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Lipid droplets (LDs) are intracellular organelles whose primary function is energy storage. Known to emerge from the endoplasmic reticulum (ER) bilayer, LDs have a unique structure with a core consisting of neutral lipids, triacylglycerol (TG) or sterol esters (SE), surrounded by a phospholipid (PL) monolayer and decorated by proteins that come and go throughout their complex lifecycle. In this Feature Article, we review recent developments in computational studies of LDs, a rapidly growing area of research. We highlight how molecular dynamics (MD) simulations have provided valuable molecular-level insight into LD targeting and LD biogenesis. Additionally, we review the physical properties of TG from different force fields compared with experimental data. Possible future directions and challenges are discussed.

Lipid droplets (LDs) are neutral lipid storage organelles surrounded by a phospholipid (PL) monolayer. LD biogenesis from the endoplasmic reticulum (ER) is driven by phase separation of neutral lipids, overcoming surface tension and membrane deformation. However, the core biophysics of the initial steps of LD formation remain relatively poorly understood. Here, we use a tunable, phenomenological coarse-grained (CG) model to study triacylglycerol (TG) nucleation in a bilayer membrane. We show that PL rigidity has a strong influence on TG lensing and membrane remodeling: When membrane rigidity increases, TG clusters remain more planar with high anisotropy but a minor degree of phase nucleation. This finding is confirmed by free energy sampling simulations that calculate the potential of mean force (PMF) as a function of the degree of nucleation and anisotropy. We also show that asymmetric tension, controlled by the number of PLs on each membrane leaflet, determines the budding direction. A TG lens buds in the direction of the monolayer containing excess PLs to allow for better PL coverage of TG, consistent with reported experiments. Finally, two governing mechanisms of the LD growth, Ostwald ripening and merging, are observed. Taken together, this study characterizes the interplay between two thermodynamic quantities during the initial LD phases, the TG bulk free energy and membrane remodeling energy.

Lipid droplets (LDs) are intracellular storage organelles composed of neutral lipids, such as triacylglycerol (TG), surrounded by a phospholipid (PL) monolayer decorated with specific proteins. Herein, we investigate the mechanism of protein association during LD and bilayer membrane expansion. We find that the neutral lipids play a dynamic role in LD expansion by further intercalating with the PL monolayer to create more surface-oriented TG molecules (SURF-TG). This interplay both reduces high surface tension incurred during LD budding or growth and also creates expansion-specific surface features for protein recognition. We then show that the auto-inhibitory (AI) helix of CTP:phosphocholine cytidylyltransferase, a protein known to target expanding monolayers and bilayers, preferentially associates with large packing defects in a sequence-specific manner. Despite the presence of three phenylalanines, the initial binding with bilayers is predominantly mediated by the sole tryptophan due to its preference for membrane interfaces. Subsequent association is dependent on the availability of large, neighboring defects that can accommodate the phenylalanines, which are more probable in the stressed systems. Tryptophan, once fully associated, preferentially interacts with the glycerol moiety of SURF-TG in LDs. The calculation of AI binding free energy, hydrogen bonding and depth analysis, and in-silico mutation experiments support the findings. Hence, SURF-TG can both reduce surface tension and mediate protein association, facilitating class II protein recruitment during LD expansion.

Lipid droplets (LDs) are neutral lipid storing organelles surrounded by a phospholipid (PL) monolayer. At present, how LDs are formed in the endoplasmic reticulum (ER) bilayer is poorly understood. In this study, we present a revised triolein (TG) model, the main constituent of the LD core, and characterize its properties in a bilayer membrane to demonstrate the implications of its behavior in LD biogenesis. In all-atom (AA) bilayer simulations, TG resides at the surface, adopting PL-like conformations (denoted in this work as SURF-TG). Free energy sampling simulation results estimate the barrier for TG relocating from the bilayer surface to the bilayer center to be [~]2 kcal/mol in the absence of an oil lens. Conical SURF-TG is able to modulate membrane properties by increasing PL ordering, decreasing bending modulus, and creating local negative curvature. The other conical lipid, dioleoyl-glycerol (DAG), also reduces the membrane bending modulus and populates the negative curvature regions. A phenomenological coarse-grained (CG) model is also developed to observe larger scale SURF-TG-mediated membrane deformation. The CG simulations confirm that TG nucleates between the bilayer leaflets at a critical concentration when SURF-TG is evenly distributed. However, when one monolayer contains more SURF-TG, the membrane bends toward the other leaflet. The central conclusion of this study is that SURF-TG is a negative curvature inducer, as well as a membrane modulator. To this end, a model is proposed in which the accumulation of SURF-TG in the luminal leaflet bends the ER bilayer toward the cytosolic side, followed by TG nucleation. TOC IMAGE O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=93 SRC="FIGDIR/small/440804v1_ufig1.gif" ALT="Figure 1"> View larger version (19K): org.highwire.dtl.DTLVardef@18fc19corg.highwire.dtl.DTLVardef@f02a31org.highwire.dtl.DTLVardef@1621324org.highwire.dtl.DTLVardef@1d6ca39_HPS_FORMAT_FIGEXP M_FIG C_FIG

Free-standing GaN wafers were fabricated by hydride vapor phase epitaxy (HVPE) using an in-situ self-separation technique. Separation of GaN happened during the high-temperature GaN growth through thermal decomposition of the decomposable buffer layer (DBL). We optimized the growth condition of DBL, which affects not only the separation property but also the crystallinity of GaN. Free standing GaN revealed negligible residual stress, narrow (0002) omega rocking curve linewidth (67ʺ), and low etch pit density (6×10 6 cm −2 ) as well. The carrier concentration of 4×10 18 cm −3 and mobility of 150 cm 2 /V s was observed from Hall effect measurement. These results showed the feasibility of the in-situ self-separation method to obtain high quality free-standing GaN wafer.

Thick GaN film was grown by hydride vapor phase epitaxy (HVPE) on spinel covered ZnO layer. Single crystal ZnO was grown on c -plane sapphire by ultra high vacuum (UHV) DC sputtering. A thin Al layer was thermally deposited on it. A spinel layer was formed by a solid phase reaction of the Al and ZnO layers, and GaN thick film was grown on it. Structural and chemical properties of the spinel layer were studied by X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS), respectively. The crystal quality of GaN was observed by atomic force microscope (AFM) and transmission electron microscope (TEM). We found that the spinel layer is helpful for the reduction of zinc and oxygen diffusion into GaN during the HVPE growth.

A new hydride vapor phase epitaxy (HVPE)-based approach for the fabrication of freestanding GaN (FS-GaN) substrates was investigated. For the direct formation of low-temperature GaN (LT-GaN) layers, the growth parameters were optimized: the polarity of ZnO, the growth temperature, and the V/III ratio. The FS-GaN layer was achieved by gas etching in an HVPE reactor. A fingerprint of Zn out-diffusion was detected in the photoluminescence measurements, especially for the thin (80 μm) FS-GaN film; however, a thicker film (400 μm) was effectively reduced by optimization of GaN growth.

We report on the MBE growth of GaSb layer by using three-step ZnTe buffer layer: a thin ZnTe buffer layer was grown at 250 °C (LT-buffer), then annealed at 330 °C (HT-annealing) and a thick buffer layer was grown at 310 °C (HT-buffer). The effect of a three-step buffer on the surface and structural quality of the GaSb layer has been investigated. Two-dimensional reciprocal space mapping (RSM) results of the GaSb layers clearly indicate that the structural deformation of the GaSb layer is greatly reduced by introducing the ZnTe buffer, which remarkably improves the crystallinity of the GaSb layer.