Accessing ns-micros side chain dynamics in ubiquitin with methyl RDCs

Area of Science:

• Biophysics
• Structural Biology
• Protein Dynamics

Background:

• Understanding protein dynamics is crucial for deciphering biological function.
• Methyl groups offer a unique probe for studying side-chain flexibility.
• Previous studies have explored ubiquitin dynamics using various techniques.

Purpose of the Study:

• To apply model-free analysis (MFA) to methyl group Residual Dipolar Couplings (RDCs) for characterizing supra-tau(c) dynamics in ubiquitin.
• To correlate methyl group mobility with residue type, backbone proximity, and solvent exposure.
• To compare RDC-derived dynamics with existing ubiquitin models and computational ensembles.

Main Methods:

• Measurement of RDCs for methyl groups in 13 different alignment media.
• Application of model-free analysis (MFA) to RDC data.
• Comparison of derived order parameters with existing ubiquitin dynamic ensembles and computational approaches.

Main Results:

• Methyl groups exhibit a wide range of mobility, from rigid to highly mobile, influenced by residue type and environment.
• Significant dynamics occur at rates slower than the correlation time tau(c), contributing substantially to overall motion.
• RDC-derived order parameters are influenced by rotameric interconversions and librational motions, aligning well with combined J-coupling and RDC methods.
• The EROS dynamic ensemble best fits the observed methyl group order parameters, suggesting its suitability for representing ubiquitin dynamics.
• MFA-derived spherical harmonics enabled rotameric searches, revealing expanded distributions indicative of concerted side-chain motions.

Conclusions:

• Methyl group RDCs analyzed with MFA provide valuable insights into supra-tau(c) protein dynamics.
• Ubiquitin side chains exhibit complex dynamics involving motions on multiple timescales.
• The findings support the use of MFA and specific dynamic ensembles for accurate protein motion characterization.