Structural basis and pathological implications of the dimeric OS9-SEL1L-HRD1 ERAD Core Complex

Area of Science:

• Molecular biology
• Structural biology
• Cellular biology

Background:

• Endoplasmic reticulum (ER)-associated degradation (ERAD) is vital for clearing misfolded proteins and maintaining ER proteostasis.
• The molecular organization and disease mechanisms of mammalian ERAD remain poorly understood.
• The SEL1L-HRD1 complex is the most conserved component of the ERAD pathway.

Purpose of the Study:

• To determine the cryo-electron microscopy (cryo-EM) structure of the core mammalian ERAD complex.
• To elucidate the molecular architecture and substrate engagement mechanism of the SEL1L-HRD1 complex.
• To understand how disease-associated mutations in SEL1L and HRD1 impact ERAD function.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the structure of the SEL1L-HRD1-OS9 complex.
• Site-directed mutagenesis to assess the functional impact of mutations.
• Crosslinking assays to validate protein-protein interactions.

Main Results:

• The first cryo-EM structure of the core mammalian ERAD complex (OS9, SEL1L, HRD1) was determined.
• The structure reveals a dimeric assembly with a claw-like OS9-SEL1L lumenal domain for substrate binding and a dimeric HRD1 transmembrane domain for translocation.
• Pathogenic mutations in SEL1L and HRD1 disrupt complex assembly, impair ERAD activity, and affect HRD1 dimerization.
• Methionine-rich crevices near HRD1 suggest substrate translocation channels.

Conclusions:

• The dimeric structure of the human SEL1L-HRD1 ERAD core complex provides key insights into protein recognition and processing.
• This structural framework explains how mutations destabilizing the ERAD machinery contribute to human diseases.
• The findings highlight the importance of complex integrity and HRD1 dimerization for ERAD function.