Dobrosotskaya Irina Y, Goldstein Joseph L, Brown Michael S, Rawson Robert B
Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9046.
J Biol Chem. 2003 Sep 12;278(37):35837-43. doi: 10.1074/jbc.M306476200. Epub 2003 Jul 3.
In mammalian cells, membrane-bound sterol regulatory element-binding proteins (SREBPs) are transported from ER to Golgi where they are processed proteolytically to generate soluble transcription factors that activate lipid synthesis. ER-to-Golgi transport requires SCAP, a sterol-regulated escort protein. In sterol-treated cells, the SCAP/SREBP complex binds to Insig-1 or Insig-2, which retains the complex in the ER, blocking SREBP processing and decreasing lipid synthesis. In Drosophila cells, the endogenous SCAP/SREBP complex is transported to Golgi, but transport is blocked by phosphatidylethanolamine instead of sterols. Here, we show that mammalian SREBP-2 is not transported to Golgi when expressed in Drosophila cells. Transport requires co-expression of mammalian SCAP. Sterols block transport of the mammalian SCAP/SREBP-2 complex, but only when mammalian Insig-1 or -2 is co-expressed. These reconstitution studies define SCAP and Insig as the minimal requirements for sterol-regulated transport of SREBPs from ER to Golgi. They indicate that insect cells can respond to sterols when proper regulatory proteins are expressed.
在哺乳动物细胞中，膜结合的固醇调节元件结合蛋白（SREBPs）从内质网（ER）转运至高尔基体，在那里它们被蛋白酶解加工以产生激活脂质合成的可溶性转录因子。从内质网到高尔基体的转运需要SCAP，一种固醇调节的护送蛋白。在经固醇处理的细胞中，SCAP/SREBP复合物与Insig-1或Insig-2结合，后者将该复合物保留在内质网中，阻止SREBP的加工并减少脂质合成。在果蝇细胞中，内源性SCAP/SREBP复合物被转运至高尔基体，但转运被磷脂乙醇胺而非固醇所阻断。在此，我们表明，哺乳动物SREBP-2在果蝇细胞中表达时不会被转运至高尔基体。转运需要共表达哺乳动物SCAP。固醇会阻断哺乳动物SCAP/SREBP-2复合物的转运，但仅在共表达哺乳动物Insig-1或-2时才会如此。这些重组研究确定了SCAP和Insig是SREBPs从内质网到高尔基体的固醇调节转运的最低要求。它们表明，当表达适当的调节蛋白时，昆虫细胞能够对固醇作出反应。
