The prevailing hypothesis of lipid droplet biogenesis proposes that neutral lipids accumulate within the lipid bilayer of the ER membrane from where they are budded off, enclosed by a protein-bearing phospholipid monolayer originating from the cytoplasmic leaflet of the ER membrane. We have used a variety of methods to investigate the nature of the sites of ER-lipid-droplet association in order to gain new insights into the mechanism of lipid droplet formation and growth. The three-dimensional perspectives provided by freeze-fracture electron microscopy demonstrate unequivocally that at sites of close association, the lipid droplet is not situated within the ER membrane; rather, both ER membranes lie external to and follow the contour of the lipid droplet, enclosing it in a manner akin to an egg cup (the ER) holding an egg (the lipid droplet). Freeze-fracture cytochemistry demonstrates that the PAT family protein adipophilin is concentrated in prominent clusters in the cytoplasmic leaflet of the ER membrane closely apposed to the lipid droplet envelope. We identify these structures as sites at which lipids and adipophilin are transferred from ER membranes to lipid droplets. These findings call for a re-evaluation of the prevailing hypothesis of lipid droplet biogenesis.

译文

脂滴生物发生的普遍假设提出,中性脂质在ER膜的脂质双层内积聚,从那里它们发芽,并被源自ER膜细胞质小叶的含蛋白质的磷脂单层包裹。我们使用了多种方法来研究ER-脂质-液滴关联位点的性质,以便对脂质液滴形成和生长的机制有新的认识。冷冻断裂电子显微镜提供的三维视角明确表明,在紧密结合的位置,脂质液滴不在ER膜内; 相反,两个ER膜都位于脂质液滴的外部并遵循其轮廓,以类似于装有鸡蛋 (脂质液滴) 的蛋杯 (ER) 的方式包围它。冷冻断裂的细胞化学表明,PAT家族蛋白脂肪蛋白集中在ER膜的细胞质小叶中与脂质液滴包膜紧密相连的突出簇中。我们将这些结构确定为脂质和脂肪蛋白从ER膜转移到脂质滴的位置。这些发现要求重新评估脂质液滴生物发生的流行假说。

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