A novel conformation of the LC3-interacting region motif revealed by the structure of a complex between LC3B and RavZ

LC3A-mediated autophagy elicits PERK-eIF2α-ATF4 axis activation and mitochondrial dysfunction: Exposing vulnerability in aggresome-positive cancer cells

The unfolded protein response pathways (UPR), autophagy, and compartmentalization of misfolded proteins into inclusion bodies are critical components of the protein quality control network. Among inclusion bodies, aggresomes are particularly intriguing due to their association with cellular survival, drug resistance, and aggresive cancer behavior. Aggresomes are molecular condensates formed when collapsed vimentin cages encircle misfolded proteins before final removal by autophagy. Yet significant gaps persist in the mechanisms governing aggresome formation and elimination in cancer cells. Understanding these mechanisms is crucial, especially considering the involvement of LC3A, a member of the MAP1LC3 family, which plays a unique role in autophagy regulation and has been reported to be epigenetically silenced in many cancers. Herein, we utilized the tetracycline-inducible expression of LC3A to investigate its role in choroid plexus carcinoma cells, which inherently exhibit the presence of aggresomes. Live cell imaging was employed to demonstrate the effect of LC3A expression on aggresome-positive cells, while SILAC-based proteomics identified LC3A-induced protein and pathway alterations. Our findings demonstrated that extended expression of LC3A is associated with cellular senescence. However, the obstruction of lysosomal degradation in this context has a deleterious effect on cellular viability. In response to LC3A-induced autophagy, we observed significant alterations in mitochondrial morphology, reflected by mitochondrial dysfunction and increased ROS production. Furthermore, LC3A expression elicited the activation of the PERK-eIF2α-ATF4 axis of the UPR, underscoring a significant change in the protein quality control network. In conclusion, our results elucidate that LC3A-mediated autophagy alters the protein quality control network, exposing a vulnerability in aggresome-positive cancer cells.

Berberine modulates cardiovascular diseases as a multitarget-mediated alkaloid with insights into its downstream signals using in silico prospective screening approaches

Atherosclerosis is potentially correlated with several cardiac disorders that are greatly associated with cellular oxidative stress generation, inflammation, endothelial cells dysfunction, and many cardiovascular complications. Berberine is a natural isoquinoline alkaloid compound that widely modulates pathogenesis of atherosclerosis through its different curative potentials. This in silico screening study was designed to confirm the potent restorative properties of berberine chloride as a multitarget-mediated alkaloid against the CVDs and their complications through screening, identifying, visualizing, and evaluating its binding models, affinities, and interactions toward several CVDs-related targets as direct and/or indirect-mediated signals via inhibiting cellular ER stress and apoptotic signals and activating autophagy pathway. The drug-likeness properties of berberine were predicted using the computational QSAR/ADMET and Lipinski's RO5 analyses as well as in silico molecular docking simulations. The potent berberine-binding modes, residues-interaction patterns, and free energies of binding scores towards several CVDs-related targets were estimated using molecular docking tools. Furthermore, the pharmacokinetic properties and toxicological features of berberine were clearly determined. According to this in silico virtual screening study, berberine chloride could restore cardiac function and improve pathogenic features of atherosclerotic CVDs through alleviating ER stress and apoptotic signals, activating autophagy, improving insulin sensitivity, decreasing hyperglycemia and dyslipidemia, increasing intracellular RCT signaling, attenuating oxidative stress and vascular inflammation, and upregulating cellular antioxidant defenses in many cardiovascular tissues. In this in silico study, berberine chloride greatly modulated several potent CVDs-related targets, including SIGMAR1, GRP78, CASP3, BECN1, PIK3C3, SQSTM1/p62, LC3B, GLUT3, INSR, LDLR, LXRα, PPARγ, IL1β, IFNγ, iNOS, COX-2, MCP-1, IL10, GPx1, and SOD3.

Mechanistic insights into the interactions of TAX1BP1 with RB1CC1 and mammalian ATG8 family proteins

TAX1BP1, a multifunctional autophagy adaptor, plays critical roles in different autophagy processes. As an autophagy receptor, TAX1BP1 can interact with RB1CC1, NAP1, and mammalian ATG8 family proteins to drive selective autophagy for relevant substrates. However, the mechanistic bases underpinning the specific interactions of TAX1BP1 with RB1CC1 and mammalian ATG8 family proteins remain elusive. Here, we find that there are two distinct binding sites between TAX1BP1 and RB1CC1. In addition to the previously reported TAX1BP1 SKICH (skeletal muscle and kidney enriched inositol phosphatase (SKIP) carboxyl homology)/RB1CC1 coiled-coil interaction, the first coiled-coil domain of TAX1BP1 can directly bind to the extreme C-terminal coiled-coil and Claw region of RB1CC1. We determine the crystal structure of the TAX1BP1 SKICH/RB1CC1 coiled-coil complex and unravel the detailed binding mechanism of TAX1BP1 SKICH with RB1CC1. Moreover, we demonstrate that RB1CC1 and NAP1 are competitive in binding to the TAX1BP1 SKICH domain, but the presence of NAP1's FIP200-interacting region (FIR) motif can stabilize the ternary TAX1BP1/NAP1/RB1CC1 complex formation. Finally, we elucidate the molecular mechanism governing the selective interactions of TAX1BP1 with ATG8 family members by solving the structure of GABARAP in complex with the non-canonical LIR (LC3-interacting region) motif of TAX1BP1, which unveils a unique binding mode between LIR and ATG8 family protein. Collectively, our findings provide mechanistic insights into the interactions of TAX1BP1 with RB1CC1 and mammalian ATG8 family proteins and are valuable for further understanding the working mode and function of TAX1BP1 in autophagy.

Atg8 family proteins, LIR/AIM motifs and other interaction modes

The Atg8 family of ubiquitin-like proteins play pivotal roles in autophagy and other processes involving vesicle fusion and transport where the lysosome/vacuole is the end station. Nuclear roles of Atg8 proteins are also emerging. Here, we review the structural and functional features of Atg8 family proteins and their protein-protein interaction modes in model organisms such as yeast, Arabidopsis, C. elegans and Drosophila to humans. Although varying in number of homologs, from one in yeast to seven in humans, and more than ten in some plants, there is a strong evolutionary conservation of structural features and interaction modes. The most prominent interaction mode is between the LC3 interacting region (LIR), also called Atg8 interacting motif (AIM), binding to the LIR docking site (LDS) in Atg8 homologs. There are variants of these motifs like "half-LIRs" and helical LIRs. We discuss details of the binding modes and how selectivity is achieved as well as the role of multivalent LIR-LDS interactions in selective autophagy. A number of LIR-LDS interactions are known to be regulated by phosphorylation. New methods to predict LIR motifs in proteins have emerged that will aid in discovery and analyses. There are also other interaction surfaces than the LDS becoming known where we presently lack detailed structural information, like the N-terminal arm region and the UIM-docking site (UDS). More interaction modes are likely to be discovered in future studies.

Semisynthetic LC3 Probes for Autophagy Pathways Reveal a Noncanonical LC3 Interacting Region Motif Crucial for the Enzymatic Activity of Human ATG3

Macroautophagy is one of two major degradation systems in eukaryotic cells. Regulation and control of autophagy are often achieved through the presence of short peptide sequences called LC3 interacting regions (LIR) in autophagy-involved proteins. Using a combination of new protein-derived activity-based probes prepared from recombinant LC3 proteins, along with protein modeling and X-ray crystallography of the ATG3-LIR peptide complex, we identified a noncanonical LIR motif in the human E2 enzyme responsible for LC3 lipidation, ATG3. The LIR motif is present in the flexible region of ATG3 and adopts an uncommon β-sheet structure binding to the backside of LC3. We show that the β-sheet conformation is crucial for its interaction with LC3 and used this insight to design synthetic macrocyclic peptide-binders to ATG3. CRISPR-enabled in cellulo studies provide evidence that LIR^ATG3 is required for LC3 lipidation and ATG3∼LC3 thioester formation. Removal of LIR^ATG3 negatively impacts the rate of thioester transfer from ATG7 to ATG3.

In vitro production of N-degron fused proteins and its application

The N-degron pathway, first discovered several decades ago by Varshavsky's laboratory, controls the half-life of target proteins depending on their N-terminal residues. In vivo cell biology studies have established the physiological role of the N-degron pathway. However, in vitro studies such as biochemical assays and structural biology studies are relatively limited. The N-degron substrates cannot be obtained via simple protein expression. The N-degron residues are exposed via the proteolytic process from the translated nascent polypeptide chains. Thus, methods for the fusion expression with several cleavable tags and subsequent treatment with specific proteases to design the exposed N-degron signals have been introduced. Recently, we developed a unique fusion technique using microtubule-associated protein 1A/1B light chain 3B (LC3B), a key marker protein of autophagy, to obtain a high yield of the purified target proteins with variable N-terminal residues for various biochemical studies including enzymatic and binding assays, and crystallization of N-degron complex. This chapter describes the protocols that include the vector map designed for producing LC3B fused target proteins, methods for expression and purification of an example protein, p62/SQSMT1, using different N-terminal residues, and methods to obtain the purified ATG4B protease, which is used for processing LC3B tag and exposing the required N-terminal residues of the target protein.

Development of new tools to study membrane-anchored mammalian Atg8 proteins

ABBREVIATIONS

A:C autophagic membrane:cytosol; ALS amyotrophic lateral sclerosis; ATG4 autophagy related 4; Atg8 autophagy related 8; BafA1 bafilomycin A₁; BNIP3L/Nix BCL2 interacting protein 3 like; CALCOCO2/NDP52 calcium binding and coiled-coil domain 2; EBSS Earle's balanced salt solution; GABARAP GABA type A receptor-associated protein; GST glutathione S transferase; HKO hexa knockout; K_d dissociation constant; LIR LC3-interacting region; MAP1LC3/LC3 microtubule associated protein 1 light chain 3; NLS nuclear localization signal/sequence; PE phosphatidylethanolamine; SpHfl1 Schizosaccharomyces pombeorganic solute transmembrane transporter; SQSTM1/p62 SQSTM1/p62; TARDBP/TDP-43 TAR DNA binding protein; TKO triple knockout.

The interplay between pathogens and Atg8 family proteins: thousand-faced interactions

Autophagy is an intracellular degradation and recycling process that can also remove pathogenic intracellular bacteria and viruses from within cells (referred to as xenophagy) and activate the adaptive immune responses. But autophagy-especially Atg proteins including Atg8 family members-can also have proviral and probacterial effects. In this review, we summarize known interactions of bacterial, parasitic, and viral proteins with Atg8 family proteins and the outcome of these interactions on pathogen replication, autophagy, or mitophagy. We discuss the value of prediction software and the research methodology in the study of pathogen protein-Atg8 family protein interactions, with selected examples of potential LC3-interacting region motif-containing SARS-CoV-2 proteins.

Deciphering the role of a membrane-targeting domain in assisting endosomal and autophagic membrane localization of a RavZ protein catalytic domain

The bacterial effector protein RavZ from a pathogen can impair autophagy in the host by delipidating the mammalian autophagy- related gene 8 (mATG8)-phosphatidylethanolamine (PE) on autophagic membranes. In RavZ, the membrane-targeting (MT) domain is an essential function. However, the molecular mechanism of this domain in regulating the intracellular localization of RavZ in cells is unclear. In this study, we found that the fusion of the green fluorescent protein (GFP) to the MT domain of RavZ (GFP-MT) resulted in localization primarily to the cytosol and nucleus, whereas the GFP-fused duplicated-MT domain (GFP-2xMT) localized to Rab5- or Rab7-positive endosomes. Similarly, GFP fusion to the catalytic domain (CA) of RavZ (GFP-CA) resulted in localization primarily to the cytosol and nucleus, even in autophagy-induced cells. However, by adding the MT domain to GFP-CA (GFP-CA-MT), the cooperation of MT and CA led to localization on the Rab5-positive endosomal membranes in a wortmannin-sensitive manner under nutrient-rich conditions, and to autophagic membranes in autophagy-induced cells. In autophagic membranes, GFP-CA-MT delipidated overexpressed or endogenous mATG8-PE. Furthermore, GFP-CAΔα3-MT, an α3 helix deletion within the CA domain, failed to localize to the endosomal or autophagic membranes and could not delipidate overexpressed mATG8-PE. Thus, the CA or MT domain alone is insufficient for stable membrane localization in cells, but the cooperation of MT and CA leads to localization to the endosomal and autophagic membranes. In autophagic membranes, the CA domain can delipidate mATG8-PE without requiring substrate recognition mediated by LC3-interacting region (LIR) motifs. [BMB Reports 2021; 54(2): 118-123].

Computational design of binder as the LC3-p62 protein-protein interaction

Cellular autophagy is an intracellular degradation pathway, which transports damaged, deformed, senescent or non-functional proteins and organelles to lysosome for digestion and degradation. Cellular autophagy is deeply evolutionarily conservedfromyeasttomammaliancells, and many homologous proteins of the autophahgy regulators are found in several species. This physiological process maintains the steady state of cells. Furtheremore, autophagy dysfunction is closely related to various diseases, such as neurodegenerative diseases, inflammation-related diseases, cardiovascular diseases, metabolic diseases, etc. The LC3 and p62 protein protein interaction (PPI) promotes the formation of autophagosomes and delivers polyubiquitinated "cargoes" to autophagic degradation. Therefore, LC3-p62 PPI plays an integral role in the formation of autophagosomes and effectively inhibits autophagy. However, there are still few studies on the LC3-p62 PPI inhibitors for its unclear molecular mechanism. Furthermore, most of these inhibitors are macromolecules with poorly active, and small molecules are particularly scarce. In this article, the computation method was used to identify the hot spot and design peptides as the binder of LC3-p62 PPI. Findings from this work provide a reference for the follow-up research of discovering small molecule inhibitors targeting LC3-p62 PPI.

Host manipulation by bacterial type III and type IV secretion system effector proteases

Proteases are powerful enzymes, which cleave peptide bonds, leading most of the time to irreversible fragmentation or degradation of their substrates. Therefore they control many critical cell fate decisions in eukaryotes. Bacterial pathogens exploit this power and deliver protease effectors through specialised secretion systems into host cells. Research over the past years revealed that the functions of protease effectors during infection are diverse, reflecting the lifestyles and adaptations to specific hosts; however, only a small number of peptidase families seem to have given rise to most of these protease virulence factors by the evolution of different substrate-binding specificities, intracellular activation and subcellular targeting mechanisms. Here, we review our current knowledge about the enzymology and function of protease effectors, which Gram-negative bacterial pathogens translocate via type III and IV secretion systems to irreversibly manipulate host processes. We highlight emerging concepts such as signalling by protease cleavage products and effector-triggered immunity, which host cells employ to detect and defend themselves against a protease attack. TAKE AWAY: Proteases irreversibly cleave proteins to control critical cell fate decisions. Gram-negative bacteria use type III and IV secretion systems to inject effectors. Protease effectors are integral weapons for the manipulation of host processes. Effectors evolved from few peptidase families to target diverse substrates. Effector-triggered immunity upon proteolytic attack emerges as host defence.

Insights on autophagosome-lysosome tethering from structural and biochemical characterization of human autophagy factor EPG5

Pivotal to the maintenance of cellular homeostasis, macroautophagy (hereafter autophagy) is an evolutionarily conserved degradation system that involves sequestration of cytoplasmic material into the double-membrane autophagosome and targeting of this transport vesicle to the lysosome/late endosome for degradation. EPG5 is a large-sized metazoan protein proposed to serve as a tethering factor to enforce autophagosome–lysosome/late endosome fusion specificity, and its deficiency causes a severe multisystem disorder known as Vici syndrome. Here, we show that human EPG5 (hEPG5) adopts an extended “shepherd’s staff” architecture. We find that hEPG5 binds preferentially to members of the GABARAP subfamily of human ATG8 proteins critical to autophagosome–lysosome fusion. The hEPG5–GABARAPs interaction, which is mediated by tandem LIR motifs that exhibit differential affinities, is required for hEPG5 recruitment to mitochondria during PINK1/Parkin-dependent mitophagy. Lastly, we find that the Vici syndrome mutation Gln336Arg does not affect the hEPG5’s overall stability nor its ability to engage in interaction with the GABARAPs. Collectively, results from our studies reveal new insights into how hEPG5 recognizes mature autophagosome and establish a platform for examining the molecular effects of Vici syndrome disease mutations on hEPG5.

Nam and Cheung et al. describe the structural and biochemical characterization of human autophagy factor EPG5 that functions in autophagosome–lysosome tethering. They show that hEPG5 adopts an extended shepherd’s staff architecture, binds preferentially to GABARAP proteins, and is recruited to mitochondria during mitophagy.

Japanese encephalitis virus capsid protein interacts with non-lipidated MAP1LC3 on replication membranes and lipid droplets

Microtubule-associated protein 1 light chain 3 (MAP1LC3) is a protein with a well-defined function in autophagy, but still incompletely understood roles in several other autophagy-independent processess. Studies have shown MAP1LC3 is a host-dependency factor for the replication of several viruses. Japanese encephalitis virus (JEV), a neurotropic flavivirus, replicates on ER-derived membranes that are marked by autophagosome-negative non-lipidated MAP1LC3 (LC3-I). Depletion of LC3 exerts a profound inhibition on virus replication and egress. Here, we further characterize the role of LC3 in JEV replication, and through immunofluorescence and immunoprecipitation show that LC3-I interacts with the virus capsid protein in infected cells. This association was observed on capsid localized to both the replication complex and lipid droplets (LDs). JEV infection decreased the number of LDs per cell indicating a link between lipid metabolism and virus replication. This capsid-LC3 interaction was independent of the autophagy adaptor protein p62/Sequestosome 1 (SQSTM1). Further, no association of capsid was seen with the Gamma-aminobutyric acid receptor-associated protein family, suggesting that this interaction was specific for LC3. High-resolution protein-protein docking studies identified a putative LC3-interacting region in capsid, 56FTAL59, and other key residues that could mediate a direct interaction between the two proteins.

Host Delipidation Mediated by Bacterial Effectors

Protein lipidation, the covalent attachment of a lipid moiety to a target protein, plays a critical role in many cellular processes in eukaryotic cells. Bacterial pathogens secrete various effectors to subvert the host signaling pathway as a mechanism of microbial pathogenesis. An increasing number of effectors from diverse bacterial pathogens function as cysteine proteases to cause irreversible delipidation of host lipidated proteins. This in turn results in disruption of crucial lipidation-mediated host signal transduction, thereby enabling pathogen survival and replication. In this review, we discuss the role of the bacterial effectors in interactions with the host and highlight our knowledge of irreversible host delipidation, with a focus on the common concerted biochemical mechanisms of the bacterial effectors.

Atg8-Family Proteins-Structural Features and Molecular Interactions in Autophagy and Beyond

Autophagy is a common name for a number of catabolic processes, which keep the cellular homeostasis by removing damaged and dysfunctional intracellular components. Impairment or misbalance of autophagy can lead to various diseases, such as neurodegeneration, infection diseases, and cancer. A central axis of autophagy is formed along the interactions of autophagy modifiers (Atg8-family proteins) with a variety of their cellular counter partners. Besides autophagy, Atg8-proteins participate in many other pathways, among which membrane trafficking and neuronal signaling are the most known. Despite the fact that autophagy modifiers are well-studied, as the small globular proteins show similarity to ubiquitin on a structural level, the mechanism of their interactions are still not completely understood. A thorough analysis and classification of all known mechanisms of Atg8-protein interactions could shed light on their functioning and connect the pathways involving Atg8-proteins. In this review, we present our views of the key features of the Atg8-proteins and describe the basic principles of their recognition and binding by interaction partners. We discuss affinity and selectivity of their interactions as well as provide perspectives for discovery of new Atg8-interacting proteins and therapeutic approaches to tackle major human diseases.

Structure and Dynamics in the ATG8 Family From Experimental to Computational Techniques

Autophagy is a conserved and essential intracellular mechanism for the removal of damaged components. Since autophagy deregulation is linked to different kinds of pathologies, it is fundamental to gain knowledge on the fine molecular and structural details related to the core proteins of the autophagy machinery. Among these, the family of human ATG8 proteins plays a central role in recruiting other proteins to the different membrane structures involved in the autophagic pathway. Several experimental structures are available for the members of the ATG8 family alone or in complex with their different biological partners, including disordered regions of proteins containing a short linear motif called LC3 interacting motif. Recently, the first structural details of the interaction of ATG8 proteins with biological membranes came into light. The availability of structural data for human ATG8 proteins has been paving the way for studies on their structure-function-dynamic relationship using biomolecular simulations. Experimental and computational structural biology can help to address several outstanding questions on the mechanism of human ATG8 proteins, including their specificity toward different interactors, their association with membranes, the heterogeneity of their conformational ensemble, and their regulation by post-translational modifications. We here summarize the main results collected so far and discuss the future perspectives within the field and the knowledge gaps. Our review can serve as a roadmap for future structural and dynamics studies of the ATG8 family members in health and disease.

Progress and Challenges in The Use of MAP1LC3 as a Legitimate Marker for Measuring Dynamic Autophagy In Vivo

Tremendous efforts have been made these last decades to increase our knowledge of intracellular degradative systems, especially in the field of autophagy. The role of autophagy in the maintenance of cell homeostasis is well documented and the existence of defects in the autophagic machinery has been largely described in diseases and aging. Determining the alterations occurring in the many forms of autophagy that coexist in cells and tissues remains complicated, as this cellular process is highly dynamic in nature and can vary from organ to organ in the same individual. Although autophagy is extensively studied, its functioning in different tissues and its links with other biological processes is still poorly understood. Several assays have been developed to monitor autophagy activity in vitro, ex vivo, and in vivo, based on different markers, the use of various inhibitors and activators, and distinct techniques. This review emphasizes the methods applied to measure (macro-)autophagy in tissue samples and in vivo via a protein, which centrally intervenes in the autophagy pathway, the microtubule-associated protein 1A/1B-light chain 3 (MAP1LC3), which is the most widely used marker and the first identified to associate with autophagosomal structures. These approaches are presented and discussed in terms of pros and cons. Some recommendations are provided to improve the reliability of the interpretation of results.

Use of the LC3B-fusion technique for biochemical and structural studies of proteins involved in the N-degron pathway

The N-degron pathway, formerly the N-end rule pathway, is a protein degradation process that determines the half-life of proteins based on their N-terminal residues. In contrast to the well-established in vivo studies over decades, in vitro studies of this pathway, including biochemical characterization and high-resolution structures, are relatively limited. In this study, we have developed a unique fusion technique using microtubule-associated protein 1A/1B light chain 3B, a key marker protein of autophagy, to tag the N terminus of the proteins involved in the N-degron pathway, which enables high yield of homogeneous target proteins with variable N-terminal residues for diverse biochemical studies including enzymatic and binding assays and substrate identification. Intriguingly, crystallization showed a markedly enhanced probability, even for the N-degron complexes. To validate our results, we determined the structures of select proteins in the N-degron pathway and compared them with the Protein Data Bank-deposited proteins. Furthermore, several biochemical applications of this technique were introduced. Therefore, this technique can be used as a general tool for the in vitro study of the N-degron pathway.

Subversion of Host Membrane Dynamics by the Legionella Dot/Icm Type IV Secretion System

Legionella species are Gram-negative ubiquitous environmental bacteria, which thrive in biofilms and parasitize protozoa. Employing an evolutionarily conserved mechanism, the opportunistic pathogens also replicate intracellularly in mammalian macrophages. This feature is a prerequisite for the pathogenicity of Legionella pneumophila, which causes the vast majority of clinical cases of a severe pneumonia, termed "Legionnaires' disease." In macrophages as well as in amoeba, L. pneumophila grows in a distinct membrane-bound compartment, the Legionella-containing vacuole (LCV). Formation of this replication-permissive pathogen compartment requires the bacterial Dot/Icm type IV secretion system (T4SS). Through the T4SS as many as 300 different "effector" proteins are injected into host cells, where they presumably subvert pivotal processes. Less than 40 Dot/Icm substrates have been characterized in detail to date, a number of which show unprecedented biological activities. Some of these effector proteins target host cell small GTPases, phosphoinositide lipids, the chelator phytate, the ubiquitination machinery, the retromer complex, the actin cytoskeleton, or the autophagy pathway. A recently discovered class of L. pneumophila effectors modulates the activity of other effectors and is termed "metaeffectors." Here, we summarize recent insight into the cellular functions and biochemical activities of L. pneumophila effectors and metaeffectors targeting the host's endocytic, retrograde, or autophagic pathways.

The C-terminal region of ATG101 bridges ULK1 and PtdIns3K complex in autophagy initiation

The initiation of macroautophagy/autophagy is tightly regulated by the upstream ULK1 kinase complex, which affects many downstream factors including the PtdIns3K complex. The phosphorylation of the right position at the right time on downstream molecules is governed by proper complex formation. One component of the ULK1 complex, ATG101, known as an accessory protein, is a stabilizer of ATG13 in cells. The WF finger region of ATG101 plays an important role in the recruitment of WIPI1 (WD repeat domain, phosphoinositide interacting protein 1) and ZFYVE1 (zinc finger FYVE-type containing 1). Here, we report that the C-terminal region identified in the structure of the human ATG101-ATG13HORMA complex is responsible for the binding of the PtdIns3K complex. This region adopts a β-strand conformation in free ATG101, but either an α-helix or random coil in our ATG101-ATG13HORMA complex, which protrudes from the core and interacts with other molecules. The C-terminal deletion of ATG101 shows a significant defect in the interaction with PtdIns3K components and subsequently impairs autophagosome formation. This result clearly presents an additional role of ATG101 for bridging the ULK1 and PtdIns3K complexes in the mammalian autophagy process. Abbreviations: ATG: autophagy related; BECN1: beclin 1; GFP: green fluorescent protein; HORMA: Hop1p/Rev7p/MAD2; HsATG13HORMA: HORMA domain of ATG13 from Homo sapiens; KO: knockout; MAD2: mitotic arrest deficient 2 like 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PIK3R4/VPS15: phosphoinositide-3-kinase regulatory subunit 4; PtdIns3K: phosphatidylinositol 3-kinase; RB1CC1/FIP200: RB1 inducible coiled-coil 1; SAXS: small-angle X-ray scattering; ScAtg13HORMA: HORMA domain of Atg13 from Sccharomyces cerevisiae; SEC-SAXS: size-exclusion chromatography with small-angle X-ray scattering; SpAtg13HORMA: HORMA domain of Atg13 from Schizosaccharomyces pombe; SQSTM1/p62: sequestosome 1; ULK1: unc51-like autophagy activating kinase 1; UVRAG: UV radiation resistance associated; WIPI1: WD repeat domain: phosphoinositide interacting 1; ZFYVE1/DFCP1: zinc finger FYVE-type containing 1.

A Structural View of Xenophagy, a Battle between Host and Microbes

The cytoplasm in mammalian cells is a battlefield between the host and invading microbes. Both the living organisms have evolved unique strategies for their survival. The host utilizes a specialized autophagy system, xenophagy, for the clearance of invading pathogens, whereas bacteria secrete proteins to defend and escape from the host xenophagy. Several molecules have been identified and their structural investigation has enabled the comprehension of these mechanisms at the molecular level. In this review, we focus on one example of host autophagy and the other of bacterial defense: the autophagy receptor, NDP52, in conjunction with the sugar receptor, galectin-8, plays a critical role in targeting the autophagy machinery against Salmonella; and the cysteine protease, RavZ secreted by Legionella pneumophila cleaves the LC3-PE on the phagophore membrane. The structure-function relationships of these two examples and the directions of future research will be discussed.