Proteolysis of adaptor protein Mmr1 during budding is necessary for mitochondrial homeostasis in Saccharomyces cerevisiae

Yeast Hsp78 plays an essential role in adapting to severe ethanol stress via mild ethanol stress pretreatment in mitochondrial protein quality control

Severe ethanol stress (10 % v/v) causes the denaturation and aggregation of certain mitochondrial proteins, such as aconitase (Aco1), forming the deposits of unfolded mitochondrial proteins (DUMPs) in the budding yeast Saccharomyces cerevisiae. Pre-exposing yeast cells to mild stress often induces adaptation to subsequent severe stress. However, whether pre-exposing yeast cells to mild ethanol stress mitigates mitochondrial protein aggregation remains unclear. Therefore, in this study, we examined the effects of pre-exposing yeast cells to mild ethanol stress on the yeast mitochondrial protein quality control (mtPQC) system under severe ethanol stress. Pretreatment with 6 % (v/v) ethanol significantly mitigated the formation of DUMPs and Aco1 aggregates under subsequent 10 % ethanol stress in wild-type cells but not in hsp78∆ and mdj1∆ cells. Pretreatment with 6 % ethanol increased the protein levels of mtPQC-related factors, Hsp78, Mdj1, and Hsp10; however, hsp78∆ cells showed significantly lower levels of Ssc1 (mtHsp70) and its co-chaperone Mdj1 than wild-type cells. Moreover, intracellular reactive oxygen species levels and the frequency of respiration-deficient mutants under 10 % ethanol stress were reduced after pretreatment with 6 % ethanol in wild-type cells but not in hsp78∆ cells. Overall, this study demonstrated that pre-exposing yeast cells to mild ethanol stress mitigated ethanol-induced mitochondrial damage by activating the mtPQC system, including HSP78 expression, providing novel insights into the effects of ethanol stress on mitochondria and the corresponding responses in yeast.

Cargo adaptors use a handhold mechanism to engage with myosin V for organelle transport

Myo2, a myosin V motor, is essential for organelle transport in budding yeast. Its attachment to and detachment from cargo are mediated by adaptor molecules. Vac17, a vacuole-specific adaptor, links Myo2 to Vac8 on the vacuole membrane, and plays a key role in the formation and dissociation of the Myo2-Vac17-Vac8 complex. Using genetics, cryo-electron microscopy and structure prediction, we find that Vac17 interacts with Myo2 through two distinct sites rather than a single interface. Similarly, the peroxisome adapter Inp2 engages two separate regions of Myo2, one of which overlaps with Vac17. These findings support a "handhold" model, in which cargo adaptors occupy multiple sites on the Myo2 tail, enhancing motor-cargo interactions and likely providing additional regulatory control over motor recruitment.

Summary

This study provides insights into how cargo adaptors bind myosin V. Genetics, cell-based assays, cryo-EM, and AlphaFold, reveal that the vacuole-specific adaptor uses a handhold mechanism to attach to two areas on the myosin V tail. Moreover, evidence is presented that other adaptors use a similar strategy.

A protein interaction map of the myosin Myo2 reveals a role for Alo1 in mitochondrial inheritance in yeast

Budding yeast cells multiply by asymmetric cell division. During this process, the cell organelles are transported by myosin motors along the actin cytoskeleton into the growing bud, and, at the same time, some organelles must be retained in the mother cell. The ordered partitioning of organelles depends on highly regulated binding of motor proteins to cargo membranes. To search for novel components involved in this process, we performed a protein fragment complementation screen using the cargo-binding domain of Myo2, the major organelle transporter in yeast, as bait and a genome-wide strain collection expressing yeast proteins as prey. One robust hit was Alo1, a poorly characterized D-arabinono-1,4-lactone oxidase located in the mitochondrial outer membrane. We found that mutants lacking Alo1 exhibited defects in mitochondrial morphology and inheritance. During oxidative stress, dysfunctional mitochondria are immobilized in the mother in wild-type cells. Intriguingly, overexpression of ALO1 restored bud-directed transport of mitochondria under these conditions. We propose that Alo1 supports the recruitment of Myo2 to mitochondria and its activity is particularly important under oxidative stress.

Spatiotemporal regulation of organelle transport by spindle position checkpoint kinase Kin4

Asymmetric cell division in Saccharomyces cerevisiae involves class V myosin-dependent transport of organelles along the polarised actin cytoskeleton to the emerging bud. Vac17 is the vacuole/lysosome-specific myosin receptor. Its timely breakdown terminates transport and results in the proper positioning of vacuoles in the bud. Vac17 breakdown is controlled by the bud-concentrated p21-activated kinase Cla4, and the E3-ubiquitin ligase Dma1. We found that the spindle position checkpoint kinase Kin4 and, to a lesser extent, its paralog Frk1 contribute to successful vacuole transport by preventing the premature breakdown of Vac17 by Cla4 and Dma1. Furthermore, Kin4 and Cla4 contribute to the regulation of peroxisome transport. We conclude that Kin4 antagonises the Cla4/Dma1 pathway to coordinate spatiotemporal regulation of organelle transport.

E3 Ligases Regulate Organelle Inheritance in Yeast

Saccharomyces cerevisiae proliferates by budding, which includes the formation of a cytoplasmic protrusion called the 'bud', into which DNA, RNA, proteins, organelles, and other materials are transported. The transport of organelles into the growing bud must be strictly regulated for the proper inheritance of organelles by daughter cells. In yeast, the RING-type E3 ubiquitin ligases, Dma1 and Dma2, are involved in the proper inheritance of mitochondria, vacuoles, and presumably peroxisomes. These organelles are transported along actin filaments toward the tip of the growing bud by the myosin motor protein, Myo2. During organelle transport, organelle-specific adaptor proteins, namely Mmr1, Vac17, and Inp2 for mitochondria, vacuoles, and peroxisomes, respectively, bridge the organelles and myosin. After reaching the bud, the adaptor proteins are ubiquitinated by the E3 ubiquitin ligases and degraded by the proteasome. Targeted degradation of the adaptor proteins is necessary to unload vacuoles, mitochondria, and peroxisomes from the actin-myosin machinery. Impairment of the ubiquitination of adaptor proteins results in the failure of organelle release from myosin, which, in turn, leads to abnormal dynamics, morphology, and function of the inherited organelles, indicating the significance of proper organelle unloading from myosin. Herein, we summarize the role and regulation of E3 ubiquitin ligases during organelle inheritance in yeast.

Mitochondrial transport in neurons and evidence for its involvement in acute neurological disorders

Ensuring mitochondrial quality is essential for maintaining neuronal homeostasis, and mitochondrial transport plays a vital role in mitochondrial quality control. In this review, we first provide an overview of neuronal mitochondrial transport, followed by a detailed description of the various motors and adaptors associated with the anterograde and retrograde transport of mitochondria. Subsequently, we review the modest evidence involving mitochondrial transport mechanisms that has surfaced in acute neurological disorders, including traumatic brain injury, spinal cord injury, spontaneous intracerebral hemorrhage, and ischemic stroke. An in-depth study of this area will help deepen our understanding of the mechanisms underlying the development of various acute neurological disorders and ultimately improve therapeutic options.

Severe ethanol stress inhibits yeast proteasome activity at moderate temperatures but not at low temperatures

Since yeast research under laboratory conditions is usually conducted at 25-30°C (moderate temperature range), most of the findings on yeast physiology are based on analyses in this temperature range. Due to inefficiencies in cultivation and analysis, insufficient information is available on yeast physiology in the low-temperature range, although alcoholic beverage production is often conducted at relatively low temperatures (around 15°C). Recently, we reported that severe ethanol stress (10% v/v) inhibits proteasomal proteolysis in yeast cells under laboratory conditions at 28°C. In this study, proteasomal proteolysis at a low temperature (15°C) was evaluated using cycloheximide chase analysis of a short-lived protein (Gic2-3HA), an auxin-inducible degron system (Paf1-AID*-6FLAG), and Spe1-3HA, which is degraded ubiquitin-independently by the proteasome. At 15°C, proteasomal proteolysis was not inhibited under severe ethanol stress, and sufficient proteasomal activity was maintained. These results provide novel insights into the effects of low temperature and ethanol on yeast physiology.

Selective retention of dysfunctional mitochondria during asymmetric cell division in yeast

Decline of mitochondrial function is a hallmark of cellular aging. To counteract this process, some cells inherit mitochondria asymmetrically to rejuvenate daughter cells. The molecular mechanisms that control this process are poorly understood. Here, we made use of matrix-targeted D-amino acid oxidase (Su9-DAO) to selectively trigger oxidative damage in yeast mitochondria. We observed that dysfunctional mitochondria become fusion-incompetent and immotile. Lack of bud-directed movements is caused by defective recruitment of the myosin motor, Myo2. Intriguingly, intact mitochondria that are present in the same cell continue to move into the bud, establishing that quality control occurs directly at the level of the organelle in the mother. The selection of healthy organelles for inheritance no longer works in the absence of the mitochondrial Myo2 adapter protein Mmr1. Together, our data suggest a mechanism in which the combination of blocked fusion and loss of motor protein ensures that damaged mitochondria are retained in the mother cell to ensure rejuvenation of the bud.

Spindle Position Checkpoint Kinase Kin4 Regulates Organelle Transport in Saccharomyces cerevisiae

Membrane-bound organelles play important, frequently essential, roles in cellular metabolism in eukaryotes. Hence, cells have evolved molecular mechanisms to closely monitor organelle dynamics and maintenance. The actin cytoskeleton plays a vital role in organelle transport and positioning across all eukaryotes. Studies in the budding yeast Saccharomyces cerevisiae (S. cerevisiae) revealed that a block in actomyosin-dependent transport affects organelle inheritance to daughter cells. Indeed, class V Myosins, Myo2, and Myo4, and many of their organelle receptors, have been identified as key factors in organelle inheritance. However, the spatiotemporal regulation of yeast organelle transport remains poorly understood. Using peroxisome inheritance as a proxy to study actomyosin-based organelle transport, we performed an automated genome-wide genetic screen in S. cerevisiae. We report that the spindle position checkpoint (SPOC) kinase Kin4 and, to a lesser extent, its paralog Frk1, regulates peroxisome transport, independent of their role in the SPOC. We show that Kin4 requires its kinase activity to function and that both Kin4 and Frk1 protect Inp2, the peroxisomal Myo2 receptor, from degradation in mother cells. In addition, vacuole inheritance is also affected in kin4/frk1-deficient cells, suggesting a common regulatory mechanism for actin-based transport for these two organelles in yeast. More broadly our findings have implications for understanding actomyosin-based transport in cells.

Regulator of Awn Elongation 3, an E3 ubiquitin ligase, is responsible for loss of awns during African rice domestication

Two species of rice have been independently domesticated from different ancestral wild species in Asia and Africa. Comparison of mutations that underlie phenotypic and physiological alterations associated with domestication traits in these species gives insights into the domestication history of rice in both regions. Asian cultivated rice, Oryza sativa, and African cultivated rice, Oryza glaberrima, have been modified and improved for common traits beneficial for humans, including erect plant architecture, nonshattering seeds, nonpigmented pericarp, and lack of awns. Independent mutations in orthologous genes associated with these traits have been documented in the two cultivated species. Contrary to this prevailing model, selection for awnlessness targeted different genes in O. sativa and O. glaberrima. We identify Regulator of Awn Elongation 3 (RAE3) a gene that encodes an E3 ubiquitin ligase and is responsible for the awnless phenotype only in O. glaberrima. A 48-bp deletion may disrupt the substrate recognition domain in RAE3 and diminish awn elongation. Sequencing analysis demonstrated low nucleotide diversity in a ~600-kb region around the derived rae3 allele on chromosome 6 in O. glaberrima compared with its wild progenitor. Identification of RAE3 sheds light on the molecular mechanism underlying awn development and provides an example of how selection on different genes can confer the same domestication phenotype in Asian and African rice.