ER-to-Golgi transport proteins link excess neuronal cargo to the IRE-1–XBP-1 unfolded protein response in C. elegans

# What the study asked

# Model and main approach Researchers used the nematode Caenorhabditis elegans. They combined genetics, imaging, and biochemical assays to test how neuronal overproduction of a gap junction protein, UNC-9 (an innexin), affects ER stress signaling and whether specific ER-to-Golgi transport proteins are required for that response.

# Key findings

• Neuronal overexpression of UNC-9 activates the IRE-1–XBP-1 branch of the ER UPR in a cell-autonomous way. The response was specifically linked to UNC-9 excess in the neurons that made it.

• Two early secretory pathway proteins, named ERGI-2 and ERGI-3, are required for this UNC-9–triggered UPR activation. Loss of either protein suppresses the IRE-1–XBP-1 response and also causes mislocalization or abnormal accumulation of UNC-9.

• ERGI-2 and ERGI-3 physically interact with both UNC-9 and the ER chaperone HSP-4/BiP. This pattern indicates these trafficking proteins may help sense or present excessive UNC-9 to the UPR machinery rather than serving only as passive transport factors.

• The role of ERGI-2 and ERGI-3 is cargo-selective. They are not required for UPR activation induced by overexpressing another innexin, UNC-7, or by unrelated overexpressed proteins. That selectivity suggests transport machinery can differentially couple certain clients to stress signaling.

• Activating the IRE-1–XBP-1 pathway mitigates abnormal UNC-9 accumulation in ergi-2 and ergi-3 mutant backgrounds, showing that UPR activation can reduce the protein burden caused by transport defects or excess cargo.

# Why this matters The study identifies specific ER-to-Golgi trafficking proteins as active regulators that link secretory-pathway demand for particular cargoes to an adaptive UPR branch. In neuronal cells, which are long-lived and highly dependent on correct protein trafficking, this selective coupling could influence how cells respond to chronic production of specific membrane proteins.

# Concrete takeaways for readers

• Cargo selectivity matters: not all overexpressed secretory proteins trigger the same dependence on ERGI-2/3 for UPR activation.
• Modulating the IRE-1–XBP-1 pathway can reduce pathological accumulation of specific proteins when trafficking is impaired.

# Next questions raised by the work

• What molecular features of UNC-9 make it dependent on ERGI-2 and ERGI-3 for coupling to the UPR?
• Do ERGI-2 and ERGI-3 act as adaptors that present cargo to BiP or directly to IRE-1?
• Are similar cargo-selective links between trafficking factors and UPR branches present in mammalian neurons or other long-lived cells?

# Bottom line This study provides genetic and biochemical evidence that two ER-to-Golgi transport proteins, ERGI-2 and ERGI-3, selectively connect the handling of an abundant neuronal cargo, UNC-9, to the IRE-1–XBP-1 unfolded protein response in C. elegans. The result is a model in which components of the secretory machinery actively shape stress signaling depending on the specific client protein burden.