Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany;
Department of Molecular Structural Biology, Max Planck Institute of Biochemistry, 82152 Martinsried, Germany.
Proc Natl Acad Sci U S A. 2019 Aug 20;116(34):16866-16871. doi: 10.1073/pnas.1903642116. Epub 2019 Aug 2.
Lipid droplets (LDs) are ubiquitous organelles comprising a central hub for cellular lipid metabolism and trafficking. This role is tightly associated with their interactions with several cellular organelles. Here, we provide a systematic and quantitative structural description of LDs in their native state in HeLa cells enabled by cellular cryoelectron microscopy. LDs consist of a hydrophobic neutral lipid mixture of triacylglycerols (TAG) and cholesteryl esters (CE), surrounded by a single monolayer of phospholipids. We show that under normal culture conditions, LDs are amorphous and that they transition into a smectic liquid-crystalline phase surrounding an amorphous core at physiological temperature under certain cell-cycle stages or metabolic scenarios. Following determination of the crystal lattice spacing of 3.5 nm and of a phase transition temperature below 43 °C, we attributed the liquid-crystalline phase to CE. We suggest that under mitotic arrest and starvation, relative CE levels increase, presumably due to the consumption of TAG metabolites for membrane synthesis and mitochondrial respiration, respectively, supported by direct visualization of LD-mitochondrial membrane contact sites. We hypothesize that the structural phase transition may have a major impact on the accessibility of lipids in LDs to enzymes or lipid transporters. These may become restricted in the smectic phase, affecting the exchange rate of lipids with surrounding membranes and lead to a different surface occupancy of LD-associated proteins. Therefore, the composition and the resulting internal structure of LDs is expected to play a key role in their function as hubs of cellular lipid flux.
脂滴 (LDs) 是普遍存在的细胞器，包含细胞脂质代谢和运输的中心枢纽。这种作用与其与几种细胞细胞器的相互作用密切相关。在这里，我们通过细胞冷冻电子显微镜，对 HeLa 细胞中天然状态下的 LD 进行了系统和定量的结构描述。LD 由三酰基甘油 (TAG) 和胆固醇酯 (CE) 的疏水中性脂质混合物组成，周围是单层磷脂。我们表明，在正常培养条件下，LD 是无定形的，并且在某些细胞周期阶段或代谢情况下，在生理温度下，它们会转变为围绕无定形核心的近晶液晶态。在确定了 3.5nm 的晶格间距和低于 43°C 的相变温度后，我们将液晶相归因于 CE。我们假设，在有丝分裂停滞和饥饿时，CE 的相对水平增加，这可能是由于 TAG 代谢物分别用于膜合成和线粒体呼吸所致，这得到了 LD-线粒体膜接触点的直接可视化的支持。我们假设，结构相变可能对 LD 中脂质对酶或脂质转运蛋白的可及性产生重大影响。在近晶相下，这些可能会受到限制，影响与周围膜的脂质交换率，并导致 LD 相关蛋白的不同表面占有率。因此，LD 的组成和由此产生的内部结构有望在其作为细胞脂质通量中心的功能中发挥关键作用。
