Tricalbin proteins regulate plasma membrane phospholipid homeostasis

ER-plasma membrane contact sites deliver ER lipids and proteins for rapid cell surface expansion

As a consequence of hypoosmotic shock, yeast cells swell rapidly and increase the surface area by ∼20% in 20 s. Approximately, 35% of this surface increase is mediated by the ER-plasma membrane contact sites, specifically the tricalbins, which are required for the delivery of both lipids and the GPI-anchored protein Crh2 from the cortical ER to the plasma membrane. Therefore, we propose a new function for the tricalbins: mediating the fusion of the ER to the plasma membrane at contact sites. This proposed fusion is triggered by calcium influx via the stretch-gated channel Cch1 and is supported by the anoctamin Ist2.

Role of cell membrane homeostasis in the pathogenicity of pathogenic filamentous fungi

The cell membrane forms a fundamental part of all living cells and participates in a variety of physiological processes, such as material exchange, stress response, cell recognition, signal transduction, cellular immunity, apoptosis, and pathogenicity. Here, we review the mechanisms and functions of the membrane structure (lipid components of the membrane and the biosynthesis of unsaturated fatty acids), membrane proteins (transmembrane proteins and proteins contributing to membrane curvature), transcriptional regulation, and cell wall components that influence the virulence and pathogenicity of filamentous fungi.

Mitochondria-ER-PM contacts regulate mitochondrial division and PI(4)P distribution

The mitochondria-ER-cortex anchor (MECA) forms a tripartite membrane contact site between mitochondria, the endoplasmic reticulum (ER), and the plasma membrane (PM). The core component of MECA, Num1, interacts with the PM and mitochondria via two distinct lipid-binding domains; however, the molecular mechanism by which Num1 interacts with the ER is unclear. Here, we demonstrate that Num1 contains a FFAT motif in its C-terminus that interacts with the integral ER membrane protein Scs2. While dispensable for Num1's functions in mitochondrial tethering and dynein anchoring, the FFAT motif is required for Num1's role in promoting mitochondrial division. Unexpectedly, we also reveal a novel function of MECA in regulating the distribution of phosphatidylinositol-4-phosphate (PI(4)P). Breaking Num1 association with any of the three membranes it tethers results in an accumulation of PI(4)P on the PM, likely via disrupting Sac1-mediated PI(4)P turnover. This work establishes MECA as an important regulatory hub that spatially organizes mitochondria, ER, and PM to coordinate crucial cellular functions.

SYNAPTOTAGMIN 4 is expressed mainly in the phloem and participates in abiotic stress tolerance in Arabidopsis

Plant synaptotagmins structurally resemble animal synaptotagmins and extended-synaptotagmins. Animal synaptotagmins are well-characterized calcium sensors in membrane trafficking, and extended-synaptotagmins mediate lipid transfer at the endoplasmic reticulum-plasma membrane contact sites. Here, we characterize SYNAPTOTAGMIN 4 (SYT4), which belongs to the six-member family in Arabidopsis. Fluorometric GUS assay showed that the SYT4 promoter was strongest in roots and the least active in rosettes and cauline leaves, which was confirmed by qPCR. In seedlings, promoter activity was influenced by several factors, such as plant growth regulators, mannitol, sucrose, polyethylene glycol and cold. GUS histochemistry revealed SYT4 promoter activity in the phloem of all organs and even almost exclusively in sieve element precursors and differentiating sieve elements. Accordingly, the SYT-GFP fusion protein also accumulated in these cells with maximal abundance in sieve element precursors. The protein formed a network in the cytoplasm, but during sieve tube differentiation, it deposited at the cell periphery and disappeared from mature tubes. Using photoconvertible fluorescence technology, we showed that a high abundance of SYT4 protein in meristematic protophloem cells was due to its extensive synthesis. SYT4 protein synthesis was interrupted in differentiating sieve elements, but protein degradation was also reduced. In addition to phloem, the fusion protein was detected in shoot and root stem cell niche as early as the late heart stage of the embryo. We isolated and molecularly and biologically characterized five syt4 T-DNA insertion alleles and subjected them to phenotype analysis. The allele with the C2B domain interrupted by an T-DNA insertion exhibits increased sensitivity to factors such as auxins, osmotics, salicylic acid, sodium chloride, and the absence of sucrose in the root growth test.

Different ER-plasma membrane tethers play opposing roles in autophagy of the cortical ER

The endoplasmic reticulum (ER) undergoes degradation by selective macroautophagy (ER-phagy) in response to starvation or the accumulation of misfolded proteins within its lumen. In yeast, actin assembly at sites of contact between the cortical ER (cER) and endocytic pits acts to displace elements of the ER from their association with the plasma membrane (PM) so they can interact with the autophagosome assembly machinery near the vacuole. A collection of proteins tether the cER to the PM. Of these, Scs2/22 and Ist2 are required for cER-phagy, most likely through their roles in lipid transport, while deletion of the tricalbins, TCB1/2/3, bypasses those requirements. An artificial ER-PM tether blocks cER-phagy in both the wild type (WT) and a strain lacking endogenous tethers, supporting the importance of cER displacement from the PM. Scs2 and Ist2 can be cross-linked to the selective cER-phagy receptor, Atg40. The COPII cargo adaptor subunit, Lst1, associates with Atg40 and is required for cER-phagy. This requirement is also bypassed by deletion of the ER-PM tethers, suggesting a role for Lst1 prior to the displacement of the cER from the PM during cER-phagy. Although pexophagy and mitophagy also require actin assembly, deletion of ER-PM tethers does not bypass those requirements. We propose that within the context of rapamycin-induced cER-phagy, Scs2/22, Ist2, and Lst1 promote the local displacement of an element of the cER from the cortex, while Tcb1/2/3 act in opposition, anchoring the cER to the plasma membrane.

Let it stick: Strategies and applications for intracellular plasma membrane targeting of proteins in Saccharomyces cerevisiae

Lipid binding domains and protein lipidations are essential features to recruit proteins to intracellular membranes, enabling them to function at specific sites within the cell. Membrane association can also be exploited to answer fundamental and applied research questions, from obtaining insights into the understanding of lipid metabolism to employing them for metabolic engineering to redirect fluxes. This review presents a broad catalog of membrane binding strategies focusing on the plasma membrane of Saccharomyces cerevisiae. Both lipid binding domains (pleckstrin homology, discoidin-type C2, kinase associated-1, basic-rich and bacterial phosphoinositide-binding domains) and co- and post-translational lipidations (prenylation, myristoylation and palmitoylation) are introduced as tools to target the plasma membrane. To provide a toolset of membrane targeting modules, respective candidates that facilitate plasma membrane targeting are showcased including their in vitro and in vivo properties. The relevance and versatility of plasma membrane targeting modules are further highlighted by presenting a selected set of use cases.

Dynamic changes in endoplasmic reticulum morphology and its contact with the plasma membrane in motor neurons in response to nerve injury

The endoplasmic reticulum (ER) extends throughout a cell and plays a critical role in maintaining cellular homeostasis. Changes in ER shape could provide a clue to explore the mechanisms that underlie the fate determination of neurons after axon injury because the ER drastically changes its morphology under neuronal stress to maintain cellular homeostasis and recover from damage. Because of their tiny structures and richness in the soma, the detailed morphology of the ER and its dynamics have not been well analysed. In this study, the focused ion beam/scanning electron microscopy (FIB/SEM) analysis was performed to explore the ultra-structures of the ER in the somata of motor neuron with axon regenerative injury models. In normal motor neurons, ER in the somata is abundantly localised near the perinucleus and represents lamella-like structures. After injury, analysis of the ER volume and ER branching points indicated a collapse of the normal distribution and a transformation from lamella-like structures to mesh-like structures. Furthermore, accompanied by ER accumulation near the plasma membrane (PM), the contact between the ER and PM (ER-PM contacts) significantly increased after injury. The accumulation of extended-synaptotagmin 1 (E-Syt1), a tethering protein of the ER and PM that regulates Ca2+-dependent lipid transfer, was also identified by immunohistochemistry and quantitative Real-time PCR after injury. These morphological alterations of ER and the increase in ER-PM contacts may be crucial events that occur in motor neurons as a resilient response for the survival after axonal injury.

Effects of biophysical membrane properties on recognition of phosphatidylserine, or phosphatidylinositol 4-phosphate by lipid biosensors LactC2, or P4M

Lipid biosensors are molecular tools used both in vivo and in vitro applications, capable of selectively detecting specific types of lipids in biological membranes. However, despite their extensive use, there is a lack of systematic characterization of their binding properties in various membrane conditions. The purpose of this study was to investigate the impact of membrane properties, such as fluidity and membrane charge, on the sensitivity of two lipid biosensors, LactC2 and P4M, to their target lipids, phosphatidylserine (PS) or phosphatidylinositol 4-phosphate (PI4P), respectively. Dual-color fluorescence cross-correlation spectroscopy, employed in this study, provided a useful technique to investigate interactions of these recombinant fluorescent biosensors with liposomes of varying compositions. The results of the study demonstrate that the binding of the LactC2 biosensor to low levels of PS in the membrane is highly supported by the presence of anionic lipids or membrane fluidity. However, at high PS levels, the presence of anionic lipids does not further enhance binding of LactC2. In contrast, neither membrane charge, nor membrane fluidity significantly affect the binding affinity of P4M to PI4P. These findings provide valuable insights into the role of membrane properties on the binding properties of lipid biosensors.

Biological roles of plant synaptotagmins

Plant synaptotagmins (SYTs) are resident proteins of the endoplasmic reticulum (ER). They are characterized by an N-terminal transmembrane region and C2 domains at the C-terminus, which tether the ER to the plasma membrane (PM). In addition to their tethering role, SYTs contain a lipid-harboring SMP domain, essential for shuttling lipids between the ER and the PM. There is now abundant literature on Arabidopsis SYT1, the best-characterized family member, which link it to biotic and abiotic responses as well as to ER morphology. Here, we review the current knowledge of SYT members, focusing on their role in stress, and discuss how these roles can be related to their tethering and lipid transport functions. Finally, we contextualize this information about SYTs with their homologs, the yeast tricalbins and the mammalian extended synaptotagmins.

Phosphatidylserine transport in cell life and death

Phosphatidylserine (PS) is a negatively charged glycerophospholipid found mainly in the plasma membrane (PM) and in the late secretory/endocytic compartments, where it regulates cellular activity and can mediate apoptosis. Export of PS from the endoplasmic reticulum, its site of synthesis, to other compartments, and its transbilayer asymmetry must therefore be precisely regulated. We review recent findings on nonvesicular transport of PS by lipid transfer proteins (LTPs) at membrane contact sites, on PS flip-flop between membrane leaflets by flippases and scramblases, and on PS nanoclustering at the PM. We also discuss emerging data on cooperation between scramblases and LTPs, how perturbation of PS distribution can lead to disease, and the specific role of PS in viral infection.

Multiple tethers of organelle contact sites are involved in α-synuclein toxicity in yeast

The protein α-synuclein (α-syn) is one of the major factors linked to Parkinson's disease, yet how its misfolding and deposition contribute to the pathology remains largely elusive. Recently, contact sites among organelles were implicated in the development of this disease. Here, we used the budding yeast Saccharomyces cerevisiae, in which organelle contact sites have been characterized extensively, as a model to investigate their role in α-syn cytotoxicity. We observed that lack of specific tethers that anchor the endoplasmic reticulum to the plasma membrane resulted in cells with increased resistance to α-syn expression. Additionally, we found that strains lacking two dual-function proteins involved in contact sites, Mdm10 and Vps39, were resistant to the expression of α-syn. In the case of Mdm10, we found that this is related to its function in mitochondrial protein biogenesis and not to its role as a contact site tether. In contrast, both functions of Vps39, in vesicular transport and as a tether of the vacuole-mitochondria contact site, were required to support α-syn toxicity. Overall, our findings support that interorganelle communication through membrane contact sites is highly relevant for α-syn-mediated toxicity.

Systematic analysis of membrane contact sites in Saccharomyces cerevisiae uncovers modulators of cellular lipid distribution

Actively maintained close appositions between organelle membranes, also known as contact sites, enable the efficient transfer of biomolecules between cellular compartments. Several such sites have been described as well as their tethering machineries. Despite these advances we are still far from a comprehensive understanding of the function and regulation of most contact sites. To systematically characterize contact site proteomes, we established a high-throughput screening approach in Saccharomyces cerevisiae based on co-localization imaging. We imaged split fluorescence reporters for six different contact sites, several of which are poorly characterized, on the background of 1165 strains expressing a mCherry-tagged yeast protein that has a cellular punctate distribution (a hallmark of contact sites), under regulation of the strong TEF2 promoter. By scoring both co-localization events and effects on reporter size and abundance, we discovered over 100 new potential contact site residents and effectors in yeast. Focusing on several of the newly identified residents, we identified three homologs of Vps13 and Atg2 that are residents of multiple contact sites. These proteins share their lipid transport domain, thus expanding this family of lipid transporters. Analysis of another candidate, Ypr097w, which we now call Lec1 (Lipid-droplet Ergosterol Cortex 1), revealed that this previously uncharacterized protein dynamically shifts between lipid droplets and the cell cortex, and plays a role in regulation of ergosterol distribution in the cell. Overall, our analysis expands the universe of contact site residents and effectors and creates a rich database to mine for new functions, tethers, and regulators.