Mechanism of annexin I-mediated membrane aggregation

Area of Science:

• Biochemistry
• Molecular Biology
• Membrane Biophysics

Background:

• Annexin I is known to interact with membranes.
• A secondary interaction site on annexin I is proposed to mediate membrane aggregation.
• The precise mechanism of annexin I-mediated membrane aggregation requires further elucidation.

Purpose of the Study:

• To investigate the properties of the secondary interaction site of annexin I involved in membrane aggregation.
• To understand the mechanism by which annexin I induces membrane aggregation.
• To evaluate the role of hydrophobic interactions and lateral association in annexin I-mediated membrane aggregation.

Main Methods:

• X-ray specular reflectivity measurements to determine the thickness and binding mode of annexin I on phospholipid monolayers.
• Surface plasmon resonance (SPR) to quantify the membrane aggregation activity and binding affinity of annexin I, its mutants, and annexin V.
• Chemical cross-linking of membrane-bound annexin I to assess molecular association.

Main Results:

• Annexin I binds to phospholipid monolayers as a monomer or monolayer, with a layer thickness of 31 ± 2 Å.
• A direct correlation exists between membrane aggregation activity and the affinity of the secondary interaction site for a secondary membrane.
• Secondary binding is primarily driven by hydrophobic interactions, contrasting with calcium-mediated electrostatic primary binding, and lateral association of annexin I precedes aggregation.

Conclusions:

• Annexin I mediates membrane aggregation through a secondary hydrophobic interaction site.
• Lateral association of annexin I molecules on the membrane surface is a prerequisite for aggregation.
• A model is proposed where a laterally aggregated annexin I monolayer interacts with a secondary membrane via its induced hydrophobic site.