The VMP1‐Beclin 1 Interaction Regulates Autophagy Induction

Abstract

The Vacuole Membrane Protein 1 -VMP1-is a pancreatitis-associated transmembrane protein whose expression triggers autophagy in several human diseases. In the current study, we unveil the mechanism through which this protein induces autophagosome formation in mammalian cells. We show that VMP1 autophagy-related function requires its 20-aminoacid C-terminus hydrophilic domain (VMP1-AtgD). This is achieved through its direct binding to the BH3 motif of Beclin 1 leading to the formation of a complex with the Class III phosphatidylinositol-3 kinase (PI3K) hVps34, a key positive regulator of autophagy, at the site where autophagosomes are generated. This interaction also concomitantly promotes the dissociation of Bcl-2, an autophagy inhibitor, from Beclin 1. Moreover, we show that the VMP1-Beclin 1-hVps34 complex favors the association of Atg16L1 and LC3 with the autophagosomal membranes. Collectively, these findings reveal that VMP1 expression recruits and activates the Class III PI3K complex at the site of autophagosome formation during mammalian autophagy. Autophagy is an evolutionarily conserved transport pathway that involves the sequestration and delivery of cytoplasmic material into the lysosome, where it is degraded and recycled 1 . This catabolic process is involved in the turnover of long-lived proteins and other cellular macromolecules, and it might play a protective role in tumor development, aging, cell death, and intracellular pathogen invasions 2,3 .Macroautophagy (hereafter autophagy) involves the formation of double-membrane autophagosomes around the targeted cargoes, which include large structures such as organelles and protein aggregates. Autophagosomes then fuse with lysosomes exposing their cargoes to the hydrolytic content of this organelles 4 . This cellular process essential to maintain cellular homeostasis is regulated in an analogous manner to secretion and endocytosis, where related molecules on distinct organelle membranes mediate the flux of vesicular transport by proteinprotein interactions. Since the discovery of yeast autophagy-related (Atg) proteins 5 , autophagosome formation has been dissected at the molecular level but a lot of questions about the molecular mechanism underlying this process remain unanswered. Autophagosomes can be considered unique organelles because they do not contain marker proteins of other subcellular compartments 6 . In mammalian cells, the sequential association of at least a subset of the Atg proteins leads to the assembly of the pre-autophagosomal structures (PAS), which is believed to be the site where the precursor structure of the autophagosomes, the phagophores, are generated 7 . The PAS and phagophore formation also requires phosphatidylinositol 3-phosphate (PI3P) 8 and it is believed to be associated to specific subdomains of the endoplasmic reticulum (ER) termed omegasomes 9,10 . Among the key mediators initiating autophagosome formation, there is a set of evolutionarily conserved Atg. gene products; the kinasecontaining Ulk1/2 complex (Atg1 in yeast)...

Expanding the view of the molecular mechanisms of autophagy pathway

“…The recruitment of PI3KC3‐C1 to autophagy initiation sites, post ULK1 complex activation and translocation, is mediated by multiple factors (Figure 3). The transmembrane protein vacuole membrane protein 1 (VMP1) interacts with the BH3 domain of BECN1 through its ATG domain, thereby recruiting the PI3KC3‐C1 to the autophagy initiation site (Molejon et al, 2013). Furthermore, ATG14L is also reported to aid translocation of PI3KC3‐C1 during starvation through its cysteine rich N‐terminal domain (Zhong et al, 2009) and c‐terminal amphiphatic helix termed as BATS domain (Barkor/ATG14L autophagosome targeting sequence) (Fan et al, 2011).…”

Autophagy and antiviral defense

“…The newly synthesized PtdIns(3)P in turn recruits the WD-repeat domain PtdIns(3)Pinteracting (WIPI, also named ATG18) family proteins and double-FYVE-containing protein 1 (DFCP1) to reconstitute an ER-associated omegasome structure (also called an IM/phagophore). [35][36][37]39,40 On the other hand, ATG9acontaining vesicle transport from the trans-Golgi network (TGN) to the ER supports the formation of the nascent membrane into an IM/phagophore, [41][42][43] and by interacting with Beclin-1, ER-localized VMP1 promotes ER-IM/ phagophore contact and enhances the kinase activity of class III PI(3)K. [44][45][46] Very recently, a growing body of literature has reported that phase separation functions in the organization of ATG1 complex on the preautophagosomal structure (PAS) and the targeting of degradative substrates for autophagy. [47][48][49] After the nucleation of the IM/phagophore, two ubiquitin-like (UBL) conjugation systems function in the expansion and elongation of an IM/phagophore, which closes to become a mature autophagosome.…”