NEMF-mediated CAT tailing facilitates translocation-associated quality control.

Area of Science:

• Cell Biology
• Molecular Biology
• Protein Biogenesis

Background:

• Ribosome stalling during co-translational translocation at the endoplasmic reticulum (ER) can lead to translocon clogging.
• Impaired ER protein biogenesis results from translocon clogging.
• Translocation-associated quality control (TAQC) is a poorly understood process that resolves translocon clogging in mammalian cells.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying the translocation-associated quality control (TAQC) process.
• To differentiate the degradation pathways for various TAQC substrates.
• To identify key factors involved in the quality control of nascent protein chains.

Main Methods:

• Genome-wide CRISPR screen
• Live-cell imaging
• Analysis of protein degradation pathways (lysosomal, proteasomal, ERAD)

Main Results:

• TAQC substrates from mRNAs with ribosome-stalling poly(A) sequences are degraded by lysosomes and the proteasome.
• Defective nascent chains from nonstop (NS) mRNAs are degraded via an unconventional ER-associated protein degradation (ERAD) pathway.
• This ERAD pathway involves ER-to-Golgi trafficking, KDEL-mediated retrieval at the Golgi, and the tRNA-binding factor NEMF.
• NEMF appends an aggregation-prone carboxyl tail to stalled NS nascent chains, termed CAT tailing.

Conclusions:

• NEMF-mediated CAT tailing targets a subset of TAQC substrates for ERAD through Golgi retrieval.
• This novel pathway safeguards ER homeostasis by clearing defective nascent chains.
• The study reveals distinct degradation routes for TAQC substrates, highlighting the complexity of protein quality control in the ER.