Structure and function of beta 2-glycoprotein I: with special reference to the interaction with phospholipid

Area of Science:

• Biochemistry
• Immunology
• Molecular Biology

Background:

• Beta 2-Glycoprotein I (beta 2-GPI) acts as a cofactor in anticardiolipin antibody recognition of cardiolipin.
• This interaction is implicated in autoimmune diseases like systemic lupus erythematosus.

Purpose of the Study:

• To investigate the binding of various beta 2-GPI forms to cardiolipin-containing liposomes.
• To analyze the influence of pH and ionic strength on these interactions.

Main Methods:

• Utilized various forms of bovine beta 2-GPI: intact, desialylated (Asialo beta 2-GPI), N-terminal domain (domain I), and nicked variants.
• Examined binding to phospholipid liposomes under varying pH and ionic strength conditions.
• Determined dissociation constants (Kd) to quantify binding affinities.

Main Results:

• Beta 2-GPI exhibited strong binding (Kd 10(-8) M) to cardiolipin liposomes at neutral pH and low ionic strength.
• Binding occurred with phosphatidylglycerol, phosphatidylserine, phosphatidic acid, and phosphatidylinositol, but not phosphatidylcholine.
• Domain I and Asialo beta 2-GPI bound cardiolipin with Kd values of 10(-6) M and 10(-8) M, respectively.
• Nicked forms showed reduced affinity for cardiolipin compared to intact forms, similar to domain I.

Conclusions:

• Beta 2-GPI's interaction with cardiolipin is dependent on its structural integrity and specific phospholipid composition.
• Environmental factors like pH and ionic strength modulate beta 2-GPI binding affinity.
• Understanding these interactions provides insights into the pathogenesis of antiphospholipid syndrome.