High-Speed Atomic Force Microscopy Tracks Toxin Action

Abstract

In this issue of Biophysical Journal, the article ''Real-Time Visualization of Assembling of a Sphingomyelin-Specific Toxin on Planar Lipid Membranes'' by Yilmaz et al. (1) describes a high-speed atomic force microscopy (HS-AFM) analysis of the assembly and the conformational changes of pore-forming toxin lysenin (2) on a supported lipid bilayer. Pore-forming toxins are ubiquitous in bacteria, fungi, and animals, inducing cell death through membrane permeation. They have been structurally (3) and functionally intensively studied (4). However, essential information concerning conforma-tional changes, assembly dynamics, and membrane insertion of the toxin action has remained unknown, probably due to the lack of a technique that can analyze molecules with both high spatial and temporal resolution. Using HS-AFM, pioneered by the authors (5), they study the addition of lysenin toxins onto supported lipid bilayers and report the subsequent steps of toxin action: Monomer diffusion is extremely fast, and HS-AFM imaging could not localize or track single subunits (which would quickly result in oligomer and cluster formations). These clusters are initially heterogeneously arranged and display a variety of short-distance orientations. The authors are able to capture pore sub-elements like half-rings within these initial clusters, probably the unique observation of assembly intermediates of the toxin self-assembly process. Subsequently, lysenin self-assembly adjusts interactions , a process that is rather slow and takes place from seconds to up to minutes, leading to the formation of hexagonal close-packed assemblies, reminiscent of hexagonal two-dimensional crystals. The authors state ''Initially, both individual and small domains of lysenin clusters formed randomly at different locations on the membrane. While some of these domains grew continuously, most of them reorganized by subsecond dissociation/reassociation of the clusters.''