Manuel Muñiz

COPI-mediated retrograde transport is essential for maintaining homeostasis between the endoplasmic reticulum (ER) and the Golgi in eukaryotic cells. However, its spatiotemporal dynamics remain unclear. In budding yeast, ER export occurs through a “hug-and-kiss” mechanism, wherein mobile cis -Golgi network (CGN) elements transiently approach ER exit sites (ERES) to capture cargo before departing to deliver it to the Golgi. Here, using high-resolution 3D and 4D live imaging, we show that the COPI-mediated recycling of a cargo receptor from the CGN back to the ER occurs during the hug-and-kiss event. Furthermore, this recycling is complete before the CGN detaches from the ERES, suggesting that COPI-mediated retrograde transport is spatiotemporally coupled to ER export during a single and transient CGN-ERES association. Consistent with this model, when COPI function is impaired, the receptor fails to return efficiently to the ER and instead remains in the CGN after separation from the ERES. Additionally, COPI impairment prolonged CGN-ERES association, suggesting that COPI function also facilitates CGN detachment from ERES. Thus, we propose that rapid COPI-mediated recycling drives CGN maturation directly at the ERES interface, enabling the CGN to detach and proceed along the secretory pathway.

Newly synthesized secretory proteins and lipids are transported from the endoplasmic reticulum (ER) to the Golgi prior to their ultimate destinations, which is tightly regulated during adaptation to environmental stress. However, regulatory pathways governing the formation of COPII vesicles budded from the ER remain insufficiently explored. Here, we present evidence indicating that COPII-mediated vesicle transport is transcriptionally controlled through the phosphatidic acid (PA)-dependent Opi1-Ino2/Ino4 regulatory circuit. Our analysis shows that YIP3, a target gene of Ino2/Ino4, exerts a negative regulatory impact on COPII-mediated vesicle transport. We demonstrate that Ino2/Ino4, but not Yip3 modulates Sar1 activation, the initial step in COPII vesicle formation, whereas Yip3 hinders Sec16 assembly on the ER membrane, thereby implying that Ino2/Ino4 governs COPII vesicle formation at multiple steps. Finally, we show that under ER stress conditions which are accompanied by elevated PA, vesicular transport is restricted in a PA and Yip3-dependent manner. Thus, this study provides the first evidence for an ER sensing system that transcriptionally fine-tunes vesicle formation in response to alterations in lipid composition of the ER membrane during ER stress.

Lipophagy is an important microautophagic process that degrades lipid droplets (LDs) to mobilize stored lipids as an energy source during nutrient starvation. However, the molecular mechanisms regulating lipophagy in response to nutrient starvation remain poorly understood. We found that budding yeast mutants defective in glycosylphosphatidylinositol (GPI) lipid remodeling exhibited aberrant accumulation of lipid droplets (LDs) and neutral lipids under glucose starvation. Our data suggest that the accumulation results from a failure of vacuolar liquid-ordered (Lo) domain-mediated lipophagy. Furthermore, we demonstrated that glycosylphosphatidylinositol-anchored proteins (GPI-APs) localize to vacuoles in response to glucose depletion and that a mutant defective in endocytosis has defects in both vacuolar Lo domain formation and lipophagy. These results imply that GPI lipid remodeling is required for Lo domain-mediated lipophagy upon glucose starvation. We propose that endocytosis functions to supply the lipid portion of GPI-APs, remodeled to C26 diacylglycerol, to the vacuolar membrane for Lo domain formation. Summary Statement: Our data suggest that the endocytic transport of GPI-APs remodeled with C26 diacylglycerol to the vacuole is required for vacuolar Lo domain formation and subsequent lipophagy in response to glucose deprivation. This reveals the essential role of GPI lipid remodeling in ensuring lipophagy to adapt to changes in nutrient availability.

Newly synthesized secretory proteins and many lipids are transported from the endoplasmic reticulum (ER) to the Golgi prior to their ultimate destinations. The ER-to-Golgi transport must be tightly regulated during adaptation to environmental stress. However, the sensing mechanism and regulatory pathways governing the consecutive formation, budding and transportation of COPII vesicles from the ER remain insufficiently explored. Here, we present evidence indicating that COPII-mediated vesicle transport is transcriptionally controlled through the phosphatidic acid-dependent Opi1-Ino2/Ino4 regulatory circuit. Our analysis indicates that YIP3 , a target gene of Ino2/Ino4, exerts a negative regulatory impact on COPII-mediated vesicle transport. Furthermore, we demonstrated that Ino2/Ino4 but not Yip3 modulates Sar1 activation, the initial step in COPII vesicle formation, whereas Yip3 hinders Sec16 assembly on the ER membrane, thereby implying that Ino2/Ino4 governs COPII-mediated trafficking at multiple steps. Thus, this study provides the first evidence for an ER sensing system that transcriptionally fine-tunes multiple steps of anterograde vesicular transport in response to alterations in lipid composition of the ER membrane.

Cells sense and control the number and quality of their organelles, but the underlying mechanisms of this regulation are not understood. Our recent research in the yeast Saccharomyces cerevisiae has shown that long acyl chain ceramides in the endoplasmic reticulum (ER) membrane and the lipid moiety of glycosylphosphatidylinositol (GPI) anchor determine the sorting of GPI-anchored proteins in the ER. Here, we show that a mutant strain, which produces shorter ceramides than the wild-type strain, displays a different count of Golgi cisternae. Moreover, deletions of proteins that remodel the lipid portion of GPI anchors resulted in an abnormal number of Golgi cisternae. Thus, our study reveals that protein sorting in the ER plays a critical role in maintaining Golgi biogenesis. The endoplasmic reticulum (ER) serves not only as a site of coat protein complex II vesicle formation but also as a site of protein sorting in budding yeast. Our data show that mutant strains defective in protein sorting in the ER affect the numbers of Golgi cisternae. This finding suggests that protein sorting plays a critical role in maintaining Golgi biogenesis. image

GPI anchoring is an essential post-translational modification in eukaryotes that links proteins to the plasma membrane. In this issue, Liu et al. (2023. J. Cell Biol.https://doi.org/10.1083/jcb.202208159) suggest, for the first time, a regulation on demand of the GPI glycolipid precursor biosynthesis.

Multisubunit Tethering Complexes (MTCs) are a set of conserved protein complexes that tether vesicles at the acceptor membrane. Interactions with other components of the trafficking machinery regulate MTCs through mechanisms that are partially understood. Here, we systematically investigate the interactome that regulates MTCs. We report that P4‐ATPases, a family of lipid flippases, interact with MTCs that participate in the anterograde and retrograde transport at the Golgi, such as TRAPPIII. We use the P4‐ATPase Drs2 as a paradigm to investigate the mechanism and biological relevance of this interplay during transport of Atg9 vesicles. Binding of Trs85, the sole‐specific subunit of TRAPPIII, to the N‐terminal tail of Drs2 stabilizes TRAPPIII on membranes loaded with Atg9 and is required for Atg9 delivery during selective autophagy, a role that is independent of P4‐ATPase canonical functions. This mechanism requires a conserved I(S/R)TTK motif that also mediates the interaction of the P4‐ATPases Dnf1 and Dnf2 with MTCs, suggesting a broader role of P4‐ATPases in MTC regulation.