- Email: [email protected]
Regulation of Syntaxin 17 during Autophagosome Maturation
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Spotlight
Regulation of Syntaxin 17 during Autophagosome Maturation Christophe Viret1,2,3,4,5,* and Mathias Faure1,2,3,4,5,6,* During macroautophagy, cytosolic elements are confined in autophagosomes before fusion with endolysosomes for degradation or recycling. Recruitment of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) factor syntaxin 17 (STX17) is instrumental for this maturation step. Two recent studies indicate that the kinase ULK1 and the apoptosis modulator BRUCE both regulate STX17 engagement during autophagosome maturation in mammalian cells. Macroautophagy or autophagy is a hierarchized, multistep process that ensures the degradation of cytosolic elements via the lysosomal pathway. These elements include misfolded/damaged cellular components as well as those of microbial origin. Autophagy operates by the formation of a specialized membrane (initiation) that grows (elongation) to form a doublemembraned vesicle, the autophagosome, in which the targets of autophagy are confined via autophagy receptors that bind to cargo and to ATG8 family members (LC3s, GABARAPs) on the elongating membrane. Constituted autophagosomes then fuse with acidic vesicles of the endolysosomal compartment for degradation of the cargo (maturation). This latter step involves various autophagy factors whose regulation is incompletely understood. Among these,
the autophagosomal factor STX17 is crucial for the maturation step through interaction with SNAP29 and the lysosomal factor VAMP8. Two recent studies provide new insights into the regulation of STX17 during autophagosome maturation. Because the conventional protein kinase Ca (PKCa) can negatively influence autophagy through phosphorylation of the microtubule-associated protein 1A/1B-light chain 3 (LC3) factor [1], Wang et al. searched for other PKCa targets among autophagy factors, and found that Unc-51-like autophagy activating kinase 1 (ULK1) displayed a marked phosphorylation signal corresponding to the conserved serine 423 (S423) [2]. ULK1 was poorly phosphorylated on S423 in cells expressing a dominant negative form of PKCa, or after PKCa silencing, indicating that PKCa is the effective upstream kinase involved. Phosphorylation of S423 influenced neither the interaction of ULK1 with BECN1/ATG13/FIP200 nor its kinase activity on BECN1/ATG13, leaving the function of the ULK1 complex during autophagy initiation unaffected. Engineered cells expressing wild-type (WT) ULK1, ULK1 S423A (absence of phosphorylation), or ULK1 S423D (mimic of phosphorylation) revealed that the level of lipidated LC3 and LC3 puncta formation was higher in the presence of S423D relative to WT and S423A. Inhibiting lysosomal acidification increased the mobilization level of LC3 in all conditions, but only marginally in S423D cells. ULK1 S423D also delayed autophagosome to lysosome fusion in vitro, indicating that ULK1 phosphorylation regulates maturation. The fusion of autophagosomes to lysosomes occurs through the Qa-type SNARE STX17 on autophagosomes, which forms a membrane complex with
the R-SNARE vesicle-associated membrane protein 8 (VAMP8) on lysosomes via the Qbc-SNARE synaptosome-associated protein 29 (SNAP29) [3]. In fact, ULK1 can bind to STX17 both in vitro and in vivo. Comparatively, ULK1 S423D was a weak binder relative to WT or S423A. The ULK1–STX17 interaction was increased when ULK1 was treated with a phosphatase, and decreased when treated with PKCa. Formation of ULK1 puncta that colocalized with STX17 and LC3 could be observed for WT and S423A, but not for S423D, confirming that S423 unphosphorylated ULK1 interacts with STX17. Regarding the STX17–SNAP29 interaction, it was the highest in the presence of ULK1 S423A relative to WT and S423D, indicating that ULK1 S423A promotes maturation by increasing the affinity of SXT17 for SNAP29. Unphosphorylated S423 on ULK1 seems to recruit STX17 to autophagosomes, and STX17 shows greater affinity for SNAP29 when it is located on the surface of autophagosomes. VAMP8 recruitment was also the highest in ULK1 S423A-expressing cells. Thus, ULK1 phosphorylation regulates the fusion of autophagosomes to lysosomes by modulating the formation of the STX17–SNAP29–VAMP8 complex. Unlike WT ULK1, which was sensitive to lysosomal degradation, the degradation of S423A mutant ULK1 involved the proteasome. Via its 227-QDLRL-231 motif, ULK1 can bind to HSC70, an adapter for chaperone-mediated autophagy (CMA). ULK1 S423D was the most efficient at binding HSC70. All ULK1 variants colocalized with the lysosomal marker lysosome-associated membrane protein 2 (LAMP2), but S423D showed the strongest signal, indicating that the phosphorylation status of ULK1 affects its stability. Because unphosphorylated S423 is involved in autophagosome maturation, phosphorylation of ULK1 by PKCa, and
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the associated degradation by CMA, is likely to attenuate autophagy under normal conditions. Indeed, CMA compensates for the reduction of autophagy, and autophagy is reduced when CMA is enhanced [4,5]. Together with mTORC1, PKCa thus contributes to moderate autophagy under nutrient-rich conditions by regulating the phosphorylation and CMA-mediated degradation of ULK1. By showing that ULK1 regulates autophagosome–lysosome fusion mediated by STX17, these findings reveal a dual role for ULK1 during autophagy, and indicate that PKCa represents an important regulator of the balance of autophagy.
and a reduction of autolysosomes. Autophagosome accumulation was confirmed by electron microscopy analysis. Thus, BRUCE appears to be an important regulator of the fusion of GABARAP/GABARAPL1-bearing autophagosomes with lysosomes. By investigating whether BRUCE could interact with factors instrumental for
Proteasomemediated degradaƟon
autophagosome maturation, Ebner et al. found that BRUCE selectively interacted with STX17 by engaging a region that included the BIR domain. Unlike LC3–LAMP2 colocalization, LC3–STX17 colocalization was enhanced in cells lacking BRUCE as a result of altered maturation. A non-catalytic BRUCE mutant capable of binding
PKCα ULK1
ULK1 STX17
ULK1
STX17 LC3
AP
STX17
ULK1 P
HSC70
ULK1 P HSC70
SNAP29 During a search for novel regulators of CMA-mediated starvation-induced autophagy among degradaƟon ubiquitin modifiers, Ebner et al. [6] idenPhagophore tified BRUCE/Apollon/Birc6, an inhibitor LC3 AP STX17 SNAP29 of apoptosis (IAP) protein [7], as a new BRUCE VAMP8 positive regulator of autophagosome– GAB/L1 lysosome fusion in mammalian cells. LYS BRUCE colocalized with LAMP2, but BRUCE VAMP8 with no signs of any contribution to lysoLAMP2 some formation or function. BRUCE did LYS not contribute to autophagosome bioAL genesis, nor did it appear to constitute a LAMP2 substrate for autophagy. Among ATG8 family members, BRUCE preferentially interacted with GABARAP and GABARFigure 1. Regulatory Insights into the Function of Soluble N-Ethylmaleimide-Sensitive Factor APL1 without engaging its baculoviral Attachment Protein Receptor (SNARE) STX17 during Autophagosome Maturation. Upon closure, IAP repeat (BIR) or C-terminal ubiqui- the elongating phagophore forms the autophagosome (AP) that encapsulates the targeted cargo and carries tin-conjugating (UBC) domains. Such the LC3 marker. Progression along autophagy flux involves fusion with lysosomes (LYS) to form autolysosomes (AL) where cargo degradation takes place. Assembly of the STX17–SNAP29–VAMP8 complex is instrumental interactions did not involve engagement for the fusion process. The ULK1 kinase, which contributes to autophagy initiation (not depicted), appears to of LC3-interacting regions (LIR) also play a role in autophagosome maturation by regulating the formation of the STX17–SNAP29–VAMP8 because GABARAP/GABARAPL1, that complex. Through direct and transient interaction with STX17, involving the 157–275 region of STX17 and the is unable to bind canonical LIRs, could 279–525 region of ULK1, ULK1 promotes STX17 recruitment to autophagosomes and enhances the affinity of STX17 for the SNAP29 factor that is important for binding VAMP8 on lysosomes. Importantly, the prostill interact with BRUCE. BRUCE defi- maturation role of ULK1 is attenuated upon phosphorylation (P) of its serine 423 by the kinase PKCa (possibly ciency inhibited autophagy flux induced balanced by phosphatase activities). Phosphorylated ULK1 (ULK1P) has an enhanced ability to interact with by either starvation or by inducers of HSC70, which is an adapter for chaperone-mediated autophagy (CMA), by using a KFERQ-like motif (227selective autophagy that targets mito- QDLRL-231). This is in contrast to non-phosphorylated ULK1, which is sensitive to proteasome-mediated degradation. Thus, PKCa can negatively modulate autophagosome maturation and thereby influence the chondria (mitophagy). In cells lacking autophagy/CMA balance in mammalian cells. The BRUCE/Apollon/Birc6 IAP protein (BRUCE) also selectively BRUCE, there was a marked reduction interacts with STX17 and SNAP29, and promotes autolysosome formation. This interaction involves the 1– in the amount of LAMP2-positive 1648 N-terminal portion of BRUCE that includes the BIR domain. BRUCE is present on the lysosomal surface vesicles that stained positive for LC3 and is capable of selectively interacting with GABARAP and GABARPL1 (GAB/L1) members of the ATG8 family without engaging a LIR motif. Of note, the C-terminal ubiquitin-conjugating (UBC) domain of BRUCE is not or GABARAP/GABARAPL1, suggesting involved in the positive regulation of autophagosome maturation. For clarity, the process of cargo targeting is an accumulation of autophagosomes not represented. 2
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RAB7, ATG14L, PLEKHM1, or HOPS complex components [8,9]. The basis of an increased affinity of STX17 for SNAP29 after ULK1–STX17 interaction also remains unclear. In addition, more studies will be necessary to understand the role of the N-terminal domain of BRUCE in its interaction with STX17/ SNAP29. Finally, it will be important to determine whether the regulation of STX17 by ULK1 and BRUCE extends to autophagosome maturation taking place during xenophagic responses. Answering these questions may contribute to a refined description of the function of STX17, and more generally of the SXT17–SNAP29–VAMP8 axis, during Together, the mechanistic studies of autolysosome formation. Wang et al. [2] and Ebner et al. [6] bring 1 CIRI, International Center for Infectiology Research, insights into the function of STX17 during Université de Lyon, France autolysosome formation in mammalian 2INSERM, U1111, Lyon, France 3 cells (Figure 1). It will be of interest to 4CNRS, UMR5308, Lyon, France Ecole Normale Supérieure de Lyon, Lyon, France determine whether STX17 regulation by 5Université Lyon 1, Centre International de Recherche en ULK1 influences the engagement or Infectiologie, 21 Av T. Garnier, 69007 Lyon, France 6 recruitment of other important players Equipe FRM (Fondation pour la Recherche Médicale), Paris, France in autophagosome maturation, such as
to GABARAP/GABARAPL1 and STX17, and of colocalizing with LAMP2, rescued autophagosome maturation in BRUCE-deficient cells, indicating that the ubiquitin-conjugating activity of BRUCE is not involved in the regulation of autophagosome maturation. By interacting with STX17 and GABARAP/ GABARAPL1 on autophagosomes, BRUCE engagement represents an important contribution to autolysosome formation. Thus, in addition to its ability to modulate apoptosis, BRUCE can positively regulate autophagosome maturation, and thereby autophagy flux, in mammalian cells.
*Correspondence: [email protected] (C. Viret) and [email protected] (M. Faure). https://doi.org/https://doi.org/10.1016/j.tcb.2018.10.003 References 1. Jiang, H. et al. (2010) Protein kinase C inhibits autophagy and phosphorylates LC3. Biochem. Biophys. Res. Commun. 395, 471–476 2. Wang, C. et al. (2018) Phosphorylation of ULK1 affects autophagosome fusion and links chaperone-mediated autophagy to macroautophagy. Nat. Commun. 9, 3492 3. Itakura, E. et al. (2012) The hairpin-type tail-anchored SNARE syntaxin 17 targets to autophagosomes for fusion with endosomes/lysosomes. Cell 151, 1256–1269 4. Kaushik, S. et al. (2008) Constitutive activation of chaperone-mediated autophagy in cells with impaired macroautophagy. Mol. Biol. Cell 19, 2179–2192 5. Wu, H. et al. (2015) Crosstalk between macroautophagy and chaperone-mediated autophagy: implications for the treatment of neurological diseases. Mol. Neurobiol. 52, 1284–1296 6. Ebner, P. et al. (2018) The IAP family member BRUCE regulates autophagosome–lysosome fusion. Nat. Commun. 9, 599 7. Hao, Y. et al. (2004) Apollon ubiquitinates SMAC and caspase-9, and has an essential cytoprotection function. Nat. Cell Biol. 6, 849–860 8. Nakamura, S. and Yoshimori, T. (2017) New insights into autophagosome–lysosome fusion. J. Cell Sci. 130, 1209– 1216 9. Yu, S. and Melia, T.M. (2017) The coordination of membrane fission and fusion at the end of autophagosome maturation. Curr. Opin. Cell Biol. 47, 92–98
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Spotlight
Regulation of Syntaxin 17 during Autophagosome Maturation Christophe Viret1,2,3,4,5,* and Mathias Faure1,2,3,4,5,6,* During macroautophagy, cytosolic elements are confined in autophagosomes before fusion with endolysosomes for degradation or recycling. Recruitment of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) factor syntaxin 17 (STX17) is instrumental for this maturation step. Two recent studies indicate that the kinase ULK1 and the apoptosis modulator BRUCE both regulate STX17 engagement during autophagosome maturation in mammalian cells. Macroautophagy or autophagy is a hierarchized, multistep process that ensures the degradation of cytosolic elements via the lysosomal pathway. These elements include misfolded/damaged cellular components as well as those of microbial origin. Autophagy operates by the formation of a specialized membrane (initiation) that grows (elongation) to form a doublemembraned vesicle, the autophagosome, in which the targets of autophagy are confined via autophagy receptors that bind to cargo and to ATG8 family members (LC3s, GABARAPs) on the elongating membrane. Constituted autophagosomes then fuse with acidic vesicles of the endolysosomal compartment for degradation of the cargo (maturation). This latter step involves various autophagy factors whose regulation is incompletely understood. Among these,
the autophagosomal factor STX17 is crucial for the maturation step through interaction with SNAP29 and the lysosomal factor VAMP8. Two recent studies provide new insights into the regulation of STX17 during autophagosome maturation. Because the conventional protein kinase Ca (PKCa) can negatively influence autophagy through phosphorylation of the microtubule-associated protein 1A/1B-light chain 3 (LC3) factor [1], Wang et al. searched for other PKCa targets among autophagy factors, and found that Unc-51-like autophagy activating kinase 1 (ULK1) displayed a marked phosphorylation signal corresponding to the conserved serine 423 (S423) [2]. ULK1 was poorly phosphorylated on S423 in cells expressing a dominant negative form of PKCa, or after PKCa silencing, indicating that PKCa is the effective upstream kinase involved. Phosphorylation of S423 influenced neither the interaction of ULK1 with BECN1/ATG13/FIP200 nor its kinase activity on BECN1/ATG13, leaving the function of the ULK1 complex during autophagy initiation unaffected. Engineered cells expressing wild-type (WT) ULK1, ULK1 S423A (absence of phosphorylation), or ULK1 S423D (mimic of phosphorylation) revealed that the level of lipidated LC3 and LC3 puncta formation was higher in the presence of S423D relative to WT and S423A. Inhibiting lysosomal acidification increased the mobilization level of LC3 in all conditions, but only marginally in S423D cells. ULK1 S423D also delayed autophagosome to lysosome fusion in vitro, indicating that ULK1 phosphorylation regulates maturation. The fusion of autophagosomes to lysosomes occurs through the Qa-type SNARE STX17 on autophagosomes, which forms a membrane complex with
the R-SNARE vesicle-associated membrane protein 8 (VAMP8) on lysosomes via the Qbc-SNARE synaptosome-associated protein 29 (SNAP29) [3]. In fact, ULK1 can bind to STX17 both in vitro and in vivo. Comparatively, ULK1 S423D was a weak binder relative to WT or S423A. The ULK1–STX17 interaction was increased when ULK1 was treated with a phosphatase, and decreased when treated with PKCa. Formation of ULK1 puncta that colocalized with STX17 and LC3 could be observed for WT and S423A, but not for S423D, confirming that S423 unphosphorylated ULK1 interacts with STX17. Regarding the STX17–SNAP29 interaction, it was the highest in the presence of ULK1 S423A relative to WT and S423D, indicating that ULK1 S423A promotes maturation by increasing the affinity of SXT17 for SNAP29. Unphosphorylated S423 on ULK1 seems to recruit STX17 to autophagosomes, and STX17 shows greater affinity for SNAP29 when it is located on the surface of autophagosomes. VAMP8 recruitment was also the highest in ULK1 S423A-expressing cells. Thus, ULK1 phosphorylation regulates the fusion of autophagosomes to lysosomes by modulating the formation of the STX17–SNAP29–VAMP8 complex. Unlike WT ULK1, which was sensitive to lysosomal degradation, the degradation of S423A mutant ULK1 involved the proteasome. Via its 227-QDLRL-231 motif, ULK1 can bind to HSC70, an adapter for chaperone-mediated autophagy (CMA). ULK1 S423D was the most efficient at binding HSC70. All ULK1 variants colocalized with the lysosomal marker lysosome-associated membrane protein 2 (LAMP2), but S423D showed the strongest signal, indicating that the phosphorylation status of ULK1 affects its stability. Because unphosphorylated S423 is involved in autophagosome maturation, phosphorylation of ULK1 by PKCa, and
Trends in Cell Biology, Month Year, Vol. xx, No. yy
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the associated degradation by CMA, is likely to attenuate autophagy under normal conditions. Indeed, CMA compensates for the reduction of autophagy, and autophagy is reduced when CMA is enhanced [4,5]. Together with mTORC1, PKCa thus contributes to moderate autophagy under nutrient-rich conditions by regulating the phosphorylation and CMA-mediated degradation of ULK1. By showing that ULK1 regulates autophagosome–lysosome fusion mediated by STX17, these findings reveal a dual role for ULK1 during autophagy, and indicate that PKCa represents an important regulator of the balance of autophagy.
and a reduction of autolysosomes. Autophagosome accumulation was confirmed by electron microscopy analysis. Thus, BRUCE appears to be an important regulator of the fusion of GABARAP/GABARAPL1-bearing autophagosomes with lysosomes. By investigating whether BRUCE could interact with factors instrumental for
Proteasomemediated degradaƟon
autophagosome maturation, Ebner et al. found that BRUCE selectively interacted with STX17 by engaging a region that included the BIR domain. Unlike LC3–LAMP2 colocalization, LC3–STX17 colocalization was enhanced in cells lacking BRUCE as a result of altered maturation. A non-catalytic BRUCE mutant capable of binding
PKCα ULK1
ULK1 STX17
ULK1
STX17 LC3
AP
STX17
ULK1 P
HSC70
ULK1 P HSC70
SNAP29 During a search for novel regulators of CMA-mediated starvation-induced autophagy among degradaƟon ubiquitin modifiers, Ebner et al. [6] idenPhagophore tified BRUCE/Apollon/Birc6, an inhibitor LC3 AP STX17 SNAP29 of apoptosis (IAP) protein [7], as a new BRUCE VAMP8 positive regulator of autophagosome– GAB/L1 lysosome fusion in mammalian cells. LYS BRUCE colocalized with LAMP2, but BRUCE VAMP8 with no signs of any contribution to lysoLAMP2 some formation or function. BRUCE did LYS not contribute to autophagosome bioAL genesis, nor did it appear to constitute a LAMP2 substrate for autophagy. Among ATG8 family members, BRUCE preferentially interacted with GABARAP and GABARFigure 1. Regulatory Insights into the Function of Soluble N-Ethylmaleimide-Sensitive Factor APL1 without engaging its baculoviral Attachment Protein Receptor (SNARE) STX17 during Autophagosome Maturation. Upon closure, IAP repeat (BIR) or C-terminal ubiqui- the elongating phagophore forms the autophagosome (AP) that encapsulates the targeted cargo and carries tin-conjugating (UBC) domains. Such the LC3 marker. Progression along autophagy flux involves fusion with lysosomes (LYS) to form autolysosomes (AL) where cargo degradation takes place. Assembly of the STX17–SNAP29–VAMP8 complex is instrumental interactions did not involve engagement for the fusion process. The ULK1 kinase, which contributes to autophagy initiation (not depicted), appears to of LC3-interacting regions (LIR) also play a role in autophagosome maturation by regulating the formation of the STX17–SNAP29–VAMP8 because GABARAP/GABARAPL1, that complex. Through direct and transient interaction with STX17, involving the 157–275 region of STX17 and the is unable to bind canonical LIRs, could 279–525 region of ULK1, ULK1 promotes STX17 recruitment to autophagosomes and enhances the affinity of STX17 for the SNAP29 factor that is important for binding VAMP8 on lysosomes. Importantly, the prostill interact with BRUCE. BRUCE defi- maturation role of ULK1 is attenuated upon phosphorylation (P) of its serine 423 by the kinase PKCa (possibly ciency inhibited autophagy flux induced balanced by phosphatase activities). Phosphorylated ULK1 (ULK1P) has an enhanced ability to interact with by either starvation or by inducers of HSC70, which is an adapter for chaperone-mediated autophagy (CMA), by using a KFERQ-like motif (227selective autophagy that targets mito- QDLRL-231). This is in contrast to non-phosphorylated ULK1, which is sensitive to proteasome-mediated degradation. Thus, PKCa can negatively modulate autophagosome maturation and thereby influence the chondria (mitophagy). In cells lacking autophagy/CMA balance in mammalian cells. The BRUCE/Apollon/Birc6 IAP protein (BRUCE) also selectively BRUCE, there was a marked reduction interacts with STX17 and SNAP29, and promotes autolysosome formation. This interaction involves the 1– in the amount of LAMP2-positive 1648 N-terminal portion of BRUCE that includes the BIR domain. BRUCE is present on the lysosomal surface vesicles that stained positive for LC3 and is capable of selectively interacting with GABARAP and GABARPL1 (GAB/L1) members of the ATG8 family without engaging a LIR motif. Of note, the C-terminal ubiquitin-conjugating (UBC) domain of BRUCE is not or GABARAP/GABARAPL1, suggesting involved in the positive regulation of autophagosome maturation. For clarity, the process of cargo targeting is an accumulation of autophagosomes not represented. 2
Trends in Cell Biology, Month Year, Vol. xx, No. yy
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RAB7, ATG14L, PLEKHM1, or HOPS complex components [8,9]. The basis of an increased affinity of STX17 for SNAP29 after ULK1–STX17 interaction also remains unclear. In addition, more studies will be necessary to understand the role of the N-terminal domain of BRUCE in its interaction with STX17/ SNAP29. Finally, it will be important to determine whether the regulation of STX17 by ULK1 and BRUCE extends to autophagosome maturation taking place during xenophagic responses. Answering these questions may contribute to a refined description of the function of STX17, and more generally of the SXT17–SNAP29–VAMP8 axis, during Together, the mechanistic studies of autolysosome formation. Wang et al. [2] and Ebner et al. [6] bring 1 CIRI, International Center for Infectiology Research, insights into the function of STX17 during Université de Lyon, France autolysosome formation in mammalian 2INSERM, U1111, Lyon, France 3 cells (Figure 1). It will be of interest to 4CNRS, UMR5308, Lyon, France Ecole Normale Supérieure de Lyon, Lyon, France determine whether STX17 regulation by 5Université Lyon 1, Centre International de Recherche en ULK1 influences the engagement or Infectiologie, 21 Av T. Garnier, 69007 Lyon, France 6 recruitment of other important players Equipe FRM (Fondation pour la Recherche Médicale), Paris, France in autophagosome maturation, such as
to GABARAP/GABARAPL1 and STX17, and of colocalizing with LAMP2, rescued autophagosome maturation in BRUCE-deficient cells, indicating that the ubiquitin-conjugating activity of BRUCE is not involved in the regulation of autophagosome maturation. By interacting with STX17 and GABARAP/ GABARAPL1 on autophagosomes, BRUCE engagement represents an important contribution to autolysosome formation. Thus, in addition to its ability to modulate apoptosis, BRUCE can positively regulate autophagosome maturation, and thereby autophagy flux, in mammalian cells.
*Correspondence: [email protected] (C. Viret) and [email protected] (M. Faure). https://doi.org/https://doi.org/10.1016/j.tcb.2018.10.003 References 1. Jiang, H. et al. (2010) Protein kinase C inhibits autophagy and phosphorylates LC3. Biochem. Biophys. Res. Commun. 395, 471–476 2. Wang, C. et al. (2018) Phosphorylation of ULK1 affects autophagosome fusion and links chaperone-mediated autophagy to macroautophagy. Nat. Commun. 9, 3492 3. Itakura, E. et al. (2012) The hairpin-type tail-anchored SNARE syntaxin 17 targets to autophagosomes for fusion with endosomes/lysosomes. Cell 151, 1256–1269 4. Kaushik, S. et al. (2008) Constitutive activation of chaperone-mediated autophagy in cells with impaired macroautophagy. Mol. Biol. Cell 19, 2179–2192 5. Wu, H. et al. (2015) Crosstalk between macroautophagy and chaperone-mediated autophagy: implications for the treatment of neurological diseases. Mol. Neurobiol. 52, 1284–1296 6. Ebner, P. et al. (2018) The IAP family member BRUCE regulates autophagosome–lysosome fusion. Nat. Commun. 9, 599 7. Hao, Y. et al. (2004) Apollon ubiquitinates SMAC and caspase-9, and has an essential cytoprotection function. Nat. Cell Biol. 6, 849–860 8. Nakamura, S. and Yoshimori, T. (2017) New insights into autophagosome–lysosome fusion. J. Cell Sci. 130, 1209– 1216 9. Yu, S. and Melia, T.M. (2017) The coordination of membrane fission and fusion at the end of autophagosome maturation. Curr. Opin. Cell Biol. 47, 92–98
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