bims-mecosi Biomed News
on Membrane Contact Sites
Issue of 2021‒07‒18
seven papers selected by
Verena Kohler
Stockholm University


  1. Front Cell Dev Biol. 2021 ;9 664788
      Lipids must be correctly transported within the cell to the right place at the right time in order to be fully functional. Non-vesicular lipid transport is mediated by so-called lipid transfer proteins (LTPs), which contain a hydrophobic cavity that sequesters lipid molecules. Oxysterol-binding protein (OSBP)-related proteins (ORPs) are a family of LTPs known to harbor lipid ligands, such as cholesterol and phospholipids. ORPs act as a sensor or transporter of those lipid ligands at membrane contact sites (MCSs) where two different cellular membranes are closely apposed. In particular, a characteristic functional property of ORPs is their role as a lipid exchanger. ORPs mediate counter-directional transport of two different lipid ligands at MCSs. Several, but not all, ORPs transport their lipid ligand from the endoplasmic reticulum (ER) in exchange for phosphatidylinositol 4-phosphate (PI4P), the other ligand, on apposed membranes. This ORP-mediated lipid "countertransport" is driven by the concentration gradient of PI4P between membranes, which is generated by its kinases and phosphatases. In this review, we will discuss how ORP function is tightly coupled to metabolism of phosphoinositides such as PI4P. Recent progress on the role of ORP-mediated lipid transport/countertransport at multiple MCSs in cellular functions will be also discussed.
    Keywords:  ORPs; PI4P; cholesterol; lipid countertransport; lipid transfer protein (LTP); membrane contact site (MCS); phosphatidylserine (PS); phosphoinositide
    DOI:  https://doi.org/10.3389/fcell.2021.664788
  2. J Cell Biol. 2021 Sep 06. pii: e201910161. [Epub ahead of print]220(9):
      ER-plasma membrane (PM) contacts are proposed to be held together by distinct families of tethering proteins, which in yeast include the VAP homologues Scs2/22, the extended-synaptotagmin homologues Tcb1/2/3, and the TMEM16 homologue Ist2. It is unclear whether these tethers act redundantly or whether individual tethers have specific functions at contacts. Here, we show that Ist2 directly recruits the phosphatidylserine (PS) transport proteins and ORP family members Osh6 and Osh7 to ER-PM contacts through a binding site located in Ist2's disordered C-terminal tethering region. This interaction is required for phosphatidylethanolamine (PE) production by the PS decarboxylase Psd2, whereby PS transported from the ER to the PM by Osh6/7 is endocytosed to the site of Psd2 in endosomes/Golgi/vacuoles. This role for Ist2 and Osh6/7 in nonvesicular PS transport is specific, as other tethers/transport proteins do not compensate. Thus, we identify a molecular link between the ORP and TMEM16 families and a role for endocytosis of PS in PE synthesis.
    DOI:  https://doi.org/10.1083/jcb.201910161
  3. J Cell Sci. 2021 Jul 14. pii: jcs.258819. [Epub ahead of print]
      Lipid droplets (LDs) are globular intracellular structures dedicated to the storage of neutral lipids. They are closely associated with the endoplasmic reticulum (ER) and are delineated by a monolayer of phospholipids that is continuous with the cytoplasmic leaflet of the ER membrane. LDs contain a specific set of proteins, but how these proteins are targeted to the LD surface is not fully understood. Here we devised a yeast mating-based microscopic readout to monitor the transfer of LD proteins upon zygote formation. The results of this analysis indicate that ER fusion between mating partners is required for transfer of LD proteins and that this transfer is continuous, bidirectional and affects most LDs simultaneously. These observations suggest that LDs do not fuse upon mating of yeast cells, but that they form a network that is interconnected through the ER membrane. Consistent with this, ER-localized LD proteins rapidly move onto LDs of a mating partner and this protein transfer is affected by seipin, a protein important for proper LD biogenesis and the functional connection of LDs with the ER membrane.
    Keywords:  Endoplasmic reticulum; Lipid droplets; Mating; Membrane fusion; Protein targeting; Saccharomyces cerevisiae; Seipin; Steryl esters; Triacylglycerols
    DOI:  https://doi.org/10.1242/jcs.258819
  4. Adv Exp Med Biol. 2021 ;1208 17-42
      Autophagosome formation is a regulated membrane remodeling process, which involves the generation of autophagosomal membrane precursors (vesicles), the assembly of the autophagosomal membrane precursors to form the phagophore, and phagophore elongation to complete the autophagosome. The sources of the autophagosomal membrane precursors are endomembrane compartments, such as the endoplasmic reticulum (ER), the ER-Golgi intermediate compartment (ERGIC), ER-exit sites (ERES), and endosomes. In response to stress, these structures are remodeled, to generate the early autophagosomal membrane precursors. The phagophore assembly site (PAS), which mainly localizes on the ER, harbors the site for autophagosomal membrane assembly, elongation, and completion. ATG proteins, membrane remodeling factors, and autophagic membranes follow a precise choreography to complete the overall process. In this chapter, we briefly discuss our current knowledge on the membrane origins of the autophagosome, as well as autophagosomal precursor generation, assembly, and expansion.
    Keywords:  ATG proteins; Autophagosome; Endoplasmic reticulum; Endosome; Membrane remodeling; Microfilament; Mitochondria; Phagophore; Plasma membrane
    DOI:  https://doi.org/10.1007/978-981-16-2830-6_2
  5. Front Microbiol. 2021 ;12 673509
      Viruses are obligate parasites that depend on the host cell machinery for their replication and dissemination. Cellular lipids play a central role in multiple stages of the viral life cycle such as entry, replication, morphogenesis, and egress. Most viruses reorganize the host cell membranes for the establishment of viral replication complex. These specialized structures allow the segregation of replicating viral RNA from ribosomes and protect it from host nucleases. They also facilitate localized enrichment of cellular components required for viral replication and assembly. The specific composition of the lipid membrane governs its ability to form negative or positive curvature and possess a rigid or flexible form, which is crucial for membrane rearrangement and establishment of viral replication complexes. In this review, we highlight how different viruses manipulate host lipid transfer proteins and harness their functions to enrich different membrane compartments with specific lipids in order to facilitate multiple aspects of the viral life cycle.
    Keywords:  CERT; HCV; NPC1; OSBP; lipid transfer proteins; membrane contact sites; replication; virus
    DOI:  https://doi.org/10.3389/fmicb.2021.673509
  6. RNA. 2021 Jul 08. pii: rna.078858.121. [Epub ahead of print]
      Stress granules (SGs) are membraneless organelles composed of mRNAs and RNA binding proteins which undergo assembly in response to stress-induced inactivation of translation initiation. In general, SG recruitment is limited to a subpopulation of a given mRNA species and RNA-seq analyses of purified SGs revealed that signal sequence-encoding (i.e. endoplasmic reticulum (ER)-targeted) transcripts are significantly under-represented, consistent with prior reports that ER localization can protect mRNAs from SG recruitment. Using translational profiling, cell fractionation, and single molecule mRNA imaging, we examined SG biogenesis following activation of the unfolded protein response (UPR) by 1,4-dithiothreitol (DTT) and report that gene-specific subsets of cytosolic and ER-targeted mRNAs can be recruited into SGs. Furthermore, we demonstrate that SGs form in close proximity to or directly associated with the ER membrane. ER-associated SG assembly was also observed during arsenite stress, suggesting broad roles for the ER in SG biogenesis. Recruitment of a given mRNA into SGs required stress-induced translational repression, though translational inhibition was not solely predictive of an mRNA's propensity for SG recruitment. SG formation was prevented by the transcriptional inhibitors actinomycin D or triptolide, suggesting a functional link between gene transcriptional state and SG biogenesis. Collectively these data demonstrate that ER-targeted and cytosolic mRNAs can be recruited into ER-associated SGs and this recruitment is sensitive to transcriptional inhibition. We propose that newly transcribed mRNAs exported under conditions of suppressed translation initiation are primary SG substrates, with the ER serving as the central subcellular site of SG formation.
    Keywords:  endoplasmic reticulum; mRNA; stress granule; translational regulation; unfolded protein response
    DOI:  https://doi.org/10.1261/rna.078858.121
  7. Biochim Biophys Acta Biomembr. 2021 Jul 13. pii: S0005-2736(21)00133-4. [Epub ahead of print] 183683
      Mitochondrial ultrastructure is highly adaptable and undergoes dynamic changes upon physiological and energetic cues. MICOS (mitochondrial contact site and cristae organizing system), a large oligomeric protein complex, maintains mitochondrial ultrastructure as it is required for formation of crista junctions (CJs) and contact sites. MIC13 acts as a critical bridge between two MICOS subcomplexes. Deletion of MIC13 causes loss of CJs resulting in cristae accumulating as concentric rings and specific destabilization of the MIC10-subcomplex. Mutations in MIC13 are associated with infantile lethal mitochondrial hepato-encephalopathy, yet functional regions within MIC13 were not known. To identify and characterize such regions, we systemically generated 20 amino-acids deletion variants across the length of MIC13. While deletion of many of these regions of MIC13 is dispensable for its stability, the N-terminal region and a stretch between amino acid residues 84 and 103 are necessary for the stability and functionality of MIC13. We could further locate conserved motifs within these regions and found that a GxxxG motif in the N-terminal transmembrane segment and an internal WN motif are essential for stability of MIC13, formation of the MIC10-subcomplex, interaction with MIC10- and MIC60-subcomplexes and maintenance of cristae morphology. The GxxxG motif is required for membrane insertion of MIC13. Overall, we systematically found important conserved residues of MIC13 that are required to perform the bridging between the two MICOS subcomplexes. The study improves our understanding of the basic molecular function of MIC13 and has implications for its role in the pathogenesis of a severe mitochondrial disease.
    Keywords:  Conserved motifs; Crista junction; Cristae; MICOS; Mitochondrial disease
    DOI:  https://doi.org/10.1016/j.bbamem.2021.183683