Stress granules: the last refuge of TORC1?

Sequestration of cellular native factors by biomolecular assemblies: Physiological or pathological?

In addition to native-state structures, biomolecules often form condensed supramolecular assemblies or cellular membraneless organelles that are critical for cell life. These biomolecular assemblies, generally including liquid-like droplets (condensates) and amyloid-like aggregates, can sequester or recruit their interacting partners, so as to either modulate various cellular behaviors or even cause disorders. This review article summarizes recent advances in the sequestration of native factors by biomolecular assemblies and discusses their potential consequences on cellular function, homeostasis, and disease pathology.

The roles of intrinsic disorder-based liquid-liquid phase transitions in the "Dr. Jekyll-Mr. Hyde" behavior of proteins involved in amyotrophic lateral sclerosis and frontotemporal lobar degeneration

Pathological developments leading to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are associated with misbehavior of several key proteins, such as SOD1 (superoxide dismutase 1), TARDBP/TDP-43, FUS, C9orf72, and dipeptide repeat proteins generated as a result of the translation of the intronic hexanucleotide expansions in the C9orf72 gene, PFN1 (profilin 1), GLE1 (GLE1, RNA export mediator), PURA (purine rich element binding protein A), FLCN (folliculin), RBM45 (RNA binding motif protein 45), SS18L1/CREST, HNRNPA1 (heterogeneous nuclear ribonucleoprotein A1), HNRNPA2B1 (heterogeneous nuclear ribonucleoprotein A2/B1), ATXN2 (ataxin 2), MAPT (microtubule associated protein tau), and TIA1 (TIA1 cytotoxic granule associated RNA binding protein). Although these proteins are structurally and functionally different and have rather different pathological functions, they all possess some levels of intrinsic disorder and are either directly engaged in or are at least related to the physiological liquid-liquid phase transitions (LLPTs) leading to the formation of various proteinaceous membrane-less organelles (PMLOs), both normal and pathological. This review describes the normal and pathological functions of these ALS- and FTLD-related proteins, describes their major structural properties, glances at their intrinsic disorder status, and analyzes the involvement of these proteins in the formation of normal and pathological PMLOs, with the ultimate goal of better understanding the roles of LLPTs and intrinsic disorder in the "Dr. Jekyll-Mr. Hyde" behavior of those proteins.

Intrinsic Disorder, Protein-Protein Interactions, and Disease

It is recognized now that biologically active proteins without stable tertiary structure (known as intrinsically disordered proteins, IDPs) and hybrid proteins containing ordered domains and intrinsically disordered protein regions (IDPRs) are important players found in any given proteome. These IDPs/IDPRs possess functions that complement functional repertoire of their ordered counterparts, being commonly related to recognition, as well as control and regulation of various signaling pathways. They are interaction masters, being able to utilize a wide spectrum of interaction mechanisms, ranging from induced folding to formation of fuzzy complexes where significant levels of disorder are preserved, to polyvalent stochastic interactions playing crucial roles in the liquid-liquid phase transitions leading to the formation of proteinaceous membrane-less organelles. IDPs/IDPRs are tightly controlled themselves via various means, including alternative splicing, precisely controlled expression and degradation, binding to specific partners, and posttranslational modifications. Distortions in the regulation and control of IDPs/IDPRs, as well as their aberrant interactivity are commonly associated with various human diseases. This review presents some aspects of the intrinsic disorder-based functionality and dysfunctionality, paying special attention to the normal and pathological protein-protein interactions.

Mmi1, the yeast homologue of mammalian TCTP, associates with stress granules in heat-shocked cells and modulates proteasome activity

As we have shown previously, yeast Mmi1 protein translocates from the cytoplasm to the outer surface of mitochondria when vegetatively growing yeast cells are exposed to oxidative stress. Here we analyzed the effect of heat stress on Mmi1 distribution. We performed domain analyses and found that binding of Mmi1 to mitochondria is mediated by its central alpha-helical domain (V-domain) under all conditions tested. In contrast, the isolated N-terminal flexible loop domain of the protein always displays nuclear localization. Using immunoelectron microscopy we confirmed re-location of Mmi1 to the nucleus and showed association of Mmi1 with intact and heat shock-altered mitochondria. We also show here that mmi1Δ mutant strains are resistant to robust heat shock with respect to clonogenicity of the cells. To elucidate this phenotype we found that the cytosolic Mmi1 holoprotein re-localized to the nucleus even in cells heat-shocked at 40°C. Upon robust heat shock at 46°C, Mmi1 partly co-localized with the proteasome marker Rpn1 in the nuclear region as well as with the cytoplasmic stress granules defined by Rpg1 (eIF3a). We co-localized Mmi1 also with Bre5, Ubp3 and Cdc48 which are involved in the protein de-ubiquitination machinery, protecting protein substrates from proteasomal degradation. A comparison of proteolytic activities of wild type and mmi1Δ cells revealed that Mmi1 appears to be an inhibitor of the proteasome. We conclude that one of the physiological functions of the multifunctional protein module, Mmi1, is likely in regulating degradation and/or protection of proteins thereby indirectly regulating the pathways leading to cell death in stressed cells.

Co-regulation of polar mRNA transport and lifespan in budding yeast Saccharomyces cerevisiae

Recent studies have uncovered the links between aging, rejuvenation and polar protein transport in the budding yeast Saccharomyces cerevisiae. Here, we examined a still unexplored possibility for co-regulation of polar mRNA transport and lifespan. To monitor the amount and distribution of mRNA-containing granules in mother and daughter cells, we used a fluorescent mRNA-labeling system, with MFA2 as a reporter gene. The results obtained showed that deletion of the selected longevity regulators in budding yeast had a significant impact on the polar mRNA transport. This included changes in the amount of mRNA-containing granules in cytoplasm, their aggregation and distribution between the mother and daughter cells. A significant negative correlation was found between strain-specific longevity, amount of granules and total fluorescent intensity both in mother and daughter cells. As indicated by the coefficient of determination, approximately 50-75% of variation in yeast lifespan could be attributed to the differences in polar mRNA transport.