FUNCTIONAL SIGNIFICANCE OF LIPID DROPLETS AS A NEW MEMBRANE DOMAIN

脂滴（ld）一直被认为是特殊分化细胞（如白色脂肪细胞）中的脂质储存库。但事实上，ld几乎存在于任何类型的细胞中，但它们的功能一直不明确。在本研究中，我们主要在非脂肪细胞中检测ld的生理功能，得到以下结果。1)小洞蛋白（caveolin-2β）优先定位于ld表面。小窝蛋白-2β的中心疏水部分是定位所必需的。当brefeldin a抑制内质网到高尔基体的转运时，其他小巢蛋白包括小巢蛋白-1和小巢蛋白-2α也定位在LD上。2)LD表面显示为含有游离胆固醇的磷脂单层。LD磷脂的脂肪酸组成独特，与内质网和木筏不同。3)ADRP（或adipophilin）是一种特异性定位于非脂肪细胞ldds的蛋白，通过使用来自小洞蛋白的不同序列线索靶向ldds。在ADRP分子的N端和C端发现了两个冗余信号。4)ADRP与ARF1结合，当细胞被brefeldin A或gdp结合的arf处理时，ADRP与ld分离。1.5)通过对纯化的ld进行蛋白质组学分析，Rab18被鉴定为ld特异性蛋白。Rab18的过表达消除了LD表面的ADRP，诱导LD- er复合物的形成。当ADRP的表达被RNA干扰或当细胞被brefeldin a处理时，也会形成相同的对抗。这些结果表明，lld不仅仅是储存过多的脂质，而且可能参与多种重要的功能，包括细胞内脂质运输和代谢。

Lipid droplets(LDs) have been regarded simply as a depot of lipids in specially differentiated cells, e.g., white adipose cells. But in fact LDs are found virtually in any cell type, but their function has been left ambiguous. In the present study, we attempted to examine the physiological function of LDs mainly in non-adipose cells, and obtained the following results.1)Caveolins, especially caveolin-2β, was preferentially localized in the surface of LDs. The central hydrophobic portion of caveolin-2βwas necessary for the localization. Other caveolins, including caveolin-1 and caveolin-2α were also localized to LDs when the ER-to-Golgi transport was inhibited by brefeldin A.2)The LD surface was shown to be a phospholipid monolayer containing free cholesterol. The fatty acid composition of the LD phospholipids was unique and different from that of the ER and rafts.3)ADRP (or adipophilin), a protein specifically localized to LDs in non-adipose cells, was targeted to LDs by using different sequence cues from caveolins. Two redundant signals were found in the N and C termini of the ADRP molecule.4)ADRP was shown to bind to ARF1,and was dissociated from LDs when cells were treated with brefeldin A or GDP-bound ARF1.5)By proteomic analysis of purified LDs, Rab18 was identified as an LD-specific protein.Overexpression of Rab18 eliminated ADRP from the LD surface, and induced formation of the LD-ER apposition. The same apposition was formed when the expression of ADRP was knocked down by RNA interference or when cells were treated by brefeldin A.These results indicate that LDs do not simply store excessive lipids, but may be involved in various important functions including intracellular lipid trafficking and metabolism.