Quality Control of Protein Complex Assembly by a Transmembrane Recognition Factor

Area of Science:

• Cell biology
• Molecular biology
• Protein quality control

Background:

• The inner nuclear membrane (INM) is crucial for nuclear functions and distinct from the endoplasmic reticulum (ER).
• The Asi1/Asi2/Asi3 ubiquitin ligase complex in yeast clears mislocalized ER proteins to maintain INM proteome integrity.
• Mechanisms of Asi complex selectivity and coordination with ER protein biogenesis remain unclear.

Purpose of the Study:

• To investigate the link between INM proteome identity and ER membrane protein complex assembly.
• To elucidate how the Asi complex targets mislocalized proteins for degradation.
• To understand the coordination between protein assembly and quality control in the ER.

Main Methods:

• Investigated yeast models to study protein localization and degradation.
• Utilized biochemical assays to analyze protein interactions and ubiquitination.
• Examined the role of Asi2 in substrate recognition via transmembrane domains.

Main Results:

• Proteins failing to assemble in the ER are targeted to the INM for Asi-mediated degradation.
• Asi2 directly binds to transmembrane domains of mislocalized proteins.
• Ubiquitination by Asi1/Asi3 leads to membrane extraction of substrates.
• Spatial segregation of assembly and quality control enhances efficiency and reduces orphan subunits.

Conclusions:

• A novel mechanism links ER protein complex assembly to INM quality control.
• The Asi complex acts as a crucial quality control mechanism for unassembled membrane proteins.
• Spatial separation of protein biogenesis and degradation optimizes cellular proteostasis.