Endoplasmic reticulum export sites and golgi bodies behave as single mobile secretory units in plant cells
Endoplasmic reticulum export sites and golgi bodies behave as single mobile secretory units in plant cells
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DOI：
10.1105/tpc.022673
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发表时间：
2004-07-01
期刊：
影响因子：
11.6
通讯作者：
Brandizzi, F
中科院分区：
文献类型：
--
作者：
DaSilva, LLP;Snapp, EL;Brandizzi, F
与动物相反，植物细胞包含分布在整个细胞质中的多个移动的高尔基体堆。然而，植物内质网（ER）表面上的蛋白质输出站点的分布和动态尚未被表征。一个被广泛接受的ER到高尔基体转运模型是基于COPII和COPI外壳复合物的顺序作用。COPII复合物通过ER膜上胞质组分的有序募集来组装。在这里，我们已经看到了两个早期的COPII机制，小GTP酶Sar1p和GTP交换因子Sec12p在活烟草（烟草）叶表皮细胞。通过在体内共聚焦激光扫描显微镜和荧光恢复后的光漂白实验，我们表明，Sar1p周期移动的点状结构，跟踪与高尔基体在密切的接近，但包含的区域，物理上分离的高尔基体。相比之下，Sec12 p沿着ER网络均匀分布，并且不积聚在这些结构中，这与Sec12 p不成为COPII囊泡的一部分的事实一致。我们建议，点状积累的Sar1p代表ER出口网站（ERES）。这些位点可能代表了Sar1p包被的ER膜、新生COPII膜和转运中的COPII载体的组合，这些载体尚未失去它们的外壳。ERES可以通过过量产生高尔基体膜蛋白而不是可溶性大量流动货物来诱导。少数点状Sar1p位点观察到独立的高尔基体，这些可能是新生的ERES。绝大多数ERES形成分泌单位，它们与高尔基体一起沿着ER表面移动，但移动不影响这两个细胞器之间的货物运输速率。此外，我们可以证明使用药物布雷菲德菌素A，ERES的形成是严格依赖于从高尔基体的功能性逆行运输路线。
In contrast with animals, plant cells contain multiple mobile Golgi stacks distributed over the entire cytoplasm. However, the distribution and dynamics of protein export sites on the plant endoplasmic reticulum (ER) surface have yet to be characterized. A widely accepted model for ER-to-Golgi transport is based on the sequential action of COPII and COPI coat complexes. The COPII complex assembles by the ordered recruitment of cytosolic components on the ER membrane. Here, we have visualized two early components of the COPII machinery, the small GTPase Sar1p and its GTP exchanging factor Sec12p in live tobacco (Nicotiana tabacum) leaf epidermal cells. By in vivo confocal laser scanning microscopy and fluorescence recovery after photobleaching experiments, we show that Sar1p cycles on mobile punctate structures that track with the Golgi bodies in close proximity but contain regions that are physically separated from the Golgi bodies. By contrast, Sec12p is uniformly distributed along the ER network and does not accumulate in these structures, consistent with the fact that Sec12p does not become part of a COPII vesicle. We propose that punctate accumulation of Sar1p represents ER export sites (ERES). The sites may represent a combination of Sar1p-coated ER membranes, nascent COPII membranes, and COPII vectors in transit, which have yet to lose their coats. ERES can be induced by overproducing Golgi membrane proteins but not soluble bulk-flow cargos. Few punctate Sar1p loci were observed that are independent of Golgi bodies, and these may be nascent ERES. The vast majority of ERES form secretory units that move along the surface of the ER together with the Golgi bodies, but movement does not influence the rate of cargo transport between these two organelles. Moreover, we could demonstrate using the drug brefeldin A that formation of ERES is strictly dependent on a functional retrograde transport route from the Golgi apparatus.