Title: Gramicidin Increases Lipid Flip-Flop in Symmetric and Asymmetric Lipid Vesicles

Abstract

In contrast to most transmembrane proteins, phospholipids can migrate from one leaflet of the membrane to the other. Because this spontaneous lipid translocation (flip-flop) tends to be very slow, cells facilitate the process with enzymes that catalyze the transmembrane movement and thereby regulate the transbilayer lipid distribution. Nonenzymatic membrane-spanning proteins with unrelated primary functions have also been found to accelerate lipid flip-flop in a nonspecific manner and by various hypothesized mechanisms. Using deuterated phospholipids, we examined the acceleration of flip-flop by gramicidin channels, which have well-defined structures and known functions, features that make them ideal candidates for probing the protein-membrane interactions underlying lipid flip-flop. To research compositionally and isotopically asymmetric proteoliposomes containing gramicidin, we expanded a recently developed protocol for the preparation and characterization of lipid-only asymmetric vesicles. Channel incorporation, conformation, and function were examined with small angle x-ray scattering, circular dichroism, and a stopped-flow spectrofluorometric assay, respectively. As a measure of lipid scrambling, we used differential scanning calorimetry to monitor the effect of gramicidin on the melting transition temperatures of the two bilayer leaflets. The two calorimetric peaks of the individual leaflets merged into a single peak over time, suggestive of scrambling, and the effect of the channel onmore » the transbilayer lipid distribution in both symmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and asymmetric 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles was quantified from proton NMR measurements. Our results show that gramicidin increases lipid flip-flop in a complex, concentration-dependent manner. To determine the molecular mechanism of the process, we used molecular dynamics simulations and further computational analysis of the trajectories to estimate the extent of membrane deformation. Combined, the experimental and computational approaches were found to constitute an effective means for studying the effects of transmembrane proteins on lipid distribution in both symmetric and asymmetric model membranes.« less

Authors:

Doktorova, Milka; Heberle, Frederick A.; Marquardt, Drew; Rusinova, Radda; Sanford, R. Lea; Peyear, Thasin A.; Katsaras, John; Feigenson, Gerald W.; Weinstein, Harel; Andersen, Olaf S.

Publication Date:

Fri Mar 01 00:00:00 EST 2019

Research Org.:

Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF); National Institutes of Health (NIH)

OSTI Identifier:

1603691

Alternate Identifier(s):

OSTI ID: 1561604

Grant/Contract Number:  

AC05-00OR22725

Resource Type:

Published Article

Journal Name:

Biophysical Journal

Additional Journal Information:

Journal Name: Biophysical Journal Journal Volume: 116 Journal Issue: 5; Journal ID: ISSN 0006-3495

Publisher:

Elsevier

Country of Publication:

United States

Language:

English

Subject:

59 BASIC BIOLOGICAL SCIENCES