The modular structure of SIP facilitates its role in stabilizing multiprotein assemblies

Area of Science:

• Molecular Biology
• Structural Biology
• Biochemistry

Background:

• Siah-interacting protein (SIP) acts as an adaptor protein.
• SIP links Siah-1 E3 ubiquitin ligase activity with Skp1 and Ebi F-Box protein.
• This complex facilitates the degradation of beta-catenin, a key transcriptional activator of TCF/LEF genes.

Purpose of the Study:

• To characterize the domain structure of SIP using solution NMR spectroscopy.
• To map the protein-protein interaction sites of Siah-1 and Skp1 with SIP domains.
• To understand how SIP's modular structure facilitates beta-catenin ubiquitination.

Main Methods:

• Solution NMR spectroscopy was employed to determine SIP's domain structure.
• NMR-based chemical shift perturbation assays were used to map Siah-1 and Skp1 interaction sites.
• Analysis of domain flexibility and protein-protein interaction effects on domain dynamics.

Main Results:

• SIP possesses a novel N-terminal helical hairpin domain, a CS domain, and an unstructured C-terminal SGS domain.
• Siah-1 interacts with the flexible linker between SIP's N-terminal and CS domains, restricting CS domain rotation.
• Skp1 interacts exclusively with the SIP CS domain via weak, uncoupled interactions.

Conclusions:

• SIP's modular structure is crucial for assembling the SCF-type complex.
• SIP's domain organization brings Siah-1 and Skp1 into proximity and orients them for beta-catenin polyubiquitination.
• This mechanism highlights the role of adaptor protein structure in regulating protein degradation pathways.