Biology subjects

Michaillat Mayer, L.

Publications and source records attributed to Michaillat Mayer, L..

4 recordsLinked to original sources

bioRxiv2026

The GTPase activating protein Gyp6 binds Retromer and inactivates Rab7/Ypt7 to coordinate the formation of endosomal carriers

The Retromer coat is conserved in all eukaryotes and is crucial for the correct intracellular sorting of many transmembrane receptors and lysosomal hydrolases. Retromer is an effector of the late endosomal small GTPase RAB7 and is also implicated in its inactivation required for proper endosomal maturation. Here, we explore the role of controlled GTP hydrolysis by the RAB7 ortholog Ypt7 in the formation of Retromer-coated membrane carriers in yeast. Proximity labelling and genetic ablation identify the GTPase Activating Protein (GAP) Gyp6 as a critical regulator of Ypt7 activity in the context or Retromer. Structural studies show that Retromer recruits Gyp6 through its Vps29 subunit, which recognises a specific PL motif and a secondary binding site in the C-terminal domain of Gyp6. This interaction does not occur with other yeast GAPs. Ablation of the Gyp6-Retromer interface or the catalytic activity of Gyp6 leads to the accumulation of tubular structures on endo-lysosomal compartments and to increased association of Ypt7 with Retromer and its cargo Vps10. These results support a model in which Gyp6 controls the switch from Ypt7-dependent Retromer coat assembly and cargo collection to the departure of the carrier through membrane fission and uncoating.

cell biology↗

bioRxiv2025

Hybrid endosomal coats containing different classes of sorting nexins

Endosomes are protein sorting stations where multiple coats form tubulovesicular carriers exporting proteins to the Golgi, the plasma membrane, or endo-lysosomal compartments. Distinct classes of sorting nexins are assumed to form distinct homogeneous coats that define the endosomal sorting routes and their cargos. Snx3 and the SNX-BARs Vps5-Vps17 belong to different sorting nexin classes. They form homogeneous Retromer-dependent coats that differ in structure and their modes of membrane association and cargo recognition. Here, we show the formation of hybrid coats between purified SNX-BARs, Snx3 and their cargos. Hybrid coats assemble at variable subunit ratios and diameters and show greater membrane scaffolding activity than homogeneous coats. In vivo, Snx3 and SNX-BARs colocalise and mutually impact the sorting of their respective cargos. Although simultaneous binding of Snx3- and SNX-BARs to Retromer is sterically prohibited, hybrid coats incorporate both SNXs in a common complex, probably linked by Retromer oligomerisation. We hence propose that SNX-BARs and Snx3 form Retromer-mediated hybrid coats in novel, stoichiometrically adaptable configurations that allow to adjust endosomal carriers for transporting varying ratios of cargo.

cell biology↗

bioRxiv2025

Acidocalcisome-like vacuoles constitute a feedback-controlled phosphate buffering system for the cytosol

Cells experience strong variations in the consumption and availability of inorganic phosphate (Pi). Since Pi is an essential macronutrient but excess Pi has negative impacts on nucleotide hydrolysis and metabolism, its concentration must be maintained in a suitable range. Conserved storage organelles, acidocalcisomes, provide this buffering function. We used acidocalcisome-like yeast vacuoles to study how such organelles are set up to for this task. Our combined in vitro and in vivo analyses revealed that their ATP-driven polyphosphate polymerase VTC converts cytosolic Pi into inorganic polyphosphates (polyP), which it transfers into the vacuole lumen. Luminal polyphosphatases immediately hydrolyse this polyP to establish a growing reservoir of vacuolar Pi. Product inhibition by this Pi pool silences the polyphosphatases, caps Pi accumulation, and favours vacuolar polyP storage. Upon cytosolic Pi scarcity, the declining inositol pyrophosphate levels activate the vacuolar Pi exporter Pho91 to replenish cytosolic Pi. In this way, acidocalcisome-like vacuoles constitute a feedback-regulated buffering system for cytosolic Pi, which the cells can switch between Pi accumulation, Pi release, and high-capacity phosphate storage through polyP.

cell biology↗

bioRxiv2023

Inositol pyrophosphate dynamics in yeast reveals control of the PHO starvation program through 1,5-IP8 and the SPX domain of the CDK inhibitor Pho81

Eukaryotic cells control inorganic phosphate to balance its role as essential macronutrient with its negative bioenergetic impact on reactions liberating phosphate. Phosphate homeostasis depends on the conserved INPHORS signaling pathway that utilizes inositol pyrophosphates (IPPs) and SPX receptor domains. Since cells synthesize various IPPs and SPX domains bind them promiscuously, it is unclear whether a specific IPP regulates SPX domains in vivo, or whether multiple IPPs act as a pool. In contrast to previous models, which postulated that phosphate starvation is signaled by increased production of the IPP 1-IP7, we now show that the levels of all detectable IPPs of yeast, 1-IP7, 5-IP7 and 1,5-IP8, strongly decline upon phosphate starvation. Among these, specifically the decline of 1,5-IP8 triggers the transcriptional phosphate starvation response, the PHO pathway. 1,5-IP8 inactivates the cyclin-dependent kinase inhibitor Pho81 through its SPX domain. This stimulates the cyclin-dependent kinase Pho85-Pho80 to phosphorylate the transcription factor Pho4 and repress the PHO pathway. Combining our results with observations from other systems we propose a unified model where 1,5-IP8 signals cytosolic phosphate abundance to SPX proteins in fungi, plants, and mammals. Its absence triggers starvation responses. Significance statementCytosolic Pi is of prime importance for cellular bioenergetics because Pi influences free energy of nucleotide hydrolysis and the metabolite fluxes through glycolysis and oxidative phosphorylation. Eukaryotic cells use the INPHORS pathway to signal Pi via SPX domains and their ligands, inositol pyrophosphates (IP7, IP8), which control Pi homeostasis through a network of target proteins that import, export, store or detoxify Pi. Studies with different systems failed to yield a coherent model on this regulation. We performed the first time-resolved profiling of the full isomer spectrum of inositol pyrophosphates in yeast and dissected the isomer that is relevant to intracellular Pi signaling. Our results can be combined with existing observations from plants, mammals, and other fungi to support a unified model of Pi signaling across all eukaryotic kingdoms, which is in accord with the fundamental importance of Pi management for metabolism.

cell biology↗