Lippincott-Schwartz J
Cell Biology and Metabolism Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892.
Subcell Biochem. 1993;21:95-119. doi: 10.1007/978-1-4615-2912-5_5.
Membrane traffic between the ER and Golgi is now recognized as a carefully regulated process controlled by distinct anterograde (to the Golgi) and retrograde (to the ER) pathways. These pathways link two organelles with different morphologies, structures, and localizations within the cell. The ER, which is involved in multiple cellular functions including protein biosynthesis and folding, extends to the cell periphery and forms a dynamic tubule reticulum. By contrast, the Golgi apparatus, which functions in membrane sorting and recycling events, is localized at the center of the cell near the MTOC and is comprised of compact cisternal units. The required transport into the Golgi apparatus of newly synthesized proteins exported from the ER offers a twofold advantage to the cell. First, the rate of movement of membrane and protein through the biosynthetic pathway can be controlled by the selective use of a recycling pathway. Second, membrane moving through the biosynthetic pathway enters a structure specialized for sorting of membrane to different final destinations in the cell Control of biosynthetic transport within the ER/Golgi system involves the utilization of two alternative transport pathways: anterograde (ER to Golgi) and retrograde (Golgi to ER). These two pathways share a common regulatory system involving membrane assembly/disassembly of cytosolic coatomer proteins. Thus, conditions that favor irreversible coatomer binding (i.e., GTP gamma S) inhibit retrograde transport while producing anterograde transport intermediates. Conditions that prevent coatomer binding (i.e., BFA) inhibit anterograde transport and enhance retrograde transport. The underlying biochemical machinery that normally balances anterograde and retrograde membrane traffic between the ER and Golgi is only just beginning to be understood. Any model to explain this system, however, must account for the morphologic characteristics of the membranes involved. Whereas anterograde traffic involves discontinuous "coated" structures moving from peripheral sites in the ER toward the central Golgi, retrograde traffic utilizes continuous "noncoated" tubule structures that move from a central site (i.e., the CGN) to the peripheral ER (see Figure 3). Such a system maximizes volume transport (utilizing vacuolar structures) in the anterograde direction and membrane transport (utilizing tubules) in the retrograde direction. It is therefore ideal for sorting of bulk flow lumenal contents from recycling membrane early in the biosynthetic pathway.
内质网（ER）和高尔基体之间的膜转运现在被认为是一个受到严格调控的过程，由不同的顺行（向高尔基体）和逆行（向内质网）途径控制。这些途径连接了细胞内两种形态、结构和定位不同的细胞器。内质网参与包括蛋白质生物合成和折叠在内的多种细胞功能，延伸至细胞周边并形成动态的管状网络。相比之下，高尔基体在膜分选和回收事件中发挥作用，位于细胞中心靠近微管组织中心（MTOC）的位置，由紧密的扁平囊泡单位组成。从内质网输出的新合成蛋白质进入高尔基体所需的转运为细胞带来了双重优势。首先，通过选择性地利用回收途径，可以控制膜和蛋白质在生物合成途径中的移动速度。其次，通过生物合成途径移动的膜进入一个专门用于将膜分选到细胞内不同最终目的地的结构。内质网/高尔基体系统内生物合成转运的控制涉及利用两种替代转运途径：顺行（内质网到高尔基体）和逆行（高尔基体到内质网）。这两种途径共享一个涉及胞质外衣蛋白膜组装/拆卸的共同调节系统。因此，有利于不可逆外衣蛋白结合的条件（即GTPγS）会抑制逆行转运，同时产生顺行转运中间体。阻止外衣蛋白结合的条件（即BFA）会抑制顺行转运并增强逆行转运。通常平衡内质网和高尔基体之间顺行和逆行膜转运的潜在生化机制才刚刚开始被理解。然而，任何解释这个系统的模型都必须考虑到所涉及膜的形态特征。顺行转运涉及从不连续的“有被”结构从内质网的周边部位向中央高尔基体移动，而逆行转运利用连续的“无被”管状结构，这些结构从中央部位（即顺面高尔基体网络，CGN）移动到周边内质网（见图3）。这样的系统在顺行方向上最大化体积运输（利用液泡结构），在逆行方向上最大化膜运输（利用小管）。因此，它非常适合在生物合成途径早期从回收膜中分选大量流动的管腔内容物。
