The Plasma Factor XIII Heterotetrameric Complex Structure: Unexpected Unequal Pairing within a Symmetric Complex.

Area of Science:

• Biochemistry
• Structural Biology
• Hemostasis

Background:

• Factor XIII (FXIII) is crucial for clot stability.
• FXIII circulates as FXIII-A2B2, with A subunits being catalytic and B subunits protective.
• The structure of FXIII-B subunits and the FXIII-A2B2 complex were previously unknown.

Purpose of the Study:

• To determine the all-atom structure of the FXIII-A2B2 complex.
• To characterize the assembly, activation, and dissociation mechanisms of FXIII-A2B2.
• To provide insights into FXIII-related diseases and therapeutic strategies.

Main Methods:

• Integrative hybrid approaches: atomic force microscopy, cross-linking mass spectrometry, computational modeling.
• Molecular dynamics simulations.
• Isothermal titration calorimetry.

Main Results:

• The first all-atom model of the FXIII-A2B2 complex was constructed.
• Unequal pairing of individual subunit monomers was observed within the complex.
• This unique structure is critical for FXIII assembly and activation.

Conclusions:

• The study elucidates the structural basis of FXIII function and regulation.
• Findings enhance understanding of FXIII mutations linked to bleeding and thrombotic disorders.
• Results have implications for designing novel therapeutics targeting FXIII activity.