Insights into ubiquitin chain architecture using Ub-clipping

Area of Science:

• Cellular Biology
• Biochemistry
• Proteomics

Background:

• Protein ubiquitination is a crucial post-translational modification regulating cellular processes.
• Polyubiquitin chains exhibit complex architectures with diverse modifications, forming the 'ubiquitin code'.
• Previous methods struggled to fully resolve polyubiquitin chain architecture.

Purpose of the Study:

• To introduce Ub-clipping, a novel methodology for analyzing polyubiquitin signals and architectures.
• To enable direct assessment of protein ubiquitination and complex ubiquitin chain structures.
• To investigate the role of specific ubiquitin modifications in cellular processes like mitophagy.

Main Methods:

• Development of Ub-clipping using an engineered viral protease (Lb^pro∗).
• Lb^pro∗ incompletely removes ubiquitin, leaving a GlyGly tag for analysis.
• Quantification of multiply modified branch-point ubiquitins and assessment of co-existing modifications.

Main Results:

• Ub-clipping simplifies the assessment of ubiquitination on substrates and within polyubiquitin chains.
• A significant proportion (10-20%) of ubiquitin in polymers exists as branched chains.
• Analysis of mitophagy revealed a reliance on mono- and short-chain polyubiquitin with unmodified phosphorylated moieties.

Conclusions:

• Ub-clipping provides unprecedented insight into polyubiquitin chain architecture and the ubiquitin code.
• The method facilitates the quantification of complex ubiquitin modifications and branched chains.
• Findings shed light on the specific ubiquitin signaling mechanisms in PINK1/parkin-mediated mitophagy.