Engineered Cas9 complexes establish an experimentally grounded benchmark for heterogeneous cryoEM reconstruction...

Area of Science:

• Structural Biology
• Biophysics
• Computational Biology

Background:

• Single-particle cryo-electron microscopy (cryo-EM) is vital for analyzing protein conformational and compositional heterogeneity.
• Evaluating heterogeneous reconstruction methods is challenging due to a lack of experimental benchmarks with ground-truth labels.
• Existing benchmarks often rely on retrospective validation or simplified synthetic data, failing to capture real-world complexities.

Purpose of the Study:

• To create an experimentally grounded benchmark dataset for robust evaluation of cryo-EM heterogeneous reconstruction algorithms.
• To provide a biochemically tractable model system for assessing cryo-EM data processing pipelines under realistic conditions.

Main Methods:

• Developed a benchmark using catalytically inactive *Streptococcus pyogenes* Cas9 complexes with engineered DNA extensions of varying lengths.
• Assembled, purified, vitrified, and imaged thirteen distinct complexes independently to generate per-particle ground-truth labels.
• Utilized systematic deep classification to create curated particle subsets, separating encoded structural signals from non-encoded conformational variability.

Main Results:

• Independent reconstructions successfully identified the engineered DNA-extension signal, establishing a quantitative readout (DNA-extension occupancy) that correlates with extension length.
• The benchmark revealed significant non-encoded conformational heterogeneity within the Cas9 core, reflecting practical challenges in cryo-EM analysis.
• Generated pooled particle stacks and a framework for *in silico* particle pooling to create diverse challenge datasets.

Conclusions:

• The developed benchmark dataset enables objective evaluation of heterogeneous reconstruction algorithms using experimentally realistic data.
• This resource facilitates the assessment of entire cryo-EM pipelines, including data collection and preprocessing strategies.
• The Cas9-DNA system serves as a valuable model for advancing cryo-EM methodology and analysis.