Transport of newly synthesized sterol to the sterol-enriched plasma membrane occurs via nonvesicular equilibration
Transport of newly synthesized sterol to the sterol-enriched plasma membrane occurs via nonvesicular equilibration
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DOI：
10.1021/bi048296z
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发表时间：
2005-04-19
期刊：
影响因子：
2.9
通讯作者：
Menon, AK
中科院分区：
文献类型：
--
作者：
Baumann, NA;Sullivan, DP;Menon, AK
新合成的甾醇从其合成位点内质网（ER）转运到富含甾醇的质膜（PM）的机制尚未完全了解。在哺乳动物细胞中的研究表明，新合成的胆固醇在高尔基体旁路囊泡和/或通过非囊泡过程中转运到PM。使用酵母酿酒酵母作为一个模型系统，我们现在排除了一个重要的作用，已知的囊泡运输途径运输的主要酵母甾醇，麦角甾醇，从其网站的合成PM。我们使用基于环糊精的甾醇捕获试验表明，新合成的麦角甾醇的PM运输是不变的，在Sec 18 p，几乎所有的细胞内囊泡运输事件所需的蛋白质缺陷的细胞;我们还表明，运输是不阻断在ER或囊泡与PM融合的运输囊泡的形成有缺陷的细胞。相反，我们的数据表明，运输发生的平衡（t（1/2）类似于10-15分钟）的ER和PM麦角固醇池通过一个双向的，非囊泡的过程，是饱和的野生型指数增长的酵母。为了调和平衡过程与高麦角固醇浓度的PM相对于ER，我们注意到，大部分的PM麦角固醇被发现与鞘脂在膜筏，与游离甾醇共存。我们建议，游离甾醇的浓度是相似的PM和ER，只有自由（nonraft）甾醇分子有机会获得一个非囊泡运输途径，连接两个细胞器。这是第一次描述酵母中的生物合成甾醇转运。
The mechanism by which newly synthesized sterols are transported from their site of synthesis, the endoplasmic reticulum (ER), to the sterol-enriched plasma membrane (PM) is not fully understood. Studies in mammalian cells suggest that newly synthesized cholesterol is transported to the PM in Golgi-bypassing vesicles and/or via a nonvesicular process. Using the yeast Saccharomyces cerevisiae as a model system, we now rule out an essential role for known vesicular transport pathways in transporting the major yeast sterol, ergosterol, from its site of synthesis to the PM. We use a cyclodextrin-based sterol capture assay to show that transport of newly synthesized ergosterol to the PM is unaltered in cells defective in Sec18p, a protein required for almost all intracellular vesicular trafficking events; we also show that transport is not blocked in cells that are defective in formation of transport vesicles at the ER or in vesicle fusion with the PM. Our data suggest instead that transport occurs by equilibration (t(1/2) similar to 10-15 min) of ER and PM ergosterol pools via a bidirectional, nonvesicular process that is saturated in wild-type exponentially growing yeast. To reconcile an equilibration process with the high ergosterol concentration of the PM relative to ER, we note that a large fraction of PM ergosterol is found condensed with sphingolipids in membrane rafts that coexist with free sterol. We propose that the concentration of free sterol is similar in the PM and ER and that only free (nonraft) sterol molecules have access to a nonvesicular transport pathway that connects the two organelles. This is the first description of biosynthetic sterol transport in yeast.