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(==Possible Membrane Sources of the isolation membrane== Some of the possible scenarios for IM formation are illustrated here. According to the maturation model, upon induction of autophagy, membrane may be directly derived from the ER by folding (A), or )
 
 
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Abbreviations: Atg, autophagy-related gene; ER, endoplasmic reticulum; IM, isolation membrane; PAS, preautophagosomal structure
 
Abbreviations: Atg, autophagy-related gene; ER, endoplasmic reticulum; IM, isolation membrane; PAS, preautophagosomal structure
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Recent real-time studies have revealed that newly forming IMs continue to elongate until the final sealing of the autophagosome [22]. This observation is difficult to reconcile with the direct maturation of an existing organelle into an IM. An alternative model posits that the IM is assembled de novo. In this model, nonvesicular transport (Figure 2C) or local synthesis (Figure 2D) of lipids supplies the material for the growing membrane. Currently, support for this model comes mostly in the form of negative results, namely the lack of convincing identification of vesicles or membrane cisternae fusing to the IM, despite numerous transmission and freeze-fracture electron microscopic studies. Similarly, conventional vesicular structures have not been observed by electron microscopy at the PAS in yeast, although retrograde transport of some Atg proteins has recently been demonstrated, presumably involving a vesicular mechanism [23]. Interestingly, the cytoplasm in the area of the PAS largely excludes free ribosomes that normally fill the cytosol [24], consistent with a high lipid content or membranaceous barrier in this region.
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In many cell types, induction of autophagy can be remarkably robust, resulting in a rapid appearance of numerous autophagosomes throughout the cell. Whether local synthesis or nonvesicular transport of lipids and their assembly into membranes could be efficient enough to generate sufficient amounts of membrane in a short time is unclear. It is also possible that multiple membrane pools contribute to the IM. The distinct steps of IM formation—nucleation, assembly, and elongation—may also rely on different membrane sources [10].
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==Reference==
  
 
Citation: Juhasz G, Neufeld TP (2006) Autophagy: A Forty-Year Search for a Missing Membrane Source. PLoS Biol 4(2): e36. doi:10.1371/journal.pbio.0040036
 
Citation: Juhasz G, Neufeld TP (2006) Autophagy: A Forty-Year Search for a Missing Membrane Source. PLoS Biol 4(2): e36. doi:10.1371/journal.pbio.0040036

Latest revision as of 08:31, 10 May 2011

Possible Membrane Sources of the isolation membrane

Some of the possible scenarios for IM formation are illustrated here. According to the maturation model, upon induction of autophagy, membrane may be directly derived from the ER by folding (A), or in the form of vesicular transport (B). In the assembly model, membrane may be assembled de novo at the site of IM formation, originating from nonvesicular transport, such as micellar, as shown in (C), or local synthesis (D). See text for further details.

Figure 2. Journal.pbio.0040036.g002.png

Abbreviations: Atg, autophagy-related gene; ER, endoplasmic reticulum; IM, isolation membrane; PAS, preautophagosomal structure


Recent real-time studies have revealed that newly forming IMs continue to elongate until the final sealing of the autophagosome [22]. This observation is difficult to reconcile with the direct maturation of an existing organelle into an IM. An alternative model posits that the IM is assembled de novo. In this model, nonvesicular transport (Figure 2C) or local synthesis (Figure 2D) of lipids supplies the material for the growing membrane. Currently, support for this model comes mostly in the form of negative results, namely the lack of convincing identification of vesicles or membrane cisternae fusing to the IM, despite numerous transmission and freeze-fracture electron microscopic studies. Similarly, conventional vesicular structures have not been observed by electron microscopy at the PAS in yeast, although retrograde transport of some Atg proteins has recently been demonstrated, presumably involving a vesicular mechanism [23]. Interestingly, the cytoplasm in the area of the PAS largely excludes free ribosomes that normally fill the cytosol [24], consistent with a high lipid content or membranaceous barrier in this region.

In many cell types, induction of autophagy can be remarkably robust, resulting in a rapid appearance of numerous autophagosomes throughout the cell. Whether local synthesis or nonvesicular transport of lipids and their assembly into membranes could be efficient enough to generate sufficient amounts of membrane in a short time is unclear. It is also possible that multiple membrane pools contribute to the IM. The distinct steps of IM formation—nucleation, assembly, and elongation—may also rely on different membrane sources [10].

Reference

Citation: Juhasz G, Neufeld TP (2006) Autophagy: A Forty-Year Search for a Missing Membrane Source. PLoS Biol 4(2): e36. doi:10.1371/journal.pbio.0040036

http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040036

Published: February 14, 2006

Copyright: © 2006 Juhasz and Neufeld. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in ay medium, provided the original author and source are credited.

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