Structural insights into the human HRD1 ubiquitin ligase complex

Area of Science:

• Cell biology
• Molecular biology
• Structural biology

Background:

• The endoplasmic reticulum (ER) clears defective proteins through the ER-associated protein degradation (ERAD) pathway.
• The HRD1 ubiquitin ligase complex (HRD1, SEL1L, XTP3B, OS9, Derlins) is vital for ERAD substrate processing, but its molecular mechanisms remain unclear.

Purpose of the Study:

• To determine the cryo-electron microscopy (cryo-EM) structure of the human HRD1-SEL1L-XTP3B complex.
• To elucidate the structural basis of HRD1 complex function in ERAD.

Main Methods:

• Cryo-electron microscopy (cryo-EM) at 3.3 Å resolution.
• Analysis of the HRD1-SEL1L-XTP3B complex structure.
• Investigation of complex conformational changes upon coexpression with Derlin proteins.
• Cell-based functional assays.

Main Results:

• The cryo-EM structure of the human HRD1-SEL1L-XTP3B complex (2:1:1 stoichiometry) was determined.
• A trimmed N-glycan was observed interacting with XTP3B and SEL1L.
• Complex formation with Derlin proteins induced dramatic conformational changes, breaking the HRD1 dimer and forming a new assembly with a four-helix bundle from SEL1L.
• These changes suggest Derlin engagement induces local ER membrane curvature.

Conclusions:

• The study provides the first high-resolution structure of the human HRD1-SEL1L-XTP3B complex.
• Structural insights reveal how Derlin proteins modulate the HRD1 complex assembly and function.
• This work lays the foundation for understanding the molecular mechanisms of mammalian ERAD.