Scientists discover a pathway that monitors a protein import into mitochondria

[Indigophoton]

If there's one fact that most people retain from elementary biology, it's that mitochondria are the powerhouses of the cell. As such, they break down molecules and manufacture new ones to generate the fuel necessary for life. But mitochondria rely on a stream of proteins to sustain this energy production. Nearly all their proteins are manufactured in the surrounding gel-like cytoplasm, and must be imported into the mitochondria to keep the powerhouse running.


A duo of MIT biologists has revealed what happens when a traffic jam of proteins at the surface of the mitochondria prevents proper import. They describe how the mitochondria communicate with the rest of the cell to signal a problem, and how the cell responds to protect the mitochondria. This newly-discovered molecular pathway, called mitoCPR, detects import mishaps and preserves mitochondrial function in the midst of such stress.

"This is the first mechanism identified that surveils mitochondrial protein import, and helps mitochondria when they can't get the proteins they need," says Angelika Amon, the Kathleen and Curtis Marble Professor of Cancer Research in the MIT Department of Biology, who is also a member of the Koch Institute for Integrative Cancer Research at MIT, a Howard Hughes Medical Institute Investigator, and senior author of the study. "Responses to mitochondrial stress have been established before, but this one specifically targets the surface of the mitochondria, clearing out the misfolded proteins that are stuck in the pores."

Hilla Weidberg, a postdoc in Amon's lab, is the lead author of the study, which appears in Science on April 13.

https://phys.org/news/2018-04-scientists-pathway-protein-import-mitochondria.amp

The paper, http://science.sciencemag.org/content/360/6385/eaan4146

Structured Abstract
INTRODUCTION
Mitochondria provide cells with energy and numerous essential metabolites such as lipids, amino acids, iron sulfur clusters, and heme. All mitochondrial functions rely on import of proteins into the organelle because the mitochondrial proteome is almost exclusively encoded by nuclear genes. Given the central importance of mitochondria for cell viability, it is not surprising that cells mount a nuclear response when mitochondrial functions are compromised. These mitochondria-to-nucleus signaling pathways include the mtUPR (mitochondrial unfolded protein response), which triggers expression of mitochondrial chaperones when mitochondrial protein folding is defective, and the UPRam (unfolded protein response activated by mistargeting of proteins) and mPOS (mitochondrial precursor over-accumulation stress) pathways, which reduce translation and induce degradation of unimported proteins in the cytosol when mitochondrial import is impaired. Even though mitochondrial import is central to all mitochondrial functions, no response to protein import defects had been described that protects mitochondria during this stress.

RATIONALE
To determine how cells respond to defects in mitochondrial protein import, we first developed a system in budding yeast with which to specifically inhibit this process. We found that overexpression of proteins that rely on a bipartite signal sequence for their mitochondrial localization inhibited mitochondrial import and led to the accumulation of mitochondrial precursors. Protease protection and carbonate extraction assays that were performed on isolated mitochondria revealed that these unimported proteins accumulated on the mitochondrial surface and in the import channel known as the translocase.

RESULTS
Having developed a system that allowed us to specifically inhibit mitochondrial protein import, we examined the cellular response to this defect. Transcriptome analysis of cells overexpressing bipartite signal–containing proteins identified a gene expression pattern related to the multi-drug resistance response. We termed this response mitochondrial compromised protein import response (mitoCPR). mitoCPR was triggered by protein import defects but not other mitochondrial deficiencies, such as respiratory failure, and was mediated by the transcription factor Pdr3. Our analyses further showed that mitoCPR was critical for the protection of mitochondria during import stress. Cells lacking PDR3 did not mount a mitoCPR during import stress and accumulated higher levels of unimported proteins on the organelle surface as compared with those of wild-type cells. Consequently, pdr3Δ cells exhibited decreased respiratory function and loss of mitochondrial DNA when mitochondrial import was restored. Our results also shed light on the mechanism by which mitoCPR protected mitochondria. Upon mitochondrial import stress, Pdr3 induced expression of Cis1. Coimmunoprecipitation analyses showed that Cis1 recruited the AAA+adenosine triphosphatase Msp1 to the translocase by binding to the translocase receptor Tom70. There, the two proteins mediated the clearance and proteasomal degradation of proteins that failed to be imported into mitochondria.

CONCLUSION
We discovered a mitochondrial import surveillance mechanism in budding yeast. This surveillance mechanism, mitoCPR, is activated when mitochondrial import is stalled in order to induce the removal of mitochondrial proteins accumulating on the mitochondrial surface. Clearance of precursors is critical for maintaining mitochondrial functions during import stress. We propose that mitoCPR could be especially important when the import machinery is overwhelmed, as may occur in situations that require the rapid expansion of the mitochondrial compartment.



MitoCPR protects mitochondria during import stress.
(Left) Mitochondrial protein import deficiency leads to the accumulation of mitochondrial proteins on the organelle’s surface and in the translocases. (Right) Pdr3 induces CIS1 expression. Cis1 binds to the mitochondrial import receptor Tom70 and recruits Msp1 to mediate clearance of unimported precursors from the mitochondrial surface and their proteasomal degradation. This protects mitochondrial functions during import stress.

ILLUSTRATION: ELLA MARU STUDIO