Biology subjects

Welander, P. V.

Publications and source records attributed to Welander, P. V..

3 recordsLinked to original sources

bioRxiv2022

De novo cholesterol biosynthesis in the bacterial domain

Sterols are versatile lipids primarily associated with eukaryotes. While bacteria also produce sterols and studies of bacterial biosynthesis proteins have revealed novel biosynthetic pathways and a potential evolutionary role in the origin of sterol biosynthesis, no bacterium has been shown to synthesize highly modified eukaryotic sterols, such as cholesterol. This has led to the notion that bacteria only produce biosynthetically simple sterols and has lessened the consideration of bacterial production in discussions of sterol biosynthesis. In this study, we demonstrate two phylogenetically distinct bacteria, Enhygromyxa salina and Calothrix sp. NIES-4105, are capable of de novo cholesterol production. We also identified 25-hydroxycholesterol released as a product of acid hydrolysis in extracts from both bacteria, suggesting cholesterol exists as a conjugated molecule in these organisms. We coupled our lipid extractions to bioinformatic analyses and heterologous expression experiments to identify genetic pathways driving cholesterol production in each bacterium. E. salina shares much of its cholesterol biosynthesis pathway with the canonical eukaryotic pathway, except for C-4 demethylation where we identified a unique variation on the bacterial C-4 demethylation pathway. Calothrix lacks homologs for several steps in cholesterol biosynthesis, suggesting this bacterium may harbor a novel mechanism for completing cholesterol biosynthesis. Altogether, these results demonstrate the complexity underpinning bacterial sterol biosynthesis and raise further questions about the functional and regulatory roles of sterols in bacteria.

microbiology↗

bioRxiv2022

Structural insights into bacterial sterol transport

Sterol lipids are widely present in eukaryotes and play essential roles in signaling and modulating membrane fluidity. Although rare, some bacteria also produce sterols, but their function in bacteria is not known. Moreover, many more species, including pathogens and commensal microbes, acquire or modify sterols from eukaryotic hosts through poorly understood molecular mechanisms. The aerobic methanotroph Methylococcus capsulatus was the first bacterium shown to synthesize sterols, producing a mixture of C-4 methylated sterols that are distinct from those observed in eukaryotes. C-4 methylated sterols are synthesized in the cytosol and localized to the outer membrane, suggesting that a bacterial sterol transport machinery exists. Until now, the identity of such machinery remained a mystery. In this study, we identified three novel proteins that may be the first examples of transporters for bacterial sterol lipids. The proteins, which all belong to well-studied families of bacterial metabolite transporters, are predicted to reside in the inner membrane, periplasm, and outer membrane of M. capsulatus, and may work as a conduit to move modified sterols to the outer membrane. Quantitative analysis of ligand binding revealed their remarkable specificity for 4-methylsterols, and crystallographic structures coupled with docking and molecular dynamics simulations revealed the structural bases for substrate binding by two of the putative transporters. Their striking structural divergence from eukaryotic sterol transporters signals that they form a distinct sterol transport system within the bacterial domain. Finally, bioinformatics revealed the widespread presence of similar transporters in bacterial genomes, including in some pathogens that use host sterol lipids to construct their cell envelopes. The unique folds of these bacterial sterol binding proteins should now guide the discovery of other proteins that handle this essential metabolite.

biochemistry↗

bioRxiv2022

Sterol side chain alkylation by sponge and bacterial methyltransferases

Sterol lipids are required by most eukaryotes and are readily preserved as sterane molecular fossils. These geologic steranes are broadly interpreted as biomarkers for ancient eukaryotes1,2 although diverse bacteria also produce sterols3. Steranes with side-chain methylations can act as more specific biomarkers4 if their sterol precursors are limited to particular extant eukaryotes and are absent in bacteria. An abundance of one such sterane, 24-isopropylcholestane, in late Neoproterozoic rocks has been attributed to marine demosponges and potentially represents the earliest evidence for animals on Earth5. However, debates over this interpretation6-14 continue given the potential for alternative sources of 24-isopropylcholestane and the lack of experimental evidence demonstrating the function of enzymes that methylate sterols to give the 24-isopropyl side-chain. Here we show that sterol methyltransferases from both sponges and bacteria are functional and identify three bacterial methyltransferases each capable of sequential methylations resulting in the 24-isopropyl sterol side-chain. We identified two of these propylating enzymes in a demosponge metagenome suggesting bacterial symbionts contribute to 24-isopropyl sterol biosynthesis in demosponges. Our results demonstrate yet-uncultured bacteria have the genomic capacity to synthesize side-chain alkylated sterols and should therefore be considered when interpreting side-chain alkylated sterane biomarkers in the rock record.

microbiology↗