Lipid Headgroup Charge and Acyl Chain Composition Modulate Closure of Bacterial β-Barrel Channels

Abstract

The outer membrane of Gram-negative bacteria contains β-barrel proteins that form high-conducting ion channels providing a path for hydrophilic molecules, including antibiotics. Traditionally, these proteins have been considered to exist only in an open state so that regulation of outer membrane permeability was accomplished via protein expression. However, electrophysiological recordings show that β-barrel channels respond to transmembrane voltages by characteristically switching from a high-conducting, open state, to a so-called ‘closed’ state, with reduced permeability and possibly exclusion of large metabolites. Here, we use the bacterial porin OmpF from E. coli as a model system to gain insight on the control of outer membrane permeability by bacterial porins through the modulation of their open state. Using planar bilayer electrophysiology, we perform an extensive study of the role of membrane lipids in the OmpF channel closure by voltage. We pay attention not only to the effects of charges in the hydrophilic lipid heads but also to the contribution of the hydrophobic tails in the lipid-protein interactions. Our results show that gating kinetics is governed by lipid characteristics so that each stage of a sequential closure is different from the previous one, probably because of intra- or intermonomeric rearrangements.

The Single Residue K12 Governs the Exceptional Voltage Sensitivity of Mitochondrial Voltage-Dependent Anion Channel Gating

“…We note that a wide variety of structurally dissimilar β-barrel channels are observed to gate at sufficiently high potentials. ,− Many of these occur as trimers, or have small barrel diameters, both of which provide additional mechanical stability relative to VDAC. It is thus suggestive that VDAC is the most voltage-sensitive of these channels and has the most β-barrel flexibility.…”

“…Voltage gating is a common property of β-barrel channels and has been observed in bacterial outer membrane porins 15 well as in the anthrax, 16 aerolysin, 17 and α-hemolysin 18 toxins. In contrast to these channels, where gating is observed at transmembrane potentials of 70 mV or higher, the sensitivity of VDAC gating to voltage is extraordinary, occurring at 30 mV or lower.…”

Biphasic concentration patterns in ionic transport under nanoconfinement revealed in steady-state and time-dependent properties

“…Another property often considered to characterize the response of biological channels to changes in membrane polarization, a key cellular and intracellular mechanism, is the voltage-induced gating. It refers to conformational transitions between fully conductive (open) states and other (closed) states where transport of ions or other metabolites is partially or totally inhibited 69 . Although PI NP does not show voltage-induced closures and a direct comparison is not possible, it is noteworthy that in OmpF porin, voltage gating also exhibits a biphasic scaling with concentration (Figure S4).…”

Probing membrane protein–lipid interactions

“…[16] Negatively charged lipids observed to bind differentially to the outer membrane porin, OmpF, by native MS were also found to affect OmpF channel opening/closure times. [17,18] Cardiolipin (CDL) was observed to stabilize and facilitate assembly of the NhaA Na + /H + antiporter. [19,20] In addition, pairing MS observations with functional assays has shown that PG binding increases Erwinia ligandgated ion channel (ELIC) function,[21••] PIP2 enhances coupling of Class A G protein-coupled receptors with heterotrimeric Gs proteins, [22] and PA and PE selectively bind the two-pore domain potassium channel, K2P4.1.…”