Close proximities between organelles have been described for decades. However, only recently a specific field dealing with organelle communication at membrane contact sites has gained wide ...acceptance, attracting scientists from multiple areas of cell biology. The diversity of approaches warrants a unified vocabulary for the field. Such definitions would facilitate laying the foundations of this field, streamlining communication and resolving semantic controversies. This opinion, written by a panel of experts in the field, aims to provide this burgeoning area with guidelines for the experimental definition and analysis of contact sites. It also includes suggestions on how to operationally and tractably measure and analyze them with the hope of ultimately facilitating knowledge production and dissemination within and outside the field of contact-site research.
Membrane contact sites (MCS) are close appositions between two organelles that facilitate both signaling and the passage of ions and lipids from one cellular compartment to another. Despite the fact ...that MCS have been observed for over 50 years now, we still know very little about the molecular machinery required to create them or their structure, function and regulation. In this review, we focus on the three best-characterized contact sites to date: the nucleus–vacuole junction and mitochondria–ER and plasma membrane–ER contact sites. In addition, we discuss principles arising from recent research and highlight several unanswered questions in the field.
Mitochondrial shape and network formation have been primarily associated with the well-established processes of fission and fusion. However, recent research has unveiled an intricate and multifaceted ...landscape of mitochondrial morphology that extends far beyond the conventional fission–fusion paradigm. These less-explored dimensions harbor numerous unresolved mysteries. This review navigates through diverse processes influencing mitochondrial shape and network formation, highlighting the intriguing complexities and gaps in our understanding of mitochondrial architecture. The exploration encompasses various scales, from biophysical principles governing membrane dynamics to molecular machineries shaping mitochondria, presenting a roadmap for future research in this evolving field.
Membrane contact sites enable interorganelle communication by positioning organelles in close proximity using molecular “tethers.” With a growing understanding of the importance of contact sites, the ...hunt for new contact sites and their tethers is in full swing. Determining just what is a tether has proven challenging. Here, we aim to delineate guidelines that define the prerequisites for categorizing a protein as a tether. Setting this gold standard now, while groups from different disciplines are beginning to explore membrane contact sites, will enable efficient cooperation in the growing field and help to realize a great collaborative opportunity to boost its development.
Membrane contact sites enable interorganellar communication by positioning two organelles in proximity through molecular “tethers.” Determining just what is a tether has been challenging. Eisenberg-Bord, Shai et al. review known tether machineries and aim to delineate a gold-standard definition for categorizing a protein as a tether.
While targeting of proteins synthesized in the cytosol to any organelle is complex, mitochondria present the most challenging of destinations. First, import of nuclear-encoded proteins needs to be ...balanced with production of mitochondrial-encoded ones. Moreover, as mitochondria are divided into distinct subdomains, their proteins harbor a number of different targeting signals and biophysical properties. While translocation into the mitochondrial membranes has been well studied, the cytosolic steps of protein import remain poorly understood. Here, we review current knowledge on mRNA and protein targeting to mitochondria, as well as recent advances in our understanding of the cellular programs that respond to accumulation of mitochondrial precursor proteins in the cytosol, thus linking defects in targeting-capacity to signaling.
Mitochondrial proteins synthetized in the cytosol can be targeted to mitochondria at different stages of gene expression: as mRNAs, ribosome-nascent chain complexes, or complete precursor proteins.While almost all proteins use the same entry gate to the mitochondria, before and after it they can embark on different targeting and import pathways.Delays in mitochondrial protein import or mistargeting to other organelles affect cellular homeostasis; hence, cells have evolved specific mechanisms to sense and counteract such situations.Cytosolic chaperones promote mitochondrial protein import under normal conditions, as well as play a major role in stress response pathways associated with mitochondrial protein import defects.
Uncovering the mechanisms underlying robust responses of cells to stress is crucial for our understanding of cellular physiology. Indeed, vast amounts of data have been collected on transcriptional ...responses in Saccharomyces cerevisiae. However, only a handful of pioneering studies describe the dynamics of proteins in response to external stimuli, despite the fact that regulation of protein levels and localization is an essential part of such responses. Here we characterized unprecedented proteome plasticity by systematically tracking the localization and abundance of 5,330 yeast proteins at single-cell resolution under three different stress conditions (DTT, H2O2, and nitrogen starvation) using the GFP-tagged yeast library. We uncovered a unique "fingerprint" of changes for each stress and elucidated a new response arsenal for adapting to radical environments. These include bet-hedging strategies, organelle rearrangement, and redistribution of protein localizations. All data are available for download through our online database, LOQATE (localization and quantitation atlas of yeast proteome).
Translocation into the endoplasmic reticulum (ER) is an initial and crucial biogenesis step for all secreted and endomembrane proteins in eukaryotes. ER insertion can take place through the ...well-characterized signal recognition particle (SRP)-dependent pathway or the less-studied route of SRP-independent translocation. To better understand the prevalence of the SRP-independent pathway, we systematically defined the translocational dependence of the yeast secretome. By combining hydropathy-based analysis and microscopy, we uncovered that a previously unappreciated fraction of the yeast secretome translocates without the aid of the SRP. Furthermore, we validated a family of SRP-independent substrates—the glycosylphosphatidylinositol (GPI)-anchored proteins. Studying this family, we identified a determinant for ER targeting and uncovered a network of cytosolic proteins that facilitate SRP-independent targeting and translocation. These findings highlight the underappreciated complexity of SRP-independent translocation, which enables this pathway to efficiently cope with its extensive substrate flux.
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► SRP-independent translocation in yeast is as prevalent as targeting through SRP ► The C′-terminal GPI-anchoring sequence is an ER-targeting motif ► Several cytosolic Hsp40s chaperone preinserted GPI-anchored proteins ► The GET pathway targets GPI-anchored proteins to the ER
A systematic assessment of secreted yeast proteins shows that a significant portion does not rely on SRP for translocation into the ER. For GPI-anchored proteins, a network of cytosolic chaperones and ER-targeting factors guides translocation, highlighting the varied signals and pathways that contribute to secretome biogenesis.
Mitochondria have crucial functions in the cell, including ATP generation, iron‐sulfur cluster biogenesis, nucleotide biosynthesis, and amino acid metabolism. All of these functions require tight ...regulation on mitochondrial activity and homeostasis. As mitochondria biogenesis is controlled by the nucleus and almost all mitochondrial proteins are encoded by nuclear genes, a tight communication network between mitochondria and the nucleus has evolved, which includes signaling cascades, proteins which are dual‐localized to the two compartments, and sensing of mitochondrial products by nuclear proteins. All of these enable a crosstalk between mitochondria and the nucleus that allows the ‘ground control’ to get information on mitochondria's status. Such information facilitates the creation of a cellular balance of mitochondrial status with energetic needs. This communication also allows a transcriptional response in case mitochondrial function is impaired aimed to restore mitochondrial homeostasis. As mitochondrial dysfunction is related to a growing number of genetic diseases as well as neurodegenerative conditions and aging, elucidating the mechanisms governing the mitochondrial/nuclear communication should progress a better understanding of mitochondrial dysfunctions.
Mitochondria biogenesis is controlled by the nucleus and almost all mitochondrial proteins are encoded by nuclear genes. To this end, a tight communication network between mitochondria and the nucleus must exist. This review summarizes ways by which mitochondria communicate with the nucleus that include signaling cascades, proteins which are dual‐localized to the two compartments, and sensing of mitochondrial products by nuclear proteins. We also discuss how and why this communication is essential for maintaining mitochondrial homeostasis.
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