Arrangement of Annexin A2 tetramer and its impact on the structure and diffusivity of supported lipid bilayers

TL;DR

This study uses x-ray reflectivity to elucidate how Annexin A2 tetramers bind to supported lipid bilayers, affecting their structure and diffusivity, with implications for membrane fusion and cellular transport processes.

Contribution

It reveals the conformation of Annexin A2 tetramers on lipid bilayers and their impact on membrane density and mobility, advancing understanding of membrane-protein interactions.

Findings

Annexin A2 tetramer binds in a side-by-side configuration.

Binding decreases lipid mobility significantly.

It causes asymmetric densification of the lipid bilayer.

Abstract

Annexins are a family of proteins that bind to anionic phospholipid membranes in a Ca2+-dependent manner. Annexin A2 forms heterotetramers (Anx A2t) with the S100A10 (p11) protein dimer. The tetramer is capable of bridging phospholipid membranes and it has been suggested to play a role in Ca2+-dependent exocytosis and cell-cell adhesion of metastatic cells. Here, we employ x-ray reflectivity measurements to resolve the conformation of Anx A2t upon Ca2+-dependent binding to single supported lipid bilayers (SLBs) composed of different mixtures of anionic (POPS) and neutral (POPC) phospholipids. Based on our results we propose that Anx A2t binds in a side-by-side configuration, i.e., both Anx A2 monomers bind to the bilayer with the p11 dimer positioned on top. Furthermore, we observe a strong decrease of lipid mobility upon binding of Anx A2t to SLBs with varying POPS content. X-ray reflectivity measurements indicate that binding of Anx A2t also increases the density of the SLB. Interestingly, in the protein-facing leaflet of the SLB the lipid density is higher than in the substrate-facing leaflet. This asymmetric densification of the lipid bilayer by Anx A2t and Ca2+ might have important implications for the biochemical mechanism of Anx A2t-induced endo- and exocytosis.