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

D Perini,Antonio Alcaraz,María Queralt-Martín

doi:10.3390/ijms20030674

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.

Highlights

Membrane channels are the cellular gatekeepers controlling neurotransmission, electrical signaling and the influx and efflux of nutrients and waste products [1,2]
A more gradual response can be found in other protein channels that may remain fully open for long times until a threshold value of the electric potential is met
Subsequent closed states could last for seconds or minutes [5]. Examples of this latter class are β-barrel channels in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts [6]. They typically form large pores with mild ion selectivity that allows for the controlled diffusion of ions and water-soluble metabolites, including respiratory substrates and antibiotics

Summary

Introduction

Membrane channels are the cellular gatekeepers controlling neurotransmission, electrical signaling and the influx and efflux of nutrients and waste products [1,2]. Typical switching times between open and close states are quite fast (around milliseconds in Na channels [5]). A more gradual response can be found in other protein channels that may remain fully open for long times until a threshold (relatively high) value of the electric potential is met. Subsequent closed states could last for seconds or minutes [5] Examples of this latter class are β-barrel channels in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts [6]. Each one of three OmpF monomers is thought to be functionally identical and independent in the open state and when interacting with small molecules like antibiotics [30,37].

Discussion

Findings

Materials and Methods