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1. Exogenous Fatty Acids Remodel Staphylococcus aureus Lipid Composition through Fatty Acid Kinase

   https://doi.org/10.1128/JB.00128-20  

   DeMars, Zachary; Singh, Vineet K.; Bose, Jeffrey L. (June 2020, Journal of Bacteriology)

   Stock, Ann M. (Ed.)

   ABSTRACT Staphylococcus aureus can utilize exogenous fatty acids for phospholipid synthesis. The fatty acid kinase FakA is essential for this utilization by phosphorylating exogenous fatty acids for incorporation into lipids. How FakA impacts the lipid membrane composition is unknown. In this study, we used mass spectrometry to determine the membrane lipid composition and properties of S. aureus in the absence of fakA . We found the fakA mutant to have increased abundance of lipids containing longer acyl chains. Since S. aureus does not synthesize unsaturated fatty acids, we utilized oleic acid (18:1) to track exogenous fatty acid incorporation into lipids. We observed a concentration-dependent incorporation of exogenous fatty acids into the membrane that required FakA. We also tested how FakA and exogenous fatty acids impact membrane-related physiology and identified changes in membrane potential, cellular respiration, and membrane fluidity. To mimic the host environment, we characterized the lipid composition of wild-type and fakA mutant bacteria grown in mouse skin homogenate. We show that wild-type S. aureus can incorporate exogenous unsaturated fatty acids from host tissue, highlighting the importance of FakA in the presence of host skin tissue. In conclusion, FakA is important for maintaining the composition and properties of the phospholipid membrane in the presence of exogenous fatty acids, impacting overall cell physiology. IMPORTANCE Environmental fatty acids can be harvested to supplement endogenous fatty acid synthesis to produce membranes and circumvent fatty acid biosynthesis inhibitors. However, how the inability to use these fatty acids impacts lipids is unclear. Our results reveal lipid composition changes in response to fatty acid addition and when S. aureus is unable to activate fatty acids through FakA. We identify concentration-dependent utilization of oleic acid that, when combined with previous work, provides evidence that fatty acids can serve as a signal to S. aureus . Furthermore, using mouse skin homogenates as a surrogate for in vivo conditions, we showed that S. aureus can incorporate host fatty acids. This study highlights how exogenous fatty acids impact bacterial membrane composition and function. 

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2. Neutron spin echo shows pHLIP is capable of retarding membrane thickness fluctuations

   https://doi.org/10.1016/j.bbamem.2024.184349  

   Scott, Haden L; Burns-Casamayor, Violeta; Dixson, Andrew C; Standaert, Robert F; Stanley, Christopher B; Stingaciu, Laura-Roxana; Carrillo, Jan-Michael Y; Sumpter, Bobby G; Katsaras, John; Qiang, Wei; et al (October 2024, Biochimica et Biophysica Acta (BBA) - Biomembranes)

   Cell membranes are responsible for a range of biological processes that require interactions between lipids and proteins. While the effects of lipids on proteins are becoming better understood, our knowledge of how protein conformational changes influence membrane dynamics remains rudimentary. Here, we performed experiments and computer simulations to study the dynamic response of a lipid membrane to changes in the conformational state of pH-low insertion peptide (pHLIP), which transitions from a surface-associated (SA) state at neutral or basic pH to a transmembrane (TM) α-helix under acidic conditions. Our results show that TM-pHLIP significantly slows down membrane thickness fluctuations due to an increase in effective membrane viscosity. Our findings suggest a possible membrane regulatory mechanism, where the TM helix affects lipid chain conformations, and subsequently alters membrane fluctuations and viscosity. 

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3. Raft-like lipid mixtures in the highly coarse-grained Cooke membrane model

   https://doi.org/10.1063/5.0230727  

   Varma, Malavika; Khuri-Makdisi, Farid; Deserno, Markus (September 2024, The Journal of Chemical Physics)

   Lipid rafts are nanoscopic assemblies of sphingolipids, cholesterol, and specific membrane proteins. They are believed to underlie the experimentally observed lateral heterogeneity of eukaryotic plasma membranes and implicated in many cellular processes, such as signaling and trafficking. Ternary model membranes consisting of saturated lipids, unsaturated lipids, and cholesterol are common proxies because they exhibit phase coexistence between a liquid-ordered (lo) and liquid-disordered (ld) phase and an associated critical point. However, plasma membranes are also asymmetric in terms of lipid type, lipid abundance, leaflet tension, and corresponding cholesterol distribution, suggesting that rafts cannot be examined separately from questions about elasticity, curvature torques, and internal mechanical stresses. Unfortunately, it is challenging to capture this wide range of physical phenomenology in a single model that can access sufficiently long length- and time scales. Here we extend the highly coarse-grained Cooke model for lipids, which has been extensively characterized on the curvature-elastic front, to also represent raft-like lo/ld mixing thermodynamics. In particular, we capture the shape and tie lines of a coexistence region that narrows upon cholesterol addition, terminates at a critical point, and has coexisting phases that reflect key differences in membrane order and lipid packing. We furthermore examine elasticity and lipid diffusion for both phase separated and pure systems and how they change upon the addition of cholesterol. We anticipate that this model will enable significant insight into lo/ld phase separation and the associated question of lipid rafts for membranes that have compositionally distinct leaflets that are likely under differential stress—like the plasma membrane. 

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4. Binding-induced lipid domains: Peptide-membrane interactions with PIP2 and PS

   https://doi.org/10.1016/j.bpj.2023.12.019  

   Al-Mualem, Ziareena A; Chen, Xiaobing; Shafieenezhad, Azam; Senning, Eric N; Baiz, Carlos R (July 2024, Biophysical Journal)

   Cell signaling is an important process involving complex interactions between lipids and proteins. The myristoylated alanine-rich C-kinase substrate (MARCKS) has been established as a key signaling regulator, serving a range of biological roles. Its effector domain (ED), which anchors the protein to the plasma membrane, induces domain formation in membranes containing phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidylserine (PS). The mechanisms governing the MARCKS-ED binding to membranes remain elusive. Here, we investigate the composition-dependent affinity and MARCKS-ED-binding-induced changes in interfacial environments using two-dimensional infrared spectroscopy and fluorescence anisotropy. Both negatively charged lipids facilitate the MARCKS-ED binding to lipid vesicles. Although the hydrogen-bonding structure at the lipid-water interface remains comparable across vesicles with varied lipid compositions, the dynamics of interfacial water show divergent patterns due to specific interactions between lipids and peptides. Our findings also reveal that PIP2 becomes sequestered by bound peptides, while the distribution of PS exhibits no discernible change upon peptide binding. Interestingly, PIP2 and PS become colocalized into domains both in the presence and absence of MARCKS-ED. More broadly, this work offers molecular insights into the effects of membrane composition on binding. 

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5. Lipid bilayer disruption induced by amphiphilic Janus nanoparticles: the non-monotonic effect of charged lipids

   https://doi.org/10.1039/c8sm02525h  

   Lee, Kwahun; Yu, Yan (March 2019, Soft Matter)

   In this study, we report the complex effects of charged lipids on the interaction between amphiphilic Janus nanoparticles and lipid bilayers. Janus nanoparticles are cationic on one hemisphere and hydrophobic on the other. We show that the nanoparticles, beyond threshold concentrations, induce holes in both cationic and anionic lipid bilayers mainly driven by hydrophobic interactions. However, the formation of these defects is non-monotonically dependent on ionic lipid composition. The electrostatic attraction between the particles and anionic lipid bilayers enhances particle adsorption and lowers the particle concentration threshold for defect initiation, but leads to more localized membrane disruption. Electrostatic repulsion leads to reduced particle adsorption on cationic bilayers and extensive defect formation that peaks at intermediate contents of cationic lipids. This study elucidates the significant role lipid composition plays in influencing how amphiphilic Janus nanoparticles interact with and perturb lipid membranes. 

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