Single-molecule studies on PolySUMO proteins reveal their mechanical flexibility.

Area of Science:

• Biophysics
• Structural Biology
• Protein Mechanics

Background:

• Proteins with beta-sandwich and beta-grasp topologies are mechanically stable, often attributed to terminal mechanical clamp geometry.
• Evidence suggests other interactions also contribute to mechanical stability, necessitating further investigation.

Purpose of the Study:

• To investigate the mechanical unfolding properties of ubiquitin-like proteins (SUMO1 and SUMO2) and compare them with ubiquitin.
• To understand how sequence and structural contact variations influence mechanical stability within similar protein topologies.

Main Methods:

• Single-molecule force spectroscopy was used to study the mechanical unfolding of SUMO1, SUMO2, and ubiquitin.
• Pulling speed-dependent experiments and Monte Carlo simulations were employed to analyze unfolding pathways and potential widths.

Main Results:

• SUMO1 and SUMO2 displayed two-state unfolding pathways, similar to ubiquitin.
• Unfolding forces for SUMO1 (~130 pN) and SUMO2 (~120 pN) were lower than ubiquitin (~190 pN) at 400 nm/s.
• Mechanical stabilities correlated with the number of interresidue contacts.
• Unfolding potential widths were significantly larger for SUMO1 (~0.51 nm) and SUMO2 (~0.33 nm) compared to ubiquitin (~0.19 nm), indicating greater mechanical flexibility.

Conclusions:

• SUMO1 and SUMO2 are mechanically less stable and more flexible than ubiquitin.
• Interresidue contacts play a crucial role in determining protein mechanical stability.
• Findings may explain functional differences between ubiquitin and SUMO proteins.