3D domain-swapped human cystatin C with amyloidlike intermolecular beta-sheets

Area of Science:

• Biochemistry
• Structural Biology
• Neuroscience

Background:

• Oligomerization of human cystatin C (HCC) causes amyloid deposits in brain arteries.
• A naturally occurring L68Q variant accelerates this process.
• Previous crystal structures suggested 3D domain swapping in HCC dimer formation, potentially explaining fibril formation.

Purpose of the Study:

• To investigate the structural basis of human cystatin C (HCC) fibril formation.
• To elucidate the mechanism of domain swapping in a new tetragonal crystal form of HCC.
• To relate structural findings to HCC amyloid aggregation and cross-beta structures.

Main Methods:

• Crystallization of full-length human cystatin C (HCC) in a tetragonal form.
• X-ray crystallography to determine the 3D structure of HCC dimers.
• Analysis of domain swapping and intermolecular interactions.

Main Results:

• Full-length HCC crystallized in a tetragonal form, showing dimer formation via domain swapping.
• The new structure exhibits a different overall dimer arrangement compared to previous cubic forms, due to hinge flexibility.
• Dimers pack via intermolecular beta-sheets, forming a supramolecular crystal structure with beta-strands perpendicular to a crystallographic direction.

Conclusions:

• The observed domain-swapping mechanism in the tetragonal HCC crystal form provides insight into the basis of amyloid aggregation.
• The formation of extended intermolecular beta-sheets is consistent with the cross-beta structure found in amyloid fibrils.
• This structural understanding is crucial for studying the pathogenesis of diseases involving HCC amyloid deposits.