Dynamic membrane protein topological switching upon changes in phospholipid environment

Abstract

A fundamental objective in membrane biology is to understand and predict how a protein sequence folds and orients in a lipid bilayer. Establishing the principles governing membrane protein folding is central to understanding the molecular basis for membrane proteins that display multiple topologies, the intrinsic dynamic organization of membrane proteins, and membrane protein conformational disorders resulting in disease. We previously established that lactose permease of Escherichia coli displays a mixture of topological conformations and undergoes postassembly bidirectional changes in orientation within the lipid bilayer triggered by a change in membrane phosphatidylethanolamine content, both in vivo and in vitro. However, the physiological implications and mechanism of dynamic structural reorganization of membrane proteins due to changes in lipid environment are limited by the lack of approaches addressing the kinetic parameters of transmembrane protein flipping. In this study, real-time fluorescence spectroscopy was used to determine the rates of protein flipping in the lipid bilayer in both directions and transbilayer flipping of lipids triggered by a change in proteoliposome lipid composition. Our results provide, for the first time to our knowledge, a dynamic picture of these events and demonstrate that membrane protein topological rearrangements in response to lipid modulations occur rapidly following a threshold change in proteoliposome lipid composition. Protein flipping was not accompanied by extensive lipid-dependent unfolding of transmembrane domains. Establishment of lipid bilayer asymmetry was not required but may accelerate the rate of protein flipping. Membrane protein flipping was found to accelerate the rate of transbilayer flipping of lipids.T here is limited understanding of how lipid environment affects membrane protein folding during exit from the translocon and insertion into the lipid bilayer or how dynamic changes in lipid environment affect the structure and folding of membrane proteins. Membrane protein lipid environment can change rapidly through lateral movement within a membrane, during trafficking along the secretion pathway, and locally in response to stimuli. Lipid environment is a determinant of transmembrane domain (TMD) orientation for several secondary transporters of Escherichia coli at the time of initial protein folding, as well as after final folding in the cell membrane (1). We used lactose permease (LacY), the paradigm of secondary transporters throughout nature (2), from E. coli as a model membrane protein to study how lipid-protein interactions determine inherently dynamic protein organization and function. When assembled in cells lacking phosphatidylethanolamine (PE), the net neutral and major phospholipid of E. coli (3) (Fig. 1), LacY exhibits inversion of the N-terminal (NT) six-TMD α-helical bundle with respect to the plane of the membrane bilayer and the C-terminal (CT) five-TMD bundle (4); TMD VII becomes an extramembrane domain (EMD) exposed to the periplasm a...

Stable membrane orientations of small dual-topology membrane proteins

“…Is topological dynamics only possible as long as the protein interacts with the membrane-insertion machinery? Or is dynamics possible only when starting from high-energy, topologically frustrated states where, e.g., one or more TMH is not properly inserted into the membrane, or the membrane composition changes such that a preexisting topology becomes destabilized (33,34)?…”

Cholesterol-induced conformational changes in the sterol-sensing domain of the Scap protein suggest feedback mechanism to control cholesterol synthesis

“…A precedent for lipid-induced inversion of transmembrane helices is found in the work of Dowhan and co-workers (15)(16)(17) with the lactose permease (LacY) of Escherichia coli. In elegant studies with LacY reconstituted into proteoliposomes, they demonstrated that addition of phosphatidyl ethanolamine to the proteoliposomes causes a previously periplasmic segment of LacY to become transmembrane, a change that alters the orientation of nearby transmembrane helices.…”

Understanding phospholipid function: Why are there so many lipids?

“…Heidi Vitrac, who joined our research group recently, reconstituted the dynamic properties of membrane proteins in a totally in vitro system composed of only LacY and lipids. She established that the rate of topological re-arrangement occurs on a time scale of seconds, making such changes of biological significance for any protein in any membrane potentially independent of other cellular factors (60). She also demonstrated that phosphorylation of a membrane protein extramembrane domain, which alters the charge balance between the protein and the membrane surface, results in a similar rapid change in topological organization (61).…”