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Vesicle trafficking -- vesicle transport in cell -- molecular mechanismAutophagy and intracellular trafficking -
Recent work from our laboratory showed that the WDD is specifically recognized by a novel amino acid element found in the intracellular domain of the transmembrane protein TMEM59, as well as other molecules. The interaction between the WDD of ATG16L1 and TMEM59 triggers an unconventional autophagic process that cause LC3 labelling of the same single-membrane endosomes where TMEM59 is located, thus promoting a more efficient lysosomal targeting of these vesicles.
Additionally, we found that TA alters the ability of the WDD of ATG16L1 to interact with the amino acid motif that recognizes this region.
Such alteration impairs the unconventional autophagic activity of TMEM59 and disrupts its normal intracellular trafficking and its ability to engage ATG16L1 in response to Staphylococcus aureus infection. Therefore, identification of additional motif-containing WDD-binding proteins will likely unravel the signalling pathways whose dysfunction is required to trigger the onset of Crohn disease.
To this end, we designed custom-made peptide microarrays with the aim of searching for motif-containing proteins. Notably, 23 cytokine receptors are represented between the collection of identified molecules. We selected a subgroup of these candidates based on their diverse pro- and anti-inflammatory activities.
Then, we conducted several molecular and functional assays, which will be shown in this talk. The mechanisms of autophagosome trafficking are similar to those of other organelles trafficking within cells. The machinery mainly includes cytoskeletal systems such as actin and microtubules, motor proteins such as myosins and the dynein-dynactin complex, and other proteins like LC3 on the membrane of autophagosomes.
Factors regulating autophagosome trafficking have not been widely studied. To date the main reagents identified for disrupting autophagosome trafficking include: 1.
Microtubule polymerization reagents, which disrupt microtubules by interfering with microtubule dynamics, thus directly influence microtubule-dependent autophagosome trafficking 2.
Autophagy is Autophagy and intracellular trafficking lysosome-mediated Autophhagy system ihtracellular is trafficming highly conserved pathway present in Digestive health and lactose intolerance eukaryotes. In Autophagy and intracellular trafficking Glutathione for athletes, double-membrane autophagosomes form and engulf cytoplasmic components, delivering intracelluar to the lysosome ingracellular degradation. Autophagy is essential for cell health and can Autopahgy activated to function as a trafflcking pathway in intrace,lular absence Comprehensive weight support nutrients or as a nad pathway to eliminate damaged organelles or even to eliminate invading pathogens. Autophagy was first identified as a pathway in mammalian cells using morphological techniques, but the Atg autophagy-related genes required for autophagy were identified in yeast genetic screens. Despite tremendous advances in elucidating the function of individual Atg proteins, our knowledge of how autophagosomes form and subsequently interact with the endosomal pathway has lagged behind. Recent progress toward understanding where and how both the endocytotic and autophagic pathways overlap is reviewed here. A utophagy is a lysosome-mediated pathway for the degradation of cytosolic proteins and organelles, which is essential for cell homeostasis, development, and for the prevention of several human diseases and infection Choi et al. Plants respond to pathogen attack with dynamic Lipid metabolism and glucose utilization of the endomembrane system and rapid trafcicking of membrane traffic to facilitate effective host defence. Mounting Heightened awareness state indicates the involvement of endocytic, intdacellular, and Autpohagy Digestive health and lactose intolerance pathways in immune receptor Aytophagy, signal transduction, and execution of multiple defence intracellupar including programmed cell trafficiing PCD. Autophagy is a conserved intracellular trafficking and degradation process and has been implicated in basal immunity as well as in some forms of immune receptor-mediated vacuolar cell death. However, the regulatory interplay of autophagy and other membrane trafficking pathways in PCD and defence responses remains obscure. This review therefore highlights recent advances in the understanding of autophagic and membrane trafficking during plant immunity, and discusses emerging molecular links and functional interconnections. Plants have shaped throughout their co-evolutionary battle with microbes a sophisticated innate immune system that relies on different reservoirs of immune receptors and a plethora of defence responses Jones and Dangl,Autophagy and intracellular trafficking -
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A upon autophagy induction, membrane is sequestered from pools of lipid donors such as the ER, mitochondria, Golgi, endosome and vacuole to form the pre-autophagosomal structure PAS or isolation membrane IM.
B PI in the PAS is converted to PI3P via Vps34 and Atg8-PE is enriched, allowing for IM expansion and closure. C Additional lipid membrane from Atg9-Atg27 vesicle trafficking allows for further expansion of the IM. D Autophagosome AV maturation is completed once PI3P is converted into PI by myotubularin phosphatases such as Ymr1.
The mature AV can fuse to the vacuole which has converted PI3P into PI 3,5 P2 via Fab1 kinase. PI 3,5 P2 permits fusion of AV and endosomes. Vacuole lipid homeostasis is further aided by Snx4 trafficking of PS and Atg27 from the endosome and vacuole.
During the initial stages of autophagy, essential Atg proteins are recruited to PAS structures to nucleate the gathering of membrane de novo and to generate autophagosome precursors called phagophores.
In mammalian cells, phagophores can be formed at PAS sites proximal to ER, mitochondria, or plasma membrane [ 13 , 14 ]. In yeast however, PAS structures are perivacuolar in nature, leading phagophores to originate at locations proximal to the vacuole with membrane contributions with other cellular locations like discussed below.
While mammalian cells can display many PAS structures at steady state, yeast often display one of these structures at any given time.
Initially, activated or triggered by upstream inhibition of mTOR, core autophagy proteins collect at the PAS structure. These core factors include the ULK1 complex, ATG13, FIP, ATG, yeast Atg1, Atg13, Atg17, Atg29 and Atg31 [ 13 , 14 ]. WIPI proteins are key PI3P effectors.
ATG14 is also able to mediate homotypic fusion of single-membrane vesicles at the PAS, allowing more traditional membrane vesicles delivered to the PAS to fuse and contribute to the nucleation and growth of the phagophore [ 13 , 14 ].
In this way, the phagophore has been found to accept lipid inputs from the ER, ER exit sites ERES , the Golgi, the plasma membrane and recycling endosomes for its growth and expansion Figure 2A [ 13 , 14 ]. The lipidation of Atg8 LC3 onto PE lipid molecules in the growing phagophore membrane is key for autophagosome formation.
Other Atg proteins like ATG16L1 yeast Atg16 as well as ATGATG5 are required for Atg8 lipidation with PE. In fact, recent findings identify the ATG protein ATG2A as a lipid shuttle factor that facilitates PI3P-dependent autophagosome growth [ 42 , 43 , 44 ]. Other phosphoinositides such as PI4P, PI 4,5 P2 and PI 3,5 P2 have also been suggested to play a role in the expansion of the phagophore [ 13 , 14 ].
Apart from these, as mentioned above, sphingosinephosphate S1P and ceramide, also contribute to autophagy. PE is produced from phosphatidylserine PS in mitochondria, which might be one of the sources of PE Figure 2A. PE might also be shuttled from the ER, plasma membrane and recycling endosomes.
It has been recently shown by Ma et al. Thus, maintenance of organelle lipid identity through proper trafficking and lipid homeostasis are crucial for autophagy as well. ATG9 Atg9 in yeast is a six-transmembrane protein that is required for autophagosome formation [ 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ].
ATG9 trafficking is one of the most studied topics in the autophagy field, highlighting the important role it plays in autophagy—the autophagic phenotypes of null mutants are very penetrant. While this is the case, we still lack detailed information about the function of this protein during autophagy.
Yeast studies have shown that Atg9 is localized to PAS as well as cytoplasmic vesicles of 30—60 nm diameter that bud-off of the late Golgi.
It is believed that Atg9 shuttles between the PAS and its cytoplasmic vesicle pool upon autophagy induction Figure 2C. At the PAS, Atg9 associates with Atg1, Atg2 as well as Atg Since Atg9 is a transmembrane protein, its trafficking directly affects the funneling or channeling of membrane to the PAS and the growing autophagosome.
The Atg9 vesicles are thought to originate at the Golgi and contain fusion factors such as subunits of the TRAPIII Trs85 complex that are responsible for fusion with the growing autophagosome [ 45 , 46 , 52 ]. These studies are synergistic with findings that Atg9 traffics through endosomal compartments, as this might be an intermediary step important for recycling.
This recycling model is supported by the observation that mutations in the retromer complex, when combined with mutations in tethering factor Trs85, can abrogate trafficking of Atg9 to the PAS.
Similarly the combination mutations with GARP subunits that are responsible for tethering vesicles to the Golgi from the endosome, with Trs85 result in defective autophagy. Atg9 trafficking is influenced by other autophagy proteins such as Atg27 and Atg23 [ 53 , 54 , 55 , 56 ].
Atg23 is a peripheral membrane protein that facilitates the anterograde trafficking of Atg9 from the Golgi to the PAS. Atg27 can be retrieved from the vacuole in a process that is Snx4-dependent.
Furthermore, earlier studies identified a C terminal tyrosine YSAV motif in Atg27 that is important for the proper delivery of Atg27 to the vacuole as well as to maintain Atg9 pools at the endosome that can be mobilized to different compartments during autophagy Figure 2C.
Taken together, the complexity of Atg9 trafficking synergizes with the hypothesis that a collection of different membranes such as endosome and Golgi all contribute to the lipid identity of the autophagosome.
The fusion of the autophagosome not only requires components of the fusion machinery as well as PI3P turnover, but it also requires the disassembly or retrieval of several ATG proteins from the autophagosome.
In yeast, phosphoinositide phosphatases, including those from the myotubularin protein family, like Ymr1, along with Sjl2 and Sjl3 are important for removal of PI3P from completed autophagosomes, making autophagosomes fusion-competent Figure 2D.
In mammalian cells, PI3P phosphatase MYM-3 acts similarly to Ymr1 in promoting autophagosome maturation and fusion. Fusion of autophagosomes with the lysosome is mediated by RAB7-like protein Ypt7 along with the HOPS Homotypic fusion and vacuole protein sorting tethering complex and SNARE Soluble Ethylmaleimide-Sensitive Factor Attachment Protein Receptor proteins.
Three Q—SNAREs: Vam3, Vti1 and Vam7 have been identified in yeast along with R-SNARE Ykt6 as key for this fusion step. Vamp7 is a sorting nexin family protein containing a PX domain that interacts with PI3P. Studies suggest that Vam7 interacts with AtgAtgAtg31 trimer complex via Atg17 interaction.
Interestingly, the PI3K VPS34 has also been linked with later stages of autophagy, including lysosomes returning to normal or regenerating once fusion with autophagosomes has taken place.
Other phosphorylated lipid species like PI4P and PI 4,5 P2 can also facilitate lysosomal regeneration, allowing for new rounds of autophagy to occur. This might also trigger the formation of recycling vesicles packaging ATG protein cargo for recycling off the lysosomal membrane.
For example, yeast vacuoles have unique lipid composition that is different from other membrane organelles within the cell. Vacuoles are enriched with lipids such as, myo-inositols, PI3P, and typically PI3,5P2, among other lipids such as ergosterol, diacylglycerol and some sphingolipids Figure 2D.
The specific lipid identity of vacuoles is important for the recruitment of fusion factors such as SNARES Ypt7 and HOPS that allow fusion of autophagosomes. Thus, the specific lipid identity of vacuole is important for its physiological function and its fusion with the late endosome and autophagosome.
While progress has been made in understanding the molecular underpinnings of autophagosome formation, our understanding has been primarily advanced by understanding the functions of proteins that lead and are required for this autophagic vesicle to form [ 28 , 29 , 30 , 31 ].
Because the extensive level of membrane remodeling that takes place during autophagy formation, much will be gained by investigating the process using methods that focus on the membrane and lipid biology of the process [ 23 , 24 , 25 , 26 , 27 ].
This new perspective has the potential of changing the way we have conventionally understood the remodeling of membranes for vesicle formation. The autophagy processes or steps benefiting from these contributions are also indicated.
Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3. Edited by Biba Vikas. Open access peer-reviewed chapter Intracellular Lipid Homeostasis and Trafficking in Autophagy Written By Shreya Goyal, Meaghan R.
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IntechOpen Cell Growth Edited by Biba Vikas. From the Edited Volume Cell Growth Edited by Biba Vikas and Michael Fasullo Book Details Order Print. Chapter metrics overview 1, Chapter Downloads View Full Metrics. Impact of this chapter. Abstract In eukaryotes, lipids are not only an important constituent of the plasma membrane but also used to generate specialized membrane-bound organelles, including temporary compartments with critical functions.
Keywords lipids lipid homeostasis lipid trafficking autophagy autophagosomes. Shreya Goyal Department of Biological Sciences, University of North Carolina at Charlotte, USA Meaghan R.
Robinson Department of Biology, High Point University, USA Verónica A. Table 1. Lipid content of different organelle membranes. Table 2. Organellar lipid contributions to autophagic processes and autophagosome formation. References 1. Yang Z, Klionsky DJ.
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A distinction should be made between endosomes e. Rab11 is also required for fusion of the autophagosomes with MVBs Fader et al. Shortly after autophagosomes form, they acquire proteins and enzymes that are normally found in MVB, LEs, and lysosomes Dunn b ; Tooze et al.
It has also been proposed that acidification occurs before delivery of enzymes, and presumably this enables rapid activation of the hydrolases and degradation of the inner autophagosome membrane, which facilitates its transition to an amphisome.
To become an autolysosome, the amphisome can either mature or fuse with a preexisiting lysosome, but the current data do not distinguish between these possibilities. However, the most compelling data to support the formation of autolysosomes by sequential fusion are the accumulation of autophagosomes and amphisomes caused by loss of trafficking complexes that control endosomes: COPI, which disrupts early endosomal trafficking Razi et al.
Rab7, a late endosomal Rab, is found on nascent autophagosomes, but proposed only to be required for fusion of autophagosomes with lysosomes Gutierrez et al.
Rab7 also mediates transport of autophagosomes along microtubules by binding FYCO, a Rab7—PI3P—LC3B-binding protein, proposed to mediate plus-end movement of autophagosomes Pankiv et al. In addition, only Rab7, but not Vps16 part of the HOPS-CORVET core complex; see below , is required for autophagosome maturation Ganley et al.
The autolysosome is the terminal stage of autophagy and is dependent on a stable cohort of lysosomes. The lysosome is adapted to uniquely provide a powerful degradative environment while maintaining active transporters in its limiting membrane.
The biogenesis of the lysosome and the pool of lysosomes is coordinated by TFEB transcription factor, which regulates the CLEAR network and, in addition, coordinates the lysosomal pathway with the autophagic pathway Settembre et al.
After complete fusion with LEs or autophagosomes, lysosomes are maintained by reformation from the hybrid organelle or the autolysosome, respectively Luzio et al. Interestingly, under normal conditions, lysosome reformation was detected after 30 min Bright et al.
Reformation in both cases would entail removal of endosomal or autophagosomal membrane components, including the SNAREs used for the fusion events and Rab 7 in particular after prolonged starvation Yu et al. Lysosome function and reformation after autophagy.
The activity of mTORC1 on the lysosome is coupled with amino acid production and reformation of the lysosome after amino acid starvation based on data from Lamb et al. mTORC1 integrates external signals, including amino acid and growth factor availability to control cell growth.
It is not clear how amino acids control mTORC1, but recent data indicate that it involves mTORC1 recruitment to the lysosome surface Sancak et al. In particular, the signal from amino acids within the lysosomal lumen is sensed by the V-ATPase and the Ragulator complex, leading to GTP loading and activation of the Rag GTPases, which subsequently recruit mTORC1 Sancak et al.
Although the exact mechanisms underlying this pathway remain elusive, very rapid progress is being made. During prolonged starvation, mTORC1 can be reactivated at the lysosomes by amino acid release as a result of autophagic degradation. This mechanism of ALR and autophagy inhibition provides a self-regulatory feedback mechanism whereby, once autophagy has improved the nutritional status of the cell, it is terminated to protect the cell and the autolysosome membranes are recycled to restore lysosome number.
A fundamental question in intracellular membrane trafficking is how directionality and specificity are achieved and maintained? At least two types of protein complexes are involved: tethering factors that are regulated by the Rab-family GTPases, and SNARE proteins that provide specificity and promote membrane fusion.
Tethering factors assist in delivering vesicles to their target compartment and play an important role in conferring specificity in membrane trafficking and fusion events for review, see Yu and Hughson They interact with membrane-associated Rab proteins.
In fact, most tethering factors are Rab effectors; thus, the regulation of tethering activity occurs through GTP binding and hydrolysis. In addition, evidence has been provided that these complexes act to regulate specificity in the assembly of the SNARE complexes. Tethering complexes and small GTPases implicated in endocytosis and autophagy.
See text for details. The COG is an evolutionally conserved Golgi-associated protein complex that has been characterized both in yeast and mammalian cells.
It functions as a tethering factor for vesicles that recycle within the Golgi apparatus and vesicles that recycle to the Golgi from the endosomal compartments VanRheenen et al. The complex is composed of eight subunits Cog1—8 , which can be divided into two structurally and functionally distinct subcomplexes, lobe A Cog1—4 , which is essential in yeast VanRheenen et al.
By studying the trafficking of the yeast Atg9, the only multitransmembrane autophagy protein, the involvement of lobe A in the cytosol-to-vacuole targeting CVT pathway, a selective yeast-specific autophagy pathway, as well as in starvation-induced autophagy was revealed Yen et al.
Other assays including EM analysis showing the accumulation of open membrane phagophore structures suggested that the COG complex was involved in autophagosome biogenesis, in particular, in PAS organization.
Thus far, however, no evidence is provided for the participation of mammalian COG complexes in autophagy. Moreover, it is important to determine whether a specific form of a COG complex composed of a different subunit composition is essential for autophagy.
Another tethering complex involved in autophagosome biogenesis is the octameric TRAPPIII Meiling-Wesse et al. TRAPP tethering complexes are a group of three distinct guanine-nucleotide exchange factors GEFs first identified in yeast that activate the Rab GTPase Ypt1 Barrowman et al.
TRAPPI and TRAPPII participate in Golgi-related trafficking events. Trs85 was recently identified as a unique subunit in the TRAPPIII complex found to regulate autophagy in yeast Lynch-Day et al.
The Trs85 subunit specifically directs the complex and Ypt1 to the PAS, enabling the activity of the Rab GTPase Ypt1 in both the CVT pathway and starvation-induced autophagy. Consistently, Ypt1 and the Trs85 TRAPPIII subunit localize to Atg9 peripheral structures in yeast, suggesting their involvement in membrane recruitment to the elongating autophagosomes Kakuta et al.
In mammalian cells, the exocyst complex was found to mediate the initial stages of autophagosome biogenesis Bodemann et al. Similar to the COG and TRAPPIII complexes, it contains eight different subunits Munson and Novick ; He and Guo and is involved in post-Golgi vesicle tethering to the plasma membrane.
Exo84, an exocyst subunit, binds the small Rab GTPase RalB following nutrient starvation and interacts with early factors of autophagy induction such as Beclin1, Vps34, and Ulk1 Bodemann et al.
These interactions are inhibited under normal growth conditions by the interaction of the small Rab GTPase RalA to the Sec5 subunit, which binds the aforementioned autophagy proteins as well as mTORC1. Furthermore, additional subunits of the exocyst complex interacted with core autophagy proteins in a yeast two-hybrid screen.
Accordingly, it has been hypothesized that the exocyst complex serves as a scaffold for autophagy initiation and autophagosome biogenesis at the isolation membrane.
The HOPS tethering complex has recently been reported to mediate autophagosome maturation in Drosophila and mammalian cells Lindmo et al. This complex, first identified in yeast, is a Rab7 effector involved in vacuolar-related fusion events Brett et al.
Deletion of HOPS subunits leads to inhibition of LC3 accumulation in lysosomes Liang et al. HOPS complex subunits and Rab7 were targeted to autophagosomes following overexpression of the tumor suppressor UVRAG.
A more recent study has indicated that Rab7 and the HOPS subunit Vps16 are both essential for endosomal fusion with the lysosomes, whereas only Rab7 and not Vps16 is essential for autophagosome—lysosome fusion Ganley et al.
Increasing evidence in both yeast and mammalian cells indicates the involvement of tethering factors in different stages of the autophagic process; however, the mechanism by which these complexes act in this pathway remains largely unknown. Because these factors are essential for other intracellular trafficking pathways, it is important to exclude indirect effects.
Moreover, it is possible that the tethering complexes that are specifically designated to act in autophagy are modified e. How these factors are regulated is yet another issue that requires future studies. Membrane fusion events are generally mediated by specific sets of SNARE complexes found on opposing sides of the fusing membranes Jahn and Scheller ; Sudhof and Rothman Typically, SNARE molecules are found in a tertiary complex of a Qa-, Qb-, Qc-, and R-SNARE in one membrane that needs to be separated primed before membrane fusion.
SNAP and NSF in mammals and Sec17 and Sec18 in yeast mediate this reaction, which requires the hydrolysis of ATP Wickner and Schekman SNAREs, NSF, and SNAP were implicated in the different steps of the autophagic process Fig. Early studies in yeast revealed that formation of CVT vesicles, but not autophagosomes, requires Tlg2 and Vps45 Abeliovich et al.
More recent studies in yeast revealed that the multispanning-membrane protein Atg9 cycles between the PAS and different organelles Reggiori et al. It has been suggested that Atg9 is localized within tubulovesicular structures trafficked from the Golgi to the PAS as part of its role in autophagosome formation Fig.
A similar function and compartmentalization have been discovered in mammalian cells Orsi et al. By following the GFP-Atg9-labeled structures, Klionsky and coworkers identified the yeast Q-SNAREs, Sso1, Sso2, Sec9, and Tlg2 and the R-SNARE Sec22 as being involved both in the formation of the Atg9-containing tubulovesicular structures and the overall autophagy process Nair et al.
The involvement of Sec17 and Sec18 in autophagosome formation was originally studied by Ohsumi and coworkers, who provided evidence that these proteins are needed only for autophagosome fusion with the vacuole Ishihara et al.
A more recent study suggests that Sec18 and Sec17 are also required for the formation of autophagosomes Nair et al.
These results imply a direct role for Golgi-derived membranes and SNAREs in autophagosome formation. At present, however, there is no evidence for such a role for the mammalian orthologs of these SNAREs in autophagy. SNAREs implicated in autophagosome formation and maturation.
Mammalian top and yeast bottom SNAREs involved in autophagy. All SNARES have a single transmembrane spanning domain except syntaxin 17, which has two transmembrane domains Itakura et al.
Accordingly, VAMP7, syntaxin 7, syntaxin 8, and Vti1B were found to mediate homotypic membrane fusion of plasma-membrane-derived vesicles to form phagophores Moreau et al. This process was sensitive to NEM, suggesting the requirement for NSF in early stages of autophagosome formation.
Here, too, a more direct approach is needed to study this process in detail. Indeed, a partial reconstitution of these membrane fusion events in the test tube was reported, and future studies should allow a resolution of this and other mechanistic aspects of autophagosome biogenesis.
As mentioned above, autophagosomes and amphisomes fuse with the lysosomal membrane. The exact machinery of this process is not yet fully understood. Studies in yeast indicated that Vam3, Vam7, Ykt6, and Vti1 participate in autophagosome—vacuole membrane fusion Darsow et al.
A recent study by Mizushima and coworkers identified for the first time a SNARE molecule, syntaxin 17, on the outer membrane of mature autophagosomes Itakura et al.
This study provides biochemical and morphological evidence that syntaxin 17, a unique SNARE with a carboxy-terminal hairpin transmembrane domain, is localized on the autophagosomal but not phagophore membrane, raising the possibility that SNAREs needed for autophagosome—lysosome membrane fusion are targeted to the autophagosome membrane only upon its completion and closure.
Although the exact mechanism for targeting of syntaxin 17 remains unknown, it provides a way to prevent premature fusion between phagophore and lysosome.
It has been previously suggested that the spatial separation between phagophore and lysosome is achieved by microtubules, which transport only mature autophagosomes but not phagophores Fass et al. Incorporation of the fusion machinery after formation of the mature autophagosome will also ensure that the SNARE molecules needed for fusion with the lysosome will be only localized in the outer membrane and thus avoid lysosomal degradation following membrane fusion.
To complete its fusion activity, syntaxin 17 interacts with SNAP and VAMP8 localized on the lysosomal membrane Itakura et al. The contribution of syntaxin 17 to autophagosome biogenesis was also examined by Yoshimori and coworkers, who found that syntaxin 17 acts early in autophagosome formation, recruiting Atg14, a subunit of the autophagy-specific Vps34 PI3 kinase, to the ER—mitochondria contact sites Hamasaki et al.
Consistent with Itakura et al. Clearly, elucidating the role of syntaxin 17 in autophagy is an important future research avenue. The formation and maturation of autophagosomes is a complex process, entailing vesicular trafficking and fusion events, and substantial reorganization of existing compartments, or subdomains of compartments, in particular, the ER but including the plasma membrane, Golgi, and mitochondria.
In canonical autophagy, which originates in the ER, remodeling of the ER-derived autophagosome must occur to allow for their fusion with the endosome and lysosome.
A particular unresolved issue is the balance between bulk conversion by compartment fusion e. The former would be an efficient and rapid single-fusion event, whereas the latter would be presumably a slower process, but would allow a stepwise remodeling of the maturing autophagosome.
In both cases, an unresolved issue is how recognition between fusion partners is established. Although much evidence supports a stepwise conversion of the nascent autophagosome into a hybrid autophagosome—endosome and some of the molecular components have been identified to support this process, there are still many unanswered questions that await an understanding of the full molecular mechanism.
is funded by Cancer Research UK. is the incumbent of the Harold Korda Chair of Biology and is funded by the Israeli Science Foundation ISF , the German-Israeli Foundation GIF , and the German Minerva Foundation. Additional Perspectives on Endocytosis available at www.
Copyright © by Cold Spring Harbor Laboratory Press. Endocytosis and Autophagy: Exploitation or Cooperation? Sharon A. Tooze 1 , Adi Abada 2 and Zvulun Elazar 2 1 London Research Institute, Cancer Research UK, Secretory Pathways Laboratory, London WC2A 3LY, United Kingdom 2 Weizmann Institute of Science, Department of Biological Chemistry, Rehovot , Israel Correspondence: sharon.
Previous Section Next Section. View larger version: In this window In a new window Download as PowerPoint Slide. Figure 1. Autophagosome Formation and Endosomes The early work on formation of autophagosomes performed using immunocytochemical analysis clearly showed early autophagosomes labeled with antibodies against rough ER proteins, but no plasma membrane, trans -Golgi, or endosomal proteins Dunn a.
Autophagosome Maturation and Endosomes Shortly after autophagosomes form, they acquire proteins and enzymes that are normally found in MVB, LEs, and lysosomes Dunn b ; Tooze et al.
Reformation of Autolysosomes and TOR The autolysosome is the terminal stage of autophagy and is dependent on a stable cohort of lysosomes. Figure 2. Tethering Factors A fundamental question in intracellular membrane trafficking is how directionality and specificity are achieved and maintained?
Figure 3. SNAREs Membrane fusion events are generally mediated by specific sets of SNARE complexes found on opposing sides of the fusing membranes Jahn and Scheller ; Sudhof and Rothman Figure 4.
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