Adriano Vissa

VAPs and ACBD5 tether peroxisomes to the ER for peroxisome maintenance and lipid homeostasis

Lipid exchange between the endoplasmic reticulum (ER) and peroxisomes is necessary for the synthesis and catabolism of lipids, the trafficking of cholesterol, and peroxisome biogenesis in mammalian cells. However, how lipids are exchanged between these two organelles is not understood. In this study, we report that the ER-resident VAMP-associated proteins A and B (VAPA and VAPB) interact with the peroxisomal membrane protein acyl-CoA binding domain containing 5 (ACBD5) and that this interaction is required to tether the two organelles together, thereby facilitating the lipid exchange between them. Depletion of either ACBD5 or VAP expression results in increased peroxisome mobility, suggesting that VAP–ACBD5 complex acts as the primary ER–peroxisome tether. We also demonstrate that tethering of peroxisomes to the ER is necessary for peroxisome growth, the synthesis of plasmalogen phospholipids, and the maintenance of cellular cholesterol levels. Collectively, our data highlight the importance of VAP–ACBD5–mediated contact between the ER and peroxisomes for organelle maintenance and lipid homeostasis.

Rab7 palmitoylation is required for efficient endosome-to-TGN trafficking

ABSTRACT Retromer is a multimeric protein complex that mediates endosome-to-trans-Golgi network (TGN) and endosome-to-plasma membrane trafficking of integral membrane proteins. Dysfunction of this complex has been linked to Alzheimers disease and Parkinsons disease. The recruitment of retromer to endosomes is regulated by Rab7 (also known as RAB7A) to coordinate endosome-to-TGN trafficking of cargo receptor complexes. Rab7 is also required for the degradation of internalized integral membrane proteins, such as the epidermal growth factor receptor (EGFR). We found that Rab7 is palmitoylated and that this modification is not required for membrane anchoring. Palmitoylated Rab7 colocalizes efficiently with and has a higher propensity to interact with retromer than nonpalmitoylatable Rab7. Rescue of Rab7 knockout cells by expressing wild-type Rab7 restores efficient endosome-to-TGN trafficking, while rescue with nonpalmitoylatable Rab7 does not. Interestingly, Rab7 palmitoylation does not appear to be required for the degradation of EGFR or for its interaction with its effector, Rab-interacting lysosomal protein (RILP). Overall, our results indicate that Rab7 palmitoylation is required for the spatiotemporal recruitment of retromer and efficient endosome-to-TGN trafficking of the lysosomal sorting receptors. Summary: Rab7 undergoes palmitoylation, a post-translational modification required for endosome-to-TGN trafficking but not for other Rab7 functions.

Cardiolipin synthesizing enzymes form a complex that interacts with cardiolipin-dependent membrane organizing proteins

The mitochondrial glycerophospholipid cardiolipin plays important roles in mitochondrial biology. Most notably, cardiolipin directly binds to mitochondrial proteins and helps assemble and stabilize mitochondrial multi-protein complexes. Despite their importance for mitochondrial health, how the proteins involved in cardiolipin biosynthesis are organized and embedded in mitochondrial membranes has not been investigated in detail. Here we show that human PGS1 and CLS1 are constituents of large protein complexes. We show that PGS1 forms oligomers and associates with CLS1 and PTPMT1. Using super-resolution microscopy, we observed well-organized nanoscale structures formed by PGS1. Together with the observation that cardiolipin and CLS1 are not required for PGS1 to assemble in the complex we predict the presence of a PGS1-centered cardiolipin-synthesizing scaffold within the mitochondrial inner membrane. Using an unbiased proteomic approach we found that PGS1 and CLS1 interact with multiple cardiolipin-binding mitochondrial membrane proteins, including prohibitins, stomatin-like protein 2 and the MICOS components MIC60 and MIC19. We further mapped the protein-protein interaction sites between PGS1 and itself, CLS1, MIC60 and PHB. Overall, this study provides evidence for the presence of a cardiolipin synthesis structure that transiently interacts with cardiolipin-dependent protein complexes.

Ubiquitin orchestrates proteasome dynamics between proliferation and quiescence in yeast

Proteasomes are key protease complexes responsible for protein degradation, and their localization changes with the growth conditions. This work in yeast shows that proteasomes exit the nucleus with the transition from proliferation to quiescence. Ubiquitin is a key player in proteasome dynamics and cytoplasmic proteasome granule formation.

mTORC1 controls glycogen synthase kinase 3β nuclear localization and function

Glycogen synthase kinase 3β (GSK3β) phosphorylates and regulates a wide range of substrates involved in diverse cellular functions. Some GSK3β substrates, such as c-myc and snail, are nuclear-resident transcription factors, suggesting possible control of GSK3β function by regulation of its nuclear localization. Inhibition of mechanistic target of rapamycin (mTORC1) led to partial redistribution of GSK3β from the cytosol to the nucleus, and GSK3β-dependent reduction of the expression of c-myc and snail. mTORC1 is controlled by metabolic cues, such as by AMP-activated protein kinase (AMPK) or amino acid abundance. Indeed AMPK activation or amino acid deprivation promoted GSK3β nuclear localization in an mTORC1-dependent manner. GSK3β was detected in several distinct endomembrane compartments, including lysosomes. Consistently, disruption of late endosomes/lysosomes through perturbation of Rab7 resulted in loss of GSK3β from lysosomes, and enhanced GSK3β nuclear localization as well as GSK3β-dependent reduction of c-myc levels. This indicates that GSK3β nuclear localization and function is suppressed by mTORC1, and suggests a new link between metabolic conditions sensed by mTORC1 and GSK3β-dependent regulation of transcriptional networks controlling biomass production. Summary statement (15-30 words) GSK3β nuclear localization and function is negatively regulated by the metabolic and mitogenic sensor mTORC1. mTORC1 control of GSK3β localization requires Rab7 and lysosomal membrane traffic.

Single-molecule localization microscopy of septin bundles in mammalian cells

Septins are a conserved family of GTPases that associate with numerous components of the cytoskeleton and the inner leaflet of the plasma membrane. These proteins are involved in many biological processes, including cell division and membrane trafficking, and serving as a scaffolding component of the cytoskeleton used to recruit other proteins and form diffusion barriers to maintain the composition of membrane domains. In order to carry out their cellular functions, septins undergo interactions via their NC or G interfaces to form heteromeric rod‐like structures that can polymerize into filaments and associate laterally into bundles. While electron microscopy studies of affinity‐tagged and purified Saccharomyces cerevisiae septin complexes have provided evidence for this periodic organization and in‐registry lateral bundling in vitro, the in‐vivo arrangement of stress fiber‐associated septin bundles in mammalian cells remains poorly characterized. We report here on a direct stochastic optical reconstruction microscopy and photoactivated localization microscopy study of the 2D spatial distribution of septins in mammalian cells. From simulated and experimental results, we show the effects of labeling method, labeling efficiency, and fluorescent emitter photophysics on image reconstruction and interpretation. Our experimental results are consistent with septin organization by polymerization of hetero‐octamers and an approximate 30–35 nm periodicity between subsequent units of SEPT2–SEPT2 or SEPT9–SEPT9.