Atomic Force Microscopy of Copine I and Annexin A1 on Supported Phospholipid Bilayers: Structure and Synergism

Abstract

The time- and calcium-dependent association of recombinant human copine I or annexin A1 with supported lipid bilayers composed of 25% brain PS and 75% DOPC was monitored by atomic force microscopy. Neither protein bound to featureless areas of the bilayer but both rapidly bound to small domains that appeared to be 0.5 to 0.8 nm lower than the rest of the bilayer. These domains may be enriched in PS and/or have a more disordered lipid structure. Copine I assembled into a reticular pattern made of 40nm linear elements that appeared to be one or two molecules high. In vivo such copine arrays might form a scaffold for the assembly of signalling proteins bound by copine I. Annexin A1 did not form ordered structures but appeared to promote the growth of the domains of lowered height to which it was bound. These enlarged domains created by annexin A1 provided binding sites for copine I when it was added subsequently. Therefore, in vivo, annexin A1 might recruit C2 domain-containing proteins like copine to membranes by modulating membrane structure.