The endophilin N-BAR domain perturbs the structure of lipid bilayers.

Endophilin A is a key player in clathrin-mediated endocytosis at nerve terminals and is essential for the maintenance of synaptic transmission. Endophilin consists of two regions: an SH3 domain that interacts with other endocytotic proteins and an N-BAR domain that binds and bends membranes. Here, we used atomic force microscopy (AFM) under fluid to examine the interaction of the endophilin N-BAR domain with planar supported lipid bilayers, under conditions that closely mimic the environment in which this protein normally operates. We found that when bound to lipid bilayers, the N-BAR domain formed aggregates of various sizes. The N-BAR domain also perturbed the structure of the planar bilayer, at a low concentration (0.15 microM) causing bilayer thinning, and at a 10-fold higher concentration (1.5 microM) forming thin slivers from the bilayer sheet. This bilayer sculpting effect crucially involved the central appendage domain. Reduced hydrophobicity in this domain, caused by the A66D mutation, almost abolished the ability of the endophilin N-BAR domain to bind to supported bilayers. In contrast, increased hydrophobicity, caused by the A66W mutation, switched the bilayer sculpting effect of the N-BAR domain from sliver formation to vesiculation. By following the action of the endophilin N-BAR domain under near-physiological conditions, we have been able to provide additional insights into its membrane binding and bending mechanism.