bims-mecosi 2021-09-26 papers

Issue of 2021–09–26
five papers selected by
Verena Kohler, Stockholm University

Front Cell Dev Biol. 2021 ;9 737907

Transport Pathways That Contribute to the Cellular Distribution of Phosphatidylserine.

Guillaume Lenoir, Juan Martín D'Ambrosio, Thibaud Dieudonné, Alenka Čopič.

Phosphatidylserine (PS) is a negatively charged phospholipid that displays a highly uneven distribution within cellular membranes, essential for establishment of cell polarity and other processes. In this review, we discuss how combined action of PS biosynthesis enzymes in the endoplasmic reticulum (ER), lipid transfer proteins (LTPs) acting within membrane contact sites (MCS) between the ER and other compartments, and lipid flippases and scramblases that mediate PS flip-flop between membrane leaflets controls the cellular distribution of PS. Enrichment of PS in specific compartments, in particular in the cytosolic leaflet of the plasma membrane (PM), requires input of energy, which can be supplied in the form of ATP or by phosphoinositides. Conversely, coupling between PS synthesis or degradation, PS flip-flop and PS transfer may enable PS transfer by passive flow. Such scenario is best documented by recent work on the formation of autophagosomes. The existence of lateral PS nanodomains, which is well-documented in the case of the PM and postulated for other compartments, can change the steepness or direction of PS gradients between compartments. Improvements in cellular imaging of lipids and membranes, lipidomic analysis of complex cellular samples, reconstitution of cellular lipid transport reactions and high-resolution structural data have greatly increased our understanding of cellular PS homeostasis. Our review also highlights how budding yeast has been instrumental for our understanding of the organization and transport of PS in cells.

Keywords: budding yeast; flippase; lipid domain; lipid scramblase; lipid transfer protein; membrane asymmetry; membrane contact site; phosphatidylserine

DOI: https://doi.org/10.3389/fcell.2021.737907

Sci Rep. 2021 Sep 22. 11(1): 18859

Structural basis of human PDZD8-Rab7 interaction for the ER-late endosome tethering.

Haider Khan, Lin Chen, Lingchen Tan, Young Jun Im.

The membrane contact sites (MCSs) between the ER and late endosomes (LEs) are essential for the regulation of endosomal protein sorting, dynamics, and motility. PDZD8 is an ER transmembrane protein containing a Synaptotagmin-like Mitochondrial lipid-binding Proteins (SMP) domain. PDZD8 tethers the ER to late endosomes and lysosomes by associating its C-terminal coiled-coil (CC) with the LE Rab7. To identify the structural determinants for the PDZD8-Rab7 interaction, we determined the crystal structure of the human PDZD8 CC domain in complex with the GTP-bound form of Rab7. The PDZD8 CC contains one short helix and the two helices forming an antiparallel coiled-coil. Two Rab7 molecules bind to the opposite sides of the PDZD8 CC in a 2:1 ratio. The switch I/II and interswitch regions of the GTP-loaded Rab7 form the binding interfaces, which correlates with the GTP-dependent interaction of PDZD8 and Rab7. Analysis of the protein interaction by isothermal titration calorimetry confirms that two Rab7 molecules bind the PDZD8 CC in a GTP-dependent manner. The structural model of the PDZD8 CC-Rab7 complex correlates with the recruitment of PDZD8 at the LE-ER interface and its role in lipid transport and regulation.

DOI: https://doi.org/10.1038/s41598-021-98419-5

Cell Res. 2021 Sep 24.

A new type of ERGIC-ERES membrane contact mediated by TMED9 and SEC12 is required for autophagosome biogenesis.

Shulin Li, Rui Yan, Jialu Xu, Shiqun Zhao, Xinyu Ma, Qiming Sun, Min Zhang, Ying Li, Jun-Jie Gogo Liu, Liangyi Chen, Sai Li, Ke Xu, Liang Ge.

Under stress, the endomembrane system undergoes reorganization to support autophagosome biogenesis, which is a central step in autophagy. How the endomembrane system remodels has been poorly understood. Here we identify a new type of membrane contact formed between the ER-Golgi intermediate compartment (ERGIC) and the ER-exit site (ERES) in the ER-Golgi system, which is essential for promoting autophagosome biogenesis induced by different stress stimuli. The ERGIC-ERES contact is established by the interaction between TMED9 and SEC12 which generates a short distance opposition (as close as 2-5 nm) between the two compartments. The tight membrane contact allows the ERES-located SEC12 to transactivate COPII assembly on the ERGIC. In addition, a portion of SEC12 also relocates to the ERGIC. Through both mechanisms, the ERGIC-ERES contact promotes formation of the ERGIC-derived COPII vesicle, a membrane precursor of the autophagosome. The ERGIC-ERES contact is physically and functionally different from the TFG-mediated ERGIC-ERES adjunction involved in secretory protein transport, and therefore defines a unique endomembrane structure generated upon stress conditions for autophagic membrane formation.

DOI: https://doi.org/10.1038/s41422-021-00563-0

Life Sci Alliance. 2021 Nov;pii: e202101092. [Epub ahead of print]4(11):

INPP5K and Atlastin-1 maintain the nonuniform distribution of ER-plasma membrane contacts in neurons.

Jingbo Sun, Raihanah Harion, Tomoki Naito, Yasunori Saheki.

In neurons, the ER extends throughout all cellular processes, forming multiple contacts with the plasma membrane (PM) to fine-tune neuronal physiology. However, the mechanisms that regulate the distribution of neuronal ER-PM contacts are not known. Here, we used the Caenorhabditis elegans DA9 motor neuron as our model system and found that neuronal ER-PM contacts are enriched in soma and dendrite and mostly absent in axons. Using forward genetic screen, we identified that the inositol 5-phosphatase, CIL-1 (human INPP5K), and the dynamin-like GTPase, ATLN-1 (human Atlastin-1), help to maintain the non-uniform, somatodendritic enrichment of neuronal ER-PM contacts. Mechanistically, CIL-1 acts upstream of ATLN-1 to maintain the balance between ER tubules and sheets. In mutants of CIL-1 or ATLN-1, ER sheets expand and invade into the axon. This is accompanied by the ectopic formation of axonal ER-PM contacts and defects in axon regeneration following laser-induced axotomy. As INPP5K and Atlastin-1 have been linked to neurological disorders, the unique distribution of neuronal ER-PM contacts maintained by these proteins may support neuronal resilience during the onset and progression of these diseases.

DOI: https://doi.org/10.26508/lsa.202101092

Mitochondrion. 2021 Sep 15. pii: S1567-7249(21)00121-5. [Epub ahead of print]

Mitofusin 2: A Link Between Mitochondrial Function and Substrate Metabolism?

Janna M Emery, Rudy M Ortiz.

Mitochondria are dynamic, interactive organelles that connect cellular signaling and whole-cell homeostasis. This "mitochatting" allows the cell to receive information about the mitochondria's condition before accommodating energy demands. Mitofusin 2 (Mfn2), an outer mitochondrial membrane fusion protein specializes in mediating mitochondrial homeostasis. Early studies defined the biological significance of Mfn2, latter studies highlighted its role in substrate metabolism. However, determining Mfn2 potential to contribute to energy homeostasis needs study. This review summarizes current literature on mitochondrial metabolic processes, dynamics, and evidence of interactions among Mfn2 and regulatory processes that may link Mfn2's role in maintaining mitochondrial function and substrate metabolism.

Keywords: fatty acid oxidation; fission; fusion; glycolysis; mitochondrial dynamics; mitophagy

DOI: https://doi.org/10.1016/j.mito.2021.09.003