Solid-phase inclusion as a mechanism for regulating unfolded proteins in the mitochondrial matrix

Abstract

Proteostasis declines with age, characterized by the accumulation of unfolded or damaged proteins. Recent studies suggest that proteins constituting pathological inclusions in neurodegenerative diseases also enter and accumulate in mitochondria. How unfolded proteins are managed within mitochondria remains unclear. Here, we found that excessive unfolded proteins in the mitochondrial matrix of yeast cells are consolidated into solid-phase inclusions, which we term deposits of unfolded mitochondrial proteins (DUMP). Formation of DUMP occurs in mitochondria near endoplasmic reticulum–mitochondria contact sites and is regulated by mitochondrial proteins controlling the production of cytidine 5′-diphosphate–diacylglycerol. DUMP formation is age dependent but accelerated by exogenous unfolded proteins. Many enzymes of the tricarboxylic acid cycle were enriched in DUMP. During yeast cell division, DUMP formation is necessary for asymmetric inheritance of damaged mitochondrial proteins between mother and daughter cells. We provide evidence that DUMP-like structures may be induced by excessive unfolded proteins in human cells.

Comprehensive characterization of the Hsp70 interactome reveals novel client proteins and interactions mediated by posttranslational modifications

“…Although S974A cells grew at a similar rate to WT in standard growth media, S974D cells were substantially inhibited for growth (Fig 5F). To determine whether the growth defect of the S974D mutant was due to irregular mitochondrial protein aggregation, we examined the aggregation behavior of a previously established mitoFluc reporter [34,35]. S974D mutants were unable to clear mitochondrial protein aggregates (Figs 5G and S3A).…”

“…Live-cell images were acquired using a Yokogawa CSU-10 spinning disc on the side port of a Carl Zeiss 200 m inverted microscope or a Carl Zeiss LSM-780 confocal system. Imaging quantification was described previously using imageJ [34]. For Mtw1, a Zeiss Axioimager Z2 microscope (Carl Zeiss AG, Germany) was used to image cells using a 63× 1.4NA apochromatic oil immersion lens.…”

A novel multifunctional role for Hsp70 in binding post-translational modifications on client proteins

“…Although S974A cells grew at a similar rate to WT in standard growth media, S974D cells were substantially inhibited for growth (Figure 5F). To determine whether the growth defect of the S974D mutant was due to irregular mitochondrial protein aggregation, we examined the aggregation behavior of a previously established mitoFluc reporter (Ruan et al, 2020;Ruan et al, 2017). S974D mutants were unable to clear mitochondrial protein aggregates (Figure 5F, Figure 5G).…”

Metabolic regulation of misfolded protein import into mitochondria

“…Mitochondria are vital organelles whose biogenesis and activities in energy production are tightly linked to cellular metabolic control (1,2). Metabolic stress and mitochondrial dysfunction are common factors that drive age-related degenerative diseases such as heart failure and dementia (3,4), and these diseases are often characterized by loss of proteostasis leading to the formation of protein aggregates (3,5,6). The MAGIC pathway, in which cytosolic protein aggregates are concentrated on the surface of mitochondria, disaggregated by molecular chaperones, and then imported to be degraded by mitochondrial proteases, may represent a link between mitochondrial dysfunction and loss of proteostasis (Figure 1-figure supplement 1A) (7)(8)(9).…”

“…The MAGIC pathway, in which cytosolic protein aggregates are concentrated on the surface of mitochondria, disaggregated by molecular chaperones, and then imported to be degraded by mitochondrial proteases, may represent a link between mitochondrial dysfunction and loss of proteostasis (Figure 1-figure supplement 1A) (7)(8)(9). Inhibition of this pathway causes prolonged protein aggregation in cytosol after proteotoxic stress (7,8), whereas an elevated burden of MPs in mitochondria can lead to mitochondrial damage, potentially forming a vicious cycle leading to decline in fitness (6). Therefore, understanding how mitochondrial function in facilitating cellular proteostasis is balanced with mitochondrial biogenesis and metabolism may provide key insights into the maintenance of cellular fitness under stresses related to aging.…”