Electrophysiology of Bacterial Translocons

Protein-translocating channels, or translocons, are ubiquitous membrane proteins found in all cell types as well as intracellular organelles. In bacteria, they often constitute one of the components of molecular machines that participate in membrane biogenesis, assembly of surface exposed appendages, and the secretion of protein substrates across the cell envelope. Their pore-forming properties make them directly amenable to electrophysiology, even though ionic movement is not their natural function. Several bacterial translocons have been investigated by patch-clamp or planar lipid bilayer electrophysiology. These studies have revealed that bacterial translocons are often dynamic channels, capable of sampling a large conformational landscape, and thus displaying multiplicity of conductance states. They typically form large pores, but are often gated by internal plug elements that limit the flow of ions and molecules in the resting state. Combined with structural data, electrophysiological studies have allowed the discovery of structure-function relationships that shed light on functional mechanisms. In some cases, the channel properties have been investigated in the presence of natural substrates to understand the molecular steps involved in translocation itself. This review highlights the electrophysiological investigations of SecYEG, BamA, TolC, secretins, ushers, type III secretion translocons, autotransporters and two-partner secretion systems.