Structural Insights into KCTD Protein Assembly and Cullin3 Recognition

Area of Science:

• Biochemistry
• Structural Biology
• Molecular Biology

Background:

• Cullin3 (Cul3)-based E3 ubiquitin ligase complexes are crucial for protein degradation.
• These complexes utilize BTB (bric-à-brac, tramtrack, broad complex) domain proteins as substrate adaptors.
• Previous studies elucidated Cul3 binding mechanisms for homodimeric BTB proteins, but KCTD proteins, another class of Cul3 adaptors, bind differently.

Purpose of the Study:

• To determine the structural basis for Cul3 recognition by KCTD proteins.
• To investigate the binding affinity and specificity of various KCTD proteins to Cul3.
• To elucidate the assembly of KCTD-Cul3 complexes.

Main Methods:

• X-ray crystallography was used to determine the pentameric structures of KCTD1 and KCTD9 BTB domains.
• Binding affinities of KCTD proteins to Cul3 were assessed.
• Cryo-electron microscopy (cryo-EM) was employed to confirm the assembly of KCTD9/Cul3 complexes.

Main Results:

• The crystal structures revealed the pentameric organization of KCTD1 and KCTD9 BTB domains, with plasticity observed in KCTD1 rings.
• KCTD proteins 5, 6, 9, and 17 exhibited high-affinity binding to Cul3, whereas KCTD 1 and 16 showed no detectable binding.
• Cryo-EM confirmed a 5:5 assembly stoichiometry for KCTD9/Cul3 complexes.

Conclusions:

• KCTD proteins represent a distinct class of Cul3 adaptors with unique binding determinants.
• The study provides a molecular explanation for the binding specificity within the KCTD family of Cul3 adaptors.
• These findings advance our understanding of Cul3 E3 ligase complex assembly and regulation.