Endocytosis in plants: fact or artefact?
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ABSTRACT Whilst plant cells are apparently equipped with all the necessary molecular machinery for receptor‐mediated endocytosis, the physiological role of this process in these cells remains an enigma. In this article, we consider current opinions of endocytosis in plants and define some of the problems that have impeded progress in our under‐standing of the part played by endocytosis in the vesicle trafficking pathway.Keywords:
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Key features of clathrin-associated endocytosis are outlined, and current evidence in favor of clathrin-independent endocytosis and of its structural counterpart(s) is reviewed. We next summarize our recent observations on clathrin-independent endocytosis in primary cultures of rat foetal fibroblasts, using two reversible inhibitors of the formation of endocytic clathrin-coated pits, Severe inhibition of clathrin polymerization at the plasma membrane slows down receptor-mediated endocytosis of transferrin by ten-fold, without affecting bulk-flow endocytosis of fluid and membrane. Furthermore, the size of primary endocytic vesicles, identified by ultrastructural cytochemistry; is the same in control and treated cells. Two interpretations are offered. The most provocative one proposes that clathrin plays no role in the formation of primary endocytic vesicles, and is only required to concentrate receptors in endocytic pits, accelerating thereby internalization of ligand-receptor complexes. In the second interpretation, inhibition of clathrin polymerization unmasks an accessory molecular machinery, which is not operating under control conditions. In both cases, another endocytic molecular machinery is required and remains to be identified.
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Transferrin receptor
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Dorsal root ganglion
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Endocytosis of glycosylphosphatidyl inositol (GPI)-anchored proteins (GPI-APs) and the fluid phase takes place primarily through a dynamin- and clathrin-independent, Cdc42-regulated pinocytic mechanism. This mechanism is mediated by primary carriers called clathrin-independent carriers (CLICs), which fuse to form tubular early endocytic compartments called GPI-AP enriched endosomal compartments (GEECs). Here, we show that reduction in activity or levels of ARF1 specifically inhibits GPI-AP and fluid-phase endocytosis without affecting other clathrin-dependent or independent endocytic pathways. ARF1 is activated at distinct sites on the plasma membrane, and by the recruitment of RhoGAP domain-containing protein, ARHGAP10, to the plasma membrane, modulates cell-surface Cdc42 dynamics. This results in the coupling of ARF1 and Cdc42 activity to regulate endocytosis at the plasma membrane. These findings provide a molecular basis for a crosstalk of endocytosis with secretion by the sharing of a key regulator of secretory traffic, ARF1. PMID: 18084285
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At rat calyx of Held terminals, ATP was required not only for slow endocytosis, but also for rapid phase of compensatory endocytosis. An ATP-independent form of endocytosis was recruited to accelerate membrane retrieval at increased activity and temperature. ATP-independent endocytosis primarily involved retrieval of pre-existing membrane, which depended on Ca2+ and the activity of neutral sphingomyelinase but not clathrin-coated pit maturation. ATP-independent endocytosis represents a non-canonical mechanism that can efficiently retrieve membrane at physiological conditions without competing for the limited ATP at elevated neuronal activity.Neurotransmission relies on membrane endocytosis to maintain vesicle supply and membrane stability. Endocytosis has been generally recognized as a major ATP-dependent function, which efficiently retrieves more membrane at elevated neuronal activity when ATP consumption within nerve terminals increases drastically. This paradox raises the interesting question of whether increased activity recruits ATP-independent mechanism(s) to accelerate endocytosis at the same time as preserving ATP availability for other tasks. To address this issue, we studied ATP requirement in three typical forms of endocytosis at rat calyx of Held terminals by whole-cell membrane capacitance measurements. At room temperature, blocking ATP hydrolysis effectively abolished slow endocytosis and rapid endocytosis but only partially inhibited excess endocytosis following intense stimulation. The ATP-independent endocytosis occurred at calyces from postnatal days 8-15, suggesting its existence before and after hearing onset. This endocytosis was not affected by a reduction of exocytosis using the light chain of botulinum toxin C, nor by block of clathrin-coat maturation. It was abolished by EGTA, which preferentially blocked endocytosis of retrievable membrane pre-existing at the surface, and was impaired by oxidation of cholesterol and inhibition of neutral sphingomyelinase. ATP-independent endocytosis became more significant at 34-35°C, and recovered membrane by an amount that, on average, was close to exocytosis. The results of the present study suggest that activity and temperature recruit ATP-independent endocytosis of pre-existing membrane (in addition to ATP-dependent endocytosis) to efficiently retrieve membrane at nerve terminals. This less understood endocytosis represents a non-canonical mechanism regulated by lipids such as cholesterol and sphingomyelinase.
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Endocytosis 是细胞外的材料通过膜变丑通过被搬运进房间的一个过程。这进程不是一系列蛋白质根据按年代先后的顺序，而是包括精确被调整的许多成员的复杂进程在工作的简单一步一步的进程。endocytosis 的角色广泛地被划分成二个范畴，吞噬作用和 pinocytosis，后者根据 endocytosis 物质的尺寸被划分成四种：clathrin 依赖者 endocytosis， clathrin 涂的泡的直径是 100 150 nm；caveolin 依赖者 endocytosis， caveolin 的直径蛋白质涂的泡是 50 100 nm；macropinocytosis， macropinocytosis 的直径通常是 0.5 2 μm，有时直到 5 μm；clathrin 和 caveolin 独立人士 endocytosis。包括象 dynamin，肌动朊和 Rab 蛋白质家庭一样的 endophilin A1， A2， A3，和 endocytotic 蛋白质 B， B1a，和 B1b 的许多蛋白质涉及 endocytosis 并且在不同阶段起一个重要作用。反常 endocytosis 可以涉及某些疾病的发展。
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