutes known to use the universal genetic code, in which UGA is a stop codon. Within the Mollicutes group, only the sterol-nonrequiring Acholeplasma has the capacity to synthesize saturated fatty acids de novo. Proteomic data were used in the primary annotation of the genome, validating expression of many predicted proteins. We also detected posttranslational modifications of A. laidlawii proteins: phosphorylation and acylation. Seventy-four candidate phosphorylated proteins were found: 16 candidates are proteins unique to A. laidlawii, and 11 of them are surface-anchored or integral membrane proteins, which implies the presence of active signaling pathways. Among 20 acylated proteins, 14 contained palmitic chains, and six contained stearic chains. No residue of linoleic or oleic acid was observed. Acylated proteins were components of mainly sugar and inorganic ion transport systems and were surface-anchored proteins with unknown functions.

Acylated cholesteryl galactosides are specific antigens of borrelia causing lyme disease and frequently induce antibodies in late stages of disease

Borrelia burgdorferi sensu lato is the causative agent of Lyme disease (LD), an infectious disease occurring in North America, Europe, and Asia in different clinical stages. B. burgdorferi sensu lato encompasses at least 12 species, with B. burgdorferi sensu stricto, B. garinii, and B. afzelii being of highest clinical importance. Immunologic testing for LD as well as recent vaccination strategies exclusively refer to proteinaceous antigens. However, B. burgdorferi sensu stricto exhibits glycolipid antigens, including 6-O-acylated cholesteryl beta-D-galactopyranoside (ACGal), and first the data indicated that this compound may act as an immunogen. Here we investigated whether B. garinii and B. afzelii also possess this antigen, and whether antibodies directed against these compounds are abundant among patients suffering from different stages of LD. Gas-liquid chromatography/mass spectroscopy and NMR spectroscopy showed that both B. garinii and B. afzelii exhibit ACGal in high quantities. In contrast, B. hermsii causing relapsing fever features 6-O-acylated cholesteryl beta-D-glucopyranoside (ACGlc). Sera derived from patients diagnosed for LD contained antibodies against ACGal, with 80% of patients suffering from late stage disease exhibiting this feature. Antibodies reacted with ACGal from all three B. burgdorferi species tested, but not with ACGlc from B. hermsii. These data show that ACGal is present in all clinically important B. burgdorferi species, and that specific antibodies against this compound are frequently found during LD. ACGal may thus be an interesting tool for improving diagnostics as well as for novel vaccination strategies.

Ferroplasma and relatives, recently discovered cell wall-lacking archaea making a living in extremely acid, heavy metal-rich environments

For several decades, the bacterium Acidithiobacillus (previously Thiobacillus) has been considered to be the principal acidophilic sulfur- and iron-oxidizing microbe inhabiting acidic environments rich in ores of iron and other heavy metals, responsible for the metal solubilization and leaching from such ores, and has become the paradigm of such microbes. However, during the last few years, new studies of a number of acidic environments, particularly mining waste waters, acidic pools, etc., in diverse geographical locations have revealed the presence of new cell wall-lacking archaea related to the recently described, acidophilic, ferrous-iron oxidizing Ferroplasma acidiphilum. These mesophilic and moderately thermophilic microbes, representing the family Ferroplasmaceae, were numerically significant members of the microbial consortia of the habitats studied, are able to mobilize metals from sulfide ores, e.g. pyrite, arsenopyrite and copper-containing sulfides, and are more acid-resistant than iron and sulfur oxidizing bacteria exhibiting similar eco-physiological properties. Ferroplasma cell membranes contain novel caldarchaetidylglycerol tetraether lipids, which have extremely low proton permeabilities, as a result of the bulky isoprenoid core, and which are probably a major contributor to the extreme acid tolerance of these cell wall-less microbes. Surprisingly, several intracellular enzymes, including an ATP-dependent DNA ligase have pH optima close to that of the external environment rather than of the cytoplasm. Ferroplasma spp. are probably the major players in the biogeochemical cycling of sulfur and sulfide metals in highly acidic environments, and may have considerable potential for biotechnological applications such as biomining and biocatalysis under extreme conditions.

Translocation of histone proteins across lipid bilayers and Mycoplasma membranes

We show that the three core histones H2A, H3 and H4 can transverse lipid bilayers of large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs). In contrast, the histone H2B, although able to bind to the liposomes, fails to penetrate the unilamellar and the multilamellar vesicles. Translocation across the lipid bilayer was determined using biotin-labeled histones and an ELISA-based system. Following incubation with the liposomes, external membrane-bound biotin molecules were neutralized by the addition of avidin. Penetrating biotin-histone conjugates were exposed by Triton treatment of the neutralized liposomes. The intraliposomal biotin-histone conjugates, in contrast to those attached only to the external surface, were attached to the detergent lysed lipid molecules. Thus, biotinylated histone molecules that were exposed only following detergent treatment of the liposomes were considered to be located at the inner leaflet of the lipid bilayers. The penetrating histone molecules failed to mediate translocation of BSA molecules covalently attached to them. Translocation of the core histones, including H2B, was also observed across mycoplasma cell membranes. The extent of this translocation was inversely related to the degree of membrane cholesterol. The addition of cholesterol also reduced the extent of histone penetration into the MLVs. Although able to bind biotinylated histones, human erythrocytes, erythrocyte ghosts and Escherichia coli cells were impermeable to them. Based on the present and previous data histones appear to be characterized by the same features that characterize cell penetrating peptides and proteins (CPPs).

A newly discovered cholesteryl galactoside from Borrelia burgdorferi

Two major glycolipids, which comprise approximately 36% of the total lipid mass from Borrelia burgdorferi, the etiological agent of Lyme disease, were investigated. We determined the fatty acid type, sugar identity, anomeric configuration, and substituent type and position. The structures were identified as cholesteryl 6-O-acyl-beta-d-galactopyranoside (B. burgdorferi glycolipid 1, BbGL-I), and 1,2-di-O-acyl-3-O-alpha-d-galactopyranosyl-sn-glycerol (BbGL-II). The major fatty acids were palmitate and oleate. The structures were corroborated by gas-liquid chromatography MS, matrix-assisted laser desorption/ionization time-of-flight spectroscopy, fast atom bombardment MS, detailed NMR spectrometry, and metabolic labeling. This is a previously undescribed demonstration of a cholesteryl galactoside in bacteria. Lipopolysaccharide was not detected in B. burgdorferi. The two glycolipids have several properties suggesting they may function as lipopolysaccharide: both are main components of the bacterial membrane, surface exposed, and have a three-domain structure. BbGL-I elicited specific antibodies in mice and rabbits, and BbGL-II elicited antibodies that reacted with both glycolipids.