Identification of ER scramblases with crucial roles in lipid biology
Several recent papers have provided evidence that the DedA family proteins TMEM41B and VMP1 are ER-localized phospholipid scramblases. Phospholipid synthesis at the ER occurs at the cytosolic leaflet and therefore a scramblase that allows rapid, bidirectional flip-flop should be required to achieve balanced expansion of both leaflets. These papers represent an important advance because a scramblase activity was found in ER membranes several decades ago but the proteins catalyzing this activity have been elusive. In addition, TMEM41B and VMP1 were known to play an important roles in the production of autophagic membranes, lipoprotein secretion, regulation of lipid droplet formation, survival of neurons and as host factors for coronavirus and flavivirus infection. Papers from the Chen, Reinisch and Yang labs provide convincing biochemical evidence that the purified and reconstituted VMP1 and TMEM41B catalyze energy-independent flip-flop of phospholipid in proteoliposomes. Moreover, Huang et al. found that Tmem41b is required for lipoprotein production and lipid homeostasis in the mouse. Liver-specific inactivation of Tmem41b causes a substantial loss of plasma lipids and reduction of lipoprotein production in the ER. Lipids presumably bound for export by the hepatocytes are instead stored in cytosolic lipid droplets wrapped by distorted ER membranes, leading to a fatty liver and rapid progression to nonalcoholic steatohepatitis (NASH). This phenotype is exacerbated by the failure of these Tmem41b-deficient cells to properly downregulate SREBP to suppress sterol and lipid synthesis. These ER scramblases appear to be acutely needed to support large fluxes of lipid from the ER membrane to either luminal structures in the case of lipoproteins, or cytosolic structures like lipid droplets and autophagosomes. In the latter process, Ghanbarpour et al. provide a compelling model for how a tandem protein array of the ER scramblase (TMEM41B and VMP1), a lipid transfer protein (ATG2) and an autophagosomal membrane scramblase (ATG9) collaborate to move phospholipid from the site of synthesis at the ER to the growing autophagosome membrane. The role of the scramblases is to provide balanced extraction of phospholipid from both leaflets of the ER, and balanced growth of both leaflets of the autophagosome. These papers provide an exciting new dimension in the study of membrane biogenesis.
TMEM41B acts as an ER scramblase required for lipoprotein biogenesis and lipid homeostasis. Huang et al.
https://www.sciencedirect.com/science/article/pii/S1550413121002230?via%3Dihub
TMEM41B and VMP1 are scramblases and regulate the distribution of cholesterol and phosphatidylserine. Li et al.
https://rupress.org/jcb/article/220/6/e202103105/212020/TMEM41B-and-VMP1-are-scramblases-and-regulate-the
A model for a partnership of lipid transfer proteins and scramblases in membrane expansion and organelle biogenesis. Ghanbarpour et al.
https://www.pnas.org/content/118/16/e2101562118
Plasma
Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape
The manuscript "Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape" (https://www.nature.com/articles/s41589-020-0529-6) integrates lipidomic, biophysical, and bioinformatic approaches to comprehensively describe the compositional and structural asymmetry of the mammalian plasma membrane. The authors conclude that the unique lipid compositions of the two leaflets of the plasma membrane bilayer impart distinct physical properties that affect the structure of protein transmembrane domains. Although it has been known for decades that lipids are non-randomly distributed in many living membranes, the precise lipid compositions and physical properties of the two plasma membrane leaflets remain open questions. Combining mass spectrometric lipidomics and classical enzymatic approaches, the authors define the comprehensive, quantitative compositions of the two leaflets of human red blood cell membranes. These compositions are intended to ground accurate and realistic models of both surfaces of the plasma membrane. While generally consistent with classical measurements, these measurements also reveal a robust asymmetry in lipid acyl chains, with inner leaflet lipids being much more unsaturated than outer. A combination of computational simulations and cellular experiments then showed that two leaflets of live cell plasma membranes have quantitatively different lipid packing and diffusivity. This structural asymmetry persists long after membrane internalization, suggesting asymmetry in endocytic membranes. Finally, these biophysical asymmetries appear to be reflected in structural asymmetries of protein transmembrane domains, whose physical characteristics reflect the biophysical profile of their membrane matrix. These protein asymmetries are observed only for residents of plasma membranes and endosomes, but not of biosynthetic organelles (i.e. ER and Golgi). Remarkably, protein transmembrane domains are asymmetric throughout Eukarya, implying that the lipidomic and biophysical asymmetries that experimentally detailed in this work may be conserved throughout the eukaryotic domain of life.
Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape
The manuscript "Plasma membranes are asymmetric in lipid unsaturation, packing and protein shape"
(https://www.nature.com/articles/s41589-020-0529-6) integrates lipidomic, biophysical, and
bioinformatic approaches to comprehensively describe the compositional and structural asymmetry of the mammalian plasma membrane. The authors conclude that
the unique lipid compositions of the two leaflets of the plasma membrane bilayer impart distinct physical properties that affect the structure of protein
transmembrane domains. Although it has been known for decades that lipids are non-randomly distributed in many living membranes, the precise lipid compositions
and physical properties of the two plasma membrane leaflets remain open questions. Combining mass spectrometric lipidomics and classical enzymatic approaches,
the authors define the comprehensive, quantitative compositions of the two leaflets of human red blood cell membranes. These compositions are intended to
ground accurate and realistic models of both surfaces of the plasma membrane. While generally consistent with classical measurements, these measurements also
reveal a robust asymmetry in lipid acyl chains, with inner leaflet lipids being much more unsaturated than outer. A combination of computational simulations and
cellular experiments then showed that two leaflets of live cell plasma membranes have quantitatively different lipid packing and diffusivity. This structural
asymmetry persists long after membrane internalization, suggesting asymmetry in endocytic membranes. Finally, these biophysical asymmetries appear to be reflected
in structural asymmetries of protein transmembrane domains, whose physical characteristics reflect the biophysical profile of their membrane matrix. These protein
asymmetries are observed only for residents of plasma membranes and endosomes, but not of biosynthetic organelles (i.e. ER and Golgi). Remarkably, protein
transmembrane domains are asymmetric throughout Eukarya, implying that the lipidomic and biophysical asymmetries that experimentally detailed in this work may be
conserved throughout the eukaryotic domain of life.
Lipidomic and biophysical homeostasis of mammalian membranes counteracts dietary lipid perturbations to maintain cellular fitness
A paper entitled "Lipidomic and biophysical homeostasis of mammalian membranes counteracts dietary lipid perturbations to maintain cellular fitness"
(https://www.nature.com/articles/s41467-020-15203-1) discusses mechanisms of
mammalian cellular homeostasis, in particular of membrane biophysical properties. The authors show that mammalian cells possess an autonomous, essential
response designed to restore membrane physical properties following perturbations from the diet, and that this response is mediated by cholesterol.
Membrane adaptiveness is a fundamental and ubiquitous response of non-thermoregulating organisms across the tree of life, from prokaryotes to cold-blooded
animals. In such organisms, changes in body temperature affect membrane fluidity, which must in turn be compensated by modulation of lipid composition to
maintain functional membrane phenotypes. In mammals, which do not experience large-scale changes in body temperature, the existence of such membrane
responsiveness has not been widely investigated. However, it is a well-established but under-appreciated fact that mammalian membranes are challenged by
dietary inputs, as dietary fatty acids and cholesterol are directly and robustly incorporated into cell membranes. The paper reports that such perturbations
induce near-simultaneous, ubiquitous remodeling of cellular lipidomes in various mammalian cells and that this lipid remodeling leads to re-normalization of
membrane properties. The response is mediated in part by a member of the SREBP family of lipid master regulators, with cholesterol regulation playing an
important role. Finally, inhibition of this response causes cytotoxicity when membrane homeostasis is challenged by dietary lipids.
These results reveal an essential mechanism of mammalian cellular homeostasis - analogous to prokaryotic homeoviscous adaptation - wherein cells remodel
their lipidomes in response to dietary lipid inputs in order to preserve functional membrane phenotypes.
Major Advance in Understanding the Regulation of Sphingolipid Biosynthesis
The enzyme responsible for sphingolipid biosynthesis, serine palmitoyl transferase (SPT) which condenses serine and palmitic
acid to generate sphingosine. This enzyme is inhibited by ceramide and this ceramide-mediated regulation is mediated by ORMDL
proteins. While this has been known for some time, the mechanisms of SPT-mediated sphingosine biosynthesis and its regulation
by ORMDLs have remained unresolved. Two back-to-back papers in Nature Structural and Molecular Biology by Wang et al and Li
et. al. present cyro-EM structures of human serine palmitoyl transferase (SPT) in a complex with its ORMDL3 regulator. This
exciting research, which garnered a News and Views comment
(https://www.nature.com/articles/s41594-021-00562-0) provides
a molecular and mechanistic understanding of the synthesis and regulation of sphingolipids. They show the organization of the
ORMDL3 complex with the SPT and how this organization affects sphingolipid biosynthesis. These articles are sure to attract a
great deal of attention and will have a major impact on our understanding of sphingolipid biochemistry and biology. (Note: an
ASBMB Lipid News article highlighting this paper is scheduled to appear in the October, 2021 issue of A-Today).
Structural insights into the regulation of human serine palmitoyltransferase complexes Wang et al.
https://doi.org/10.1038/s41594-020-00551-9 (2021))
Structural insights into the assembly and substrate selectivity of human SPT–ORMDL3 complex Li et al
https://doi.org/10.1038/s41594-020-00553-7 (2021)
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