Lipid droplet (LD) biogenesis begins in the endoplasmic reticulum (ER) bilayer, but how the ER topology impacts this process is unclear. An early step in LD formation is nucleation, wherein free neutral lipids, mainly triacylglycerols (TGs) and sterol esters (SEs), condense into a nascent LD. How this transition occurs is poorly known. Here, we found that LDs preferably assemble at ER tubules, with higher curvature than ER sheets, independently of the LD assembly protein seipin. Indeed, the critical TG concentration required for initiating LD assembly is lower at curved versus flat membrane regions. In agreement with this finding, flat ER regions bear higher amounts of free TGs than tubular ones and present less LDs. By using an in vitro approach, we discovered that the presence of free TGs in tubules is energetically unfavorable, leading to outflow of TGs to flat membrane regions or condensation into LDs. Accordingly, in vitro LD nucleation can be achieved by the sole increase of membrane curvature. In contrast to TGs, the presence of free SEs is favored at tubules and increasing SE levels is inhibitory to the curvature-induced nucleation of TG LDs. Finally, we found that seipin is enriched at ER tubules and controls the condensation process, preventing excessive tubule-induced nucleation. The absence of seipin provokes erratic LD nucleation events determined by the abundance of ER tubules. In summary, our data indicate that membrane curvature catalyzes LD assembly.

脂滴 (LD) 生物发生在内质网 (ER) 双层中开始，但 ER 拓扑如何影响这一过程尚不清楚。LD 形成的早期步骤是成核，其中游离的中性脂质，主要是三酰基甘油 (TG) 和甾醇酯 (SE)，凝结成新生的 LD。这种转变是如何发生的，我们知之甚少。在这里，我们发现 LD 最好在 ER 小管处组装，其曲率高于 ER 片，与 LD 组装蛋白 seipin 无关。事实上，启动 LD 组装所需的临界 TG 浓度在弯曲膜区域与平坦膜区域相比较低。与这一发现一致，平坦的 ER 区域比管状区域具有更多的游离 TG，并且呈现的 LD 更少。通过使用体外方法，我们发现小管中游离TGs的存在在能量上是不利的，导致TGs流出到平坦的膜区域或凝结成LDs。因此，仅通过增加膜曲率就可以实现体外LD 成核。与 TG 相比，游离 SE 的存在有利于小管，并且增加 SE 水平会抑制曲率诱导的 TG LD 成核。最后，我们发现 seipin 在 ER 小管中富集并控制冷凝过程，防止过度的小管诱导成核。seipin 的缺失引发了不稳定的 LD 成核事件，这由 ER 小管的丰度决定。总之，我们的数据表明膜曲率催化 LD 组装。