Scientists Uncover How Cells Maintain Protein-Folding Machinery

A team from Rockefeller University's Laboratory of Metabolic Regulation and Genetics has identified a crucial mechanism that maintains the proper environment for protein folding within a key cellular organelle. The findings, published in Nature Cell Biology, could have implications for understanding and treating certain neurodevelopmental disorders and cancers.

"We discovered a glutathione regulator in the ER that likely plays a key role in these conditions." – Kivanç Birsoy

The Discovery

Researchers have pinpointed how the antioxidant glutathione maintains the specialized, oxidized environment required for protein folding in the endoplasmic reticulum (ER). The study found that the ER actively maintains this state by importing an oxidized form of glutathione (GSSG) and exporting a reduced form (GSH).

Through genetic screening, the team identified the transporter protein SLC33A1 as the manager of this critical glutathione ratio. Subsequent structural studies confirmed that SLC33A1 is responsible for transporting GSSG into the ER.

Background: A Cellular Balancing Act

The ER is the cell's factory for folding secretory and membrane proteins destined for export. Unlike other parts of the cell, such as the mitochondria, the ER requires an oxidized environment for proteins to fold into their correct shapes.

Prior to this work, the machinery that maintains the ER's oxidized state was unknown. The same research team had previously established that precise glutathione levels are essential for mitochondrial function, leading them to investigate its role in the ER.

"Before this work, we knew the ER needed to stay oxidized to fold proteins correctly, but the machinery responsible for maintaining that balance was essentially a black box." – Mark Gad

Key Findings on Protein Quality Control

The research revealed that the precise glutathione ratio (high GSSG to GSH) is not just a background condition—it is essential for a critical proofreading step in the protein-folding process.

If this balance is disrupted and GSSG accumulates, it inhibits a key enzyme that operates a protein quality control system. This system itself relies on the correct oxidation of the ER environment. When it fails, misfolded proteins accumulate in the ER, which can ultimately lead to cell death.

"We discovered that the correct glutathione ratio is essential to a proofreading step in protein folding. It may even be its primary job." – Shanshan Liu

Connections to Human Disease

The findings shed new light on the potential causes of certain diseases:

• Huppke-Brendel Syndrome: This severe neurodevelopmental disorder is linked to mutations in the SLC33A1 gene. The researchers suggest these mutations may disrupt the glutathione balance in the ER, leading to widespread protein misfolding during sensitive periods of brain development.

• Lung Cancer: The study may also inform new approaches for treating certain lung cancers with mutations in the KEAP1 gene. These cancer cells are known to rely on high levels of glutathione synthesis. Inhibiting the SLC33A1 transporter could cause toxic GSSG accumulation, selectively killing the cancer cells.

Regarding Huppke-Brendel Syndrome, Liu noted: "Our findings raise the possibility that the dysfunction of this gene alters the delicate glutathione balance in the ER... We think this could lead to new interventions."

Looking Forward

The study underscores the importance of understanding basic cellular transport mechanisms. As Birsoy concluded:

"Our work demonstrates that defining how nutrients and metabolites are transported across cellular and organelle membranes reveals fundamental principles of cell biology while uncovering a major class of disease-relevant and therapeutically tractable proteins."

The team plans to continue exploring this critical area of cellular logistics in future research.