YUP.SCX: coaxing atomic models into medium resolution electron density maps.

Area of Science:

• Structural biology
• Biophysics
• Computational biology

Background:

• Cryo-electron microscopy (cryo-EM) provides low-to-intermediate resolution electron density maps of macromolecular complexes.
• Integrating high-resolution atomic structures with cryo-EM maps enhances model accuracy beyond map resolution.
• Existing methods for docking and refining atomic models into density maps include rigid and flexible approaches.

Purpose of the Study:

• To develop a novel refinement method for improving atomic models of macromolecular complexes.
• To optimize the fitting of atomic structures into cryo-electron microscopy density maps.
• To construct an accurate atomic model of the 70S ribosome in its pre-accommodation state.

Main Methods:

• Developed a new refinement method named YUP.SCX.
• Converted electron density maps into a component of the potential energy function.
• Incorporated stereochemical restraints and volume exclusion terms into the potential energy function.
• Utilized simulated annealing for potential energy minimization to achieve optimal fitting.

Main Results:

• Successfully constructed an atomic model of the 70S ribosome in the pre-accommodation state.
• Demonstrated that atomic displacements, up to 33Å, occurred in nearly rigid fragments.
• Observed smooth transitions between these rigid fragments along natural boundaries.

Conclusions:

• YUP.SCX provides an effective method for stereochemically restrained atomic structure refinement against cryo-EM density maps.
• The method yields accurate atomic models of macromolecular complexes, even with significant atomic displacements.
• The fragmentation observed in the 70S ribosome model suggests inherent flexibility within the complex.