LYCHOS is a human hybrid of a plant-like PIN transporter and a GPCR

Mechanisms of auxin action in plant growth and development

The phytohormone auxin is a major signal coordinating growth and development in plants. The variety of its effects arises from its ability to form local auxin maxima and gradients within tissues, generated through directional cell-to-cell transport and elaborate metabolic control. These auxin distribution patterns instruct cells in a context-dependent manner to undergo predefined developmental transitions. In this Review, we discuss advances in auxin action at the level of homeostasis and signalling. We highlight key insights into the structural basis of PIN-mediated intercellular auxin transport and explore two novel non-transcriptional auxin signalling mechanisms: one involving intracellular Ca2+ transients and another involving cell-surface auxin perception that mediates global, ultrafast phosphorylation. Furthermore, we examine emerging evidence indicating the involvement of cyclic adenosine monophosphate as a second messenger in the transcriptional auxin response. Together, these recent developments in auxin research have profoundly deepened our understanding of the complex and diverse activities of auxin in plant growth and development.

Molecular architecture of human LYCHOS involved in lysosomal cholesterol signaling

Lysosomal membrane protein LYCHOS (lysosomal cholesterol signaling) translates cholesterol abundance to mammalian target of rapamycin activation. Here we report the 2.11-Å structure of human LYCHOS, revealing a unique fusion architecture comprising a G-protein-coupled receptor (GPCR)-like domain and a transporter domain that mediates homodimer assembly. The NhaA-fold transporter harbors a previously uncharacterized intramembrane Na+ pocket. The GPCR-like domain is stabilized, by analogy to canonical GPCRs, in an inactive state through 'tethered antagonism' by a lumenal loop and strong interactions at the cytosol side preventing the hallmark swing of the sixth transmembrane helix seen in active GPCRs. A cholesterol molecule and an associated docosahexaenoic acid (DHA)-phospholipid are entrapped between the transporter and GPCR-like domains, with the DHA-phospholipid occupying a pocket previously implicated in cholesterol sensing, indicating inter-domain coupling via dynamic lipid-protein interactions. Our work provides a high-resolution framework for functional investigations of the understudied LYCHOS protein.

The role of indole-3-acetic acid and characterization of PIN transporters in complex streptophyte alga Chara braunii

Auxin, indole-3-acetic acid (IAA), is a key phytohormone with diverse morphogenic roles in land plants, but its function and transport mechanisms in algae remain poorly understood. We therefore aimed to explore the role of IAA in a complex, streptophyte algae Chara braunii. Here, we described novel responses of C. braunii to IAA and characterized two homologs of PIN auxin efflux carriers: CbPINa and CbPINc. We determined their localization in C. braunii using epitope-specific antibodies and tested their function in heterologous land plant models. Further, using phosphoproteomic analysis, we identified IAA-induced phosphorylation events. The thallus regeneration assay showed that IAA promotes thallus elongation and side branch development. Immunolocalization of CbPINa and CbPINc confirmed their presence on the plasma membrane of vegetative and generative cells of C. braunii. However, functional assays in tobacco BY-2 cells demonstrated that CbPINa affects auxin transport, whereas CbPINc does not. The IAA is effective in the acceleration of cytoplasmic streaming and the phosphorylation of evolutionary conserved targets such as homolog of RAF-like kinase. These findings suggest that, although canonical PIN-mediated auxin transport mechanisms might not be fully conserved in Chara, IAA is involved in morphogenesis and fast signaling processes.

Multiple Pendants-Bearing Triglucosides for Membrane Protein Studies: Effects of Pendant Length and Number on Micelle Interior Hydration and Protein Stability

Membrane proteins play central roles in cell physiology and are the targets of over 50% of FDA-approved drugs. In the present study, we prepared single alkyl-chained triglucosides decorated with multiple pendants, designated multiple pendant-bearing glucosides (MPGs), to enhance membrane protein stability. The new detergents feature two and four pendants of varying size at the hydrophilic-lipophilic interfaces, designated MPG-Ds and MPG-Ts, respectively. When tested with model membrane proteins, including the human adrenergic receptor (β2AR), the tetra-pendant-bearing MPGs (MPG-Ts) demonstrated superior performance compared to the dipendant analogs (MPG-Ds) and the gold standard DDM. All-atom molecular dynamics (MD) simulations results reveal that the four-pendant configuration of this detergent is remarkably effective in excluding water from the hydrophobic micelle interiors compared to the dipendant MPGs and DDM, an unprecedented feature of this new detergent. Our findings provide a novel strategy for designing water-resistant detergents, advancing the field of membrane protein research.

Structural basis for cholesterol sensing of LYCHOS and its interaction with indoxyl sulfate

The lysosome serves as an essential nutrient-sensing hub within the cell, where the mechanistic target of rapamycin complex 1 (mTORC1) is activated. Lysosomal cholesterol signaling (LYCHOS), a lysosome membrane protein, has been identified as a cholesterol sensor that couples cholesterol concentration to mTORC1 activation. However, the molecular basis is unknown. Here, we determine the cryo-electron microscopy (cryo-EM) structure of human LYCHOS at a resolution of 3.1 Å, revealing a cholesterol-like density at the interface between the permease and G-protein coupled receptor (GPCR) domains. Advanced 3D classification reveals two distinct states of LYCHOS. Comparative structural analysis between these two states demonstrated a cholesterol-related movement of GPCR domain relative to permease domain, providing structural insights into how LYCHOS senses lysosomal cholesterol levels. Additionally, we identify indoxyl sulfate (IS) as a binding ligand to the permease domain, confirmed by the LYCHOS-IS complex structure. Overall, our study provides a foundation and indicates additional directions for further investigation of the essential role of LYCHOS in the mTORC1 signaling pathway.

Mechanisms and functions of lysosomal lipid homeostasis

Lysosomes are the central degradative organelle of mammalian cells and have emerged as major intersections of cellular metabolite flux. Macromolecules derived from dietary and intracellular sources are delivered to the acidic lysosomal lumen where they are subjected to degradation by acid hydrolases. Lipids derived from lipoproteins, autophagy cargo, or autophagosomal membranes themselves constitute major lysosomal substrates. Dysregulation of lysosomal lipid processing, defective export of lipid catabolites, and lysosomal membrane permeabilization underly diseases ranging from neurodegeneration to metabolic syndromes and lysosomal storage disorders. Mammalian cells are equipped with sophisticated homeostatic control mechanisms that protect the lysosomal limiting membrane from excessive damage, prevent the spillage of luminal hydrolases into the cytoplasm, and preserve the lysosomal membrane composition in the face of constant fusion with heterotypic organelles such as endosomes and autophagosomes. In this review we discuss the molecular mechanisms that govern lysosomal lipid homeostasis and, thereby, lysosome function in health and disease.

Lysosomal Ion Channels and Transporters: Recent Findings, Therapeutic Potential, and Technical Approaches

In recent years, there has been a growing interest in lysosomal ion channels and transporters due to their critical role in maintaining lysosomal function and their involvement in a variety of diseases, particularly lysosomal storage diseases, cancer, and neurodegenerative disorders. Recent advancements in research techniques, including manual and automated patch clamp (APC) electrophysiology, solid-supported membrane-based electrophysiology (SSME), and fluorescence-based ion imaging, have further enhanced our ability to investigate lysosomal ion channels and transporters in both physiological and pathological conditions, spurring drug discovery efforts. Several pharmaceutical companies are now developing therapies aimed at modulating these channels and transporters to improve lysosomal function in disease. Small molecules targeting channels like transient receptor potential mucolipin (TRPML) 1 and TMEM175, as well as drugs modulating lysosomal pH, are currently in preclinical and clinical development. This review provides an overview of the role of lysosomal ion channels and transporters in health and disease, highlights the cutting-edge techniques used to study them, and discusses the therapeutic potential of targeting these channels and transporters in the treatment of various diseases. Furthermore, in addition to summarizing recent discoveries, we contribute novel functional data on cystinosin, TRPML1, and two-pore channel 2 (TPC2), utilizing both SSME and APC approaches.

Illuminating cholesterol-mTORC1 signaling: LYCHOS in focus

In a recent issue of Nature, Bayly-Jones et al.1 report the first cryoelectron microscopy (cryo-EM) structure of the lysosomal transmembrane protein LYCHOS, which mediates cholesterol sensing by mTORC1. LYCHOS forms a homodimer, with cholesterol engagement at the transporter-GPCR domain interface, coupled to auxin binding at the transporter-like domain, suggesting multi-domain coordination as critical for cholesterol sensing.

Over 25 years of decrypting PIN-mediated plant development

Identification of PIN exporters for auxin, the major coordinative signal in plants, some 25 years ago, signifies a landmark in our understanding of plant-specific mechanisms underlying development and adaptation. Auxin is directionally transported throughout the plant body; a unique feature already envisioned by Darwin and solidified by PINs' discovery and characterization. The PIN-based auxin distribution network with its complex regulations of PIN expression, localization and activity turned out to underlie a remarkable multitude of developmental processes and represents means to integrate endogenous and environmental signals. Given the recent anniversary, we here summarize past and current developments in this exciting field.

Bound by the love for cholesterol: A transporter meets a GPCR

In a recently published article in Nature, Bayly-Jones et al. report the cryo-EM structures of a lysosomal cholesterol sensor, LYCHOS, also known as GPR155, which reveals a unique fusion of a plant auxin-transporter-like domain with a seven-transmembrane GPCR-like domain and elucidates mechanistic insights into cellular regulation of mTORC1 activity.

Cholesterol sensing goes vegetarian

The cholesterol-sensing protein LYCHOS is a hybrid of a GPCR and a plantlike transport protein.