ATG4B contains a C-terminal LIR motif important for binding and efficient cleavage of mammalian orthologs of yeast Atg8

The cysteine protease ATG4B cleaves off one or more C-terminal residues of the inactive proform of proteins of the ortholog and paralog LC3 and GABARAP subfamilies of yeast Atg8 to expose a C-terminal glycine that is conjugated to phosphatidylethanolamine during autophagosome formation. We show that ATG4B contains a C-terminal LC3-interacting region (LIR) motif important for efficient binding to and cleavage of LC3 and GABARAP proteins. We solved the crystal structures of the GABARAPL1-ATG4B C-terminal LIR complex. Analyses of the structures and in vitro binding assays, using specific point mutants, clearly showed that the ATG4B LIR binds via electrostatic-, aromatic HP1 and hydrophobic HP2 pocket interactions. Both these interactions and the catalytic site-substrate interaction contribute to binding between LC3s or GABARAPs and ATG4B. We also reveal an unexpected role for ATG4B in stabilizing the unlipidated forms of GABARAP and GABARAPL1. In mouse embryonic fibroblast (MEF) atg4b knockout cells, GABARAP and GABARAPL1 were unstable and degraded by the proteasome. Strikingly, the LIR motif of ATG4B was required for stabilization of the unlipidated forms of GABARAP and GABARAPL1 in cells.     

Human LC3 and GABARAP subfamily members achieve functional specificity via specific structural modulations

ABSTRACT

Autophagy is a conserved adaptive cellular pathway essential to maintain a variety of physiological functions. Core components of this machinery are the six human Atg8 orthologs that initiate formation of appropriate protein complexes. While these proteins are routinely used as indicators of autophagic flux, it is presently not possible to discern their individual biological functions due to our inability to predict specific binding partners. In our attempts towards determining downstream effector functions, we developed a computational pipeline to define structural determinants of human Atg8 family members that dictate functional diversity. We found a clear evolutionary separation between human LC3 and GABARAP subfamilies and also defined a novel sequence motif responsible for their specificity. By analyzing known protein structures, we observed that functional modules or microclusters reveal a pattern of intramolecular network, including distinct hydrogen bonding of key residues (F52/Y49; a subset of HP2) that may directly modulate their interaction preferences. Multiple molecular dynamics simulations were performed to characterize how these proteins interact with a common protein binding partner, PLEKHM1. Our analysis showed remarkable differences in binding modes via intrinsic protein dynamics, with PLEKHM1-bound GABARAP complexes showing less fluctuations and higher number of contacts. We further mapped 373 genomic variations and demonstrated that distinct cancer-related mutations are likely to lead to significant structural changes. Our findings present a quantitative framework to establish factors underlying exquisite specificity of human Atg8 proteins, and thus facilitate the design of precise modulators.

Abbreviations: Atg: autophagy-related; ECs: evolutionary constraints; GABARAP: GABA type A receptor-associated protein; HsAtg8: human Atg8; HP: hydrophobic pocket; KBTBD6: kelch repeat and BTB domain containing 6; LIR: LC3-interacting region; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MD: molecular dynamics; HIV-1 Nef: human immunodeficiency virus type 1 negative regulatory factor; PLEKHM1: pleckstrin homology and RUN domain containing M1; RMSD: root mean square deviation; SQSTM1/p62: sequestosome 1; WDFY3/ALFY: WD repeat and FYVE domain containing 3     

Structural basis for extremely strong binding affinity of giant ankyrins to LC3/GABARAP and its application in the inhibition of autophagy

The Atg8/LC3/GABARAP family of proteins binds its physiological binding partners, which function in macroautophagy (hereafter autophagy), via recognition of their short linear motif, also known as the LC3-interactiong region (LIR) or Atg8-interacting motif (AIM). The AIM/LIR motif, with the consensus sequence [W/F/Y]xx[L/I/V], utilizes the aromatic and hydrophobic residues that bind on the surface of Atg8/LC3/GABARAP. Despite modest binding affinity, this interaction is essential for efficient autophagy. Here we highlight the recent paper by Li and collaborators who discovered the structural basis for a much stronger interaction between the LIR motif-containing peptides and LC3/GABARAP. Moreover, they showed that these peptides are potent and selective inhibitors of autophagy in cultured cells and in C. elegans.     

Development of LC3/GABARAP sensors containing a LIR and a hydrophobic domain to monitor autophagy

Macroautophagy allows for bulk degradation of cytosolic components in lysosomes. Overexpression of GFP/RFP-LC3/GABARAP is commonly used to monitor autophagosomes, a hallmark of autophagy, despite artifacts related to their overexpression. Here, we developed new sensors that detect endogenous LC3/GABARAP proteins at the autophagosome using an LC3-interacting region (LIR) and a short hydrophobic domain (HyD). Among HyD-LIR-GFP sensors harboring LIR motifs of 34 known LC3-binding proteins, HyD-LIR(TP)-GFP using the LIR motif from TP53INP2 allowed detection of all LC3/GABARAPs-positive autophagosomes. However, HyD-LIR(TP)-GFP preferentially localized to GABARAP/GABARAPL1-positive autophagosomes in a LIR-dependent manner. In contrast, HyD-LIR(Fy)-GFP using the LIR motif from FYCO1 specifically detected LC3A/B-positive autophagosomes. HyD-LIR(TP)-GFP and HyD-LIR(Fy)-GFP efficiently localized to autophagosomes in the presence of endogenous LC3/GABARAP levels and without affecting autophagic flux. Both sensors also efficiently localized to MitoTracker-positive damaged mitochondria upon mitophagy induction. HyD-LIR(TP)-GFP allowed live-imaging of dynamic autophagosomes upon autophagy induction. These novel autophagosome sensors can thus be widely used in autophagy research.     

Structural determinants in GABARAP required for the selective binding and recruitment of ALFY to LC3B‐positive structures

Several autophagy proteins contain an LC3‐interacting region (LIR) responsible for their interaction with Atg8 homolog proteins. Here, we show that ALFY binds selectively to LC3C and the GABARAPs through a LIR in its WD40 domain. Binding of ALFY to GABARAP is indispensable for its recruitment to LC3B‐positive structures and, thus, for the clearance of certain p62 structures by autophagy. In addition, the crystal structure of the GABARAP‐ALFY‐LIR peptide complex identifies three conserved residues in the GABARAPs that are responsible for binding to ALFY. Interestingly, introduction of these residues in LC3B is sufficient to enable its interaction with ALFY, indicating that residues outside the LIR‐binding hydrophobic pockets confer specificity to the interactions with Atg8 homolog proteins.     

Centrosome to autophagosome signaling: Specific GABARAP regulation by centriolar satellites

Yeast have one Atg8 protein; however, multiple Atg8 orthologs (LC3s and GABARAPs) are found in humans. We discovered that a population of the Atg8 ortholog GABARAP resides on the centrosome and the peri-centrosomal region. This centrosomal pool of GABARAP translocates to forming autophagosomes upon starvation to activate autophagosome formation in a non-hierarchical pathway. How this centrosome-to-phagophore delivery of GABARAP occurs was not understood. To address this, we have shown that the archetypal centriolar satellite protein PCM1 regulates recruitment of GABARAP to the centrosome. PCM1 recruits GABARAP, but not MAP1LC3B, directly to centriolar satellites through a LC3-interacting region (LIR) motif. Furthermore, PCM1, in concert with its interacting centriolar satellite E3 ligase MIB1, controls GABARAP stability, K48-linked ubiquitination and GABARAP-mediated autophagic flux.     

Phosphorylation of the LIR Domain of SCOC Modulates ATG8 Binding Affinity and Specificity

Autophagy is a highly conserved degradative pathway, essential for cellular homeostasis and implicated in diseases including cancer and neurodegeneration. Autophagy-related 8 (ATG8) proteins play a central role in autophagosome formation and selective delivery of cytoplasmic cargo to lysosomes by recruiting autophagy adaptors and receptors. The LC3-interacting region (LIR) docking site (LDS) of ATG8 proteins binds to LIR motifs present in autophagy adaptors and receptors. LIR-ATG8 interactions can be highly selective for specific mammalian ATG8 family members (LC3A-C, GABARAP, and GABARAPL1-2) and how this specificity is generated and regulated is incompletely understood.We have identified a LIR motif in the Golgi protein SCOC (short coiled-coil protein) exhibiting strong binding to GABARAP, GABARAPL1, LC3A and LC3C. The residues within and surrounding the core LIR motif of the SCOC LIR domain were phosphorylated by autophagy-related kinases (ULK1-3, TBK1) increasing specifically LC3 family binding. More distant flanking residues also contributed to ATG8 binding. Loss of these residues was compensated by phosphorylation of serine residues immediately adjacent to the core LIR motif, indicating that the interactions of the flanking LIR regions with the LDS are important and highly dynamic.Our comprehensive structural, biophysical and biochemical analyses support and provide novel mechanistic insights into how phosphorylation of LIR domain residues regulates the affinity and binding specificity of ATG8 proteins towards autophagy adaptors and receptors.     

Analysis of the native conformation of the LIR/AIM motif in the Atg8/LC3/GABARAP-binding proteins

The Atg8/LC3/GABARAP family of proteins, a group that has structural homology with ubiquitin, connects with a large set of binding partners to function in macroautophagy (hereafter autophagy). This interaction occurs primarily via a conserved motif termed the LC3-interacting region (LIR), or the Atg8-interacting motif (AIM). The consensus sequence for this motif, [W/F/Y]xx[L/I/V], can be found in many proteins, but only some of them are physiological partners containing a functional LIR/AIM. Because the structure of many full-length partners has not been, or cannot be, solved, the structural context of the LIR/AIM within the native protein conformation is not obvious. Here we suggest that the functional LIR/AIM is a short linear motif (SLiM) protein-binding module, arising from an intrinsically disordered region. This finding enables the rapid elimination of some false Atg8/LC3/GABARAP-binding proteins, and connects the exponentially growing knowledge on disordered SLiMs with autophagy.     

Structural basis of FYCO1 and MAP1LC3A interaction reveals a novel binding mode for Atg8-family proteins

FYCO1 (FYVE and coiled-coil domain containing 1) functions as an autophagy adaptor in directly linking autophagosomes with the microtubule-based kinesin motor, and plays an essential role in the microtubule plus end-directed transport of autophagic vesicles. The specific association of FYCO1 with autophagosomes is mediated by its interaction with Atg8-family proteins decorated on the outer surface of autophagosome. However, the mechanistic basis governing the interaction between FYCO1 and Atg8-family proteins is largely unknown. Here, using biochemical and structural analyses, we demonstrated that FYCO1 contains a unique LC3-interacting region (LIR), which discriminately binds to mammalian Atg8 orthologs and preferentially binds to the MAP1LC3A and MAP1LC3B. In addition to uncovering the detailed molecular mechanism underlying the FYCO1 LIR and MAP1LC3A interaction, the determined FYCO1-LIR-MAP1LC3A complex structure also reveals a unique LIR binding mode for Atg8-family proteins, and demonstrates, first, the functional relevance of adjacent sequences C-terminal to the LIR core motif for binding to Atg8-family proteins. Taken together, our findings not only provide new mechanistic insight into FYCO1-mediated transport of autophagosomes, but also expand our understanding of the interaction modes between LIR motifs and Atg8-family proteins in general.     

GABARAPL1 antibodies: target one protein, get one free!

Atg8 is a yeast protein involved in the autophagic process and in particular in the elongation of autophagosomes. In mammals, several orthologs have been identified and are classed into two subfamilies: the LC3 subfamily and the GABARAP subfamily, referred to simply as the LC3 or GABARAP families. GABARAPL1 (GABARAP-like protein 1), one of the proteins belonging to the GABARAP (GABA(A) receptor-associated protein) family, is highly expressed in the central nervous system and implicated in processes such as receptor and vesicle transport as well as autophagy. The proteins that make up the GABARAP family demonstrate conservation of their amino acid sequences and protein structures. In humans, GABARAPL1 shares 86% identity with GABARAP and 61% with GABARAPL2 (GATE-16). The identification of the individual proteins is thus very limited when working in vivo due to a lack of unique peptide sequences from which specific antibodies can be developed. Actually, and to our knowledge, there are no available antibodies on the market that are entirely specific to GABARAPL1 and the same may be true of the anti-GABARAP antibodies. In this study, we sought to examine the specificity of three antibodies targeted against different peptide sequences within GABARAPL1: CHEM-CENT (an antibody raised against a short peptide sequence within the center of the protein), PTG-NTER (an antibody raised against the N-terminus of the protein) and PTG-FL (an antibody raised against the full-length protein). The results described in this article demonstrate the importance of testing antibody specificity under the conditions for which it will be used experimentally, a caution that should be taken when studying the expression of the GABARAP family proteins.     

GABARAPL1 (GEC1) associates with autophagic vesicles

Gabarapl1 (gec1) was first described as an estrogen regulated gene which shares a high sequence homology with the gabarap gene. We previously demonstrated that GABARAPL1, like GABARAP, interacts with the GABAA receptor and tubulin and promotes tubulin polymerization. Previous work has demonstrated that the GABARAP family members (GABARAP, LC3, GATE-16 and Atg8) are not only involved in the transport of proteins or vesicles but are also implicated in various mechanisms such as autophagy, cell death, cell proliferation and tumor progression. We therefore asked whether GABARAPL1 might also play a role in autophagy. First, we showed that GABARAPL1 is cleaved at glycine 116, a residue which is conserved in other members of the family. We also demonstrated that GABARAPL1 is linked to phospholipids, delipidated by Atg4B, associated with intracellular membranes and accumulated in intracellular vesicles after inhibition of lysosomal activity. Finally, we showed that GABARAPL1 partially colocalizes with LC3 or Lysotracker green in intracellular vesicles. Taken together, our results demonstrate that GABARAPL1 associates with autophagic vesicles.     

GABARAPL1 antibodies: Target one protein,get one free!

Atg8 is a yeast protein involved in the autophagic process and in particular in the elongation of autophagosomes. In mammals, several orthologs have been identified and are classed into two subfamilies: the LC3 subfamily and the GABARAP subfamily, referred to simply as the LC3 or GABARAP families. GABARAPL1 (GABARAP-like protein 1), one of the proteins belonging to the GABARAP (GABA_A receptor-associated protein) family, is highly expressed in the central nervous system and implicated in processes such as receptor and vesicle transport as well as autophagy. The proteins that make up the GABARAP family demonstrate conservation of their amino acid sequences and protein structures. In humans, GABARAPL1 shares 86% identity with GABARAP and 61% with GABARAPL2 (GATE-16). The identification of the individual proteins is thus very limited when working in vivo due to a lack of unique peptide sequences from which specific antibodies can be developed. Actually, and to our knowledge, there are no available antibodies on the market that are entirely specific to GABARAPL1 and the same may be true of the anti-GABARAP antibodies. In this study, we sought to examine the specificity of three antibodies targeted against different peptide sequences within GABARAPL1: CHEM-CENT (an antibody raised against a short peptide sequence within the center of the protein), PTG-NTER (an antibody raised against the N-terminus of the protein) and PTG-FL (an antibody raised against the full-length protein). The results described in this article demonstrate the importance of testing antibody specificity under the conditions for which it will be used experimentally, a caution that should be taken when studying the expression of the GABARAP family proteins.     

GABARAPL1 (GEC1): Original or copycat?

The GABARAPL1 (GABARAP-LIKE 1) gene was first described as an early estrogen-regulated gene that shares a high sequence homology with GABARAP and is thus a part of the GABARAP family. GABARAPL1, like GABARAP, interacts with the GABA_A receptor and tubulin and promotes tubulin polymerization. The GABARAP family members (GABARAP, GABARAPL1 and GABARAPL2) and their close homologs (LC3 and Atg8) are not only involved in the transport of proteins or vesicles but are also implicated in various mechanisms such as autophagy, cell death, cell proliferation and tumor progression. However, despite these similarities, GABARAPL1 displays a complex regulation that is different from that of other GABARAP family members. Moreover, it presents a regulated tissue expression and is the most highly expressed gene among the family in the central nervous system. In this review article, we will outline the specific functions of this protein and also hypothesize about the roles that GABARAPL1 might have in several important biological processes such as cancer or neurodegenerative diseases.     

GABARAPL1 (GEC1): original or copycat?

The GABARAPL1 (GABARAP-LIKE 1) gene was first described as an early estrogen-regulated gene that shares a high sequence homology with GABARAP and is thus a part of the GABARAP family. GABARAPL1, like GABARAP, interacts with the GABAA receptor and tubulin and promotes tubulin polymerization. The GABARAP family members (GABARAP, GABARAPL1 and GABARAPL2) and their close homologs (LC3 and Atg8) are not only involved in the transport of proteins or vesicles but are also implicated in various mechanisms such as autophagy, cell death, cell proliferation and tumor progression. However, despite these similarities, GABARAPL1 displays a complex regulation that is different from that of other GABARAP family members. Moreover, it presents a regulated tissue expression and is the most highly expressed gene among the family in the central nervous system. In this review article, we will outline the specific functions of this protein and also hypothesize about the roles that GABARAPL1 might have in several important biological processes such as cancer or neurodegenerative diseases.     

iLIR@viral: A web resource for LIR motif-containing proteins in viruses

Macroautophagy/autophagy has been shown to mediate the selective lysosomal degradation of pathogenic bacteria and viruses (xenophagy), and to contribute to the activation of innate and adaptative immune responses. Autophagy can serve as an antiviral defense mechanism but also as a proviral process during infection. Atg8-family proteins play a central role in the autophagy process due to their ability to interact with components of the autophagy machinery as well as selective autophagy receptors and adaptor proteins. Such interactions are usually mediated through LC3-interacting region (LIR) motifs. So far, only one viral protein has been experimentally shown to have a functional LIR motif, leaving open a vast field for investigation. Here, we have developed the iLIR@viral database (http://ilir.uk/virus/) as a freely accessible web resource listing all the putative canonical LIR motifs identified in viral proteins. Additionally, we used a curated text-mining analysis of the literature to identify novel putative LIR motif-containing proteins (LIRCPs) in viruses. We anticipate that iLIR@viral will assist with elucidating the full complement of LIRCPs in viruses.     

Regulation of SQSTM1/p62 via UBA domain ubiquitination and its role in disease

Macroautophagy/autophagy can be a selective degradative process via the utilization of various autophagic receptor proteins. Autophagic receptors selectively recognize ubiquitinated cargoes and deliver them to phagophores, the precursors to autophagosomes, for their degradation. For example, SQSTM1/p62 directly binds to ubiquitinated protein aggregates via its UBA domain and sequesters them into inclusion bodies via its PB1 domain. SQSTM1also interacts with phagophores via its LC3-interacting (LIR) motif. However, a regulatory mechanism for autophagic receptors is not yet understood.     

Cloud hunting: doryphagy,a form of selective autophagy that degrades centriolar satellites

ABSTRACT

The selective clearance of cellular components by macroautophagy (hereafter autophagy) is critical for maintaining cellular homeostasis. In this punctum, we summarize and discuss our recent findings regarding a novel type of selective autophagy that targets centriolar satellites (CS) for degradation, a process we termed doryphagy from the Greek word “doryphoros”, standing for “satellite”. CS are microtubule-associated protein complexes that regulate centrosome composition. We show that CS degradation is mediated through a direct interaction between GABARAPs and an LC3-interacting region (LIR) motif in the CS protein PCM1. Autophagy-deficient systems accumulate large abnormal CS and consequently display centrosome reorganization and abnormal mitoses. Our findings provide a mechanistic link between autophagy deficiency and centrosome abnormalities and exemplify how mammalian Atg8-family proteins (mATG8s) can regulate substrate specificity.     

Human Atg8-cardiolipin interactions in mitophagy: Specific properties of LC3B,GABARAPL2 and GABARAP

The phospholipid cardiolipin (CL) has been proposed to play a role in selective mitochondrial autophagy, or mitophagy. CL externalization to the outer mitochondrial membrane would act as a signal for the human Atg8 ortholog subfamily, MAP1LC3 (LC3). The latter would mediate both mitochondrial recognition and autophagosome formation, ultimately leading to removal of damaged mitochondria. We have applied quantitative biophysical techniques to the study of CL interaction with various Atg8 human orthologs, namely LC3B, GABARAPL2 and GABARAP. We have found that LC3B interacts preferentially with CL over other di-anionic lipids, that CL-LC3B binding occurs with positive cooperativity, and that the CL-LC3B interaction relies only partially on electrostatic forces. CL-induced increased membrane fluidity appears also as an important factor helping LC3B to bind CL. The LC3B C terminus remains exposed to the hydrophilic environment after protein binding to CL-enriched membranes. In intact U87MG human glioblastoma cells rotenone-induced autophagy leads to LC3B translocation to mitochondria and subsequent delivery of mitochondria to lysosomes. We have also observed that GABARAP, but not GABARAPL2, interacts with CL in vitro. However neither GABARAP nor GABARAPL2 were translocated to mitochondria in rotenone-treated U87MG cells. Thus the various human Atg8 orthologs might play specific roles in different autophagic processes.     

Beyond Atg8 binding: The role of AIM/LIR motifs in autophagy

Selective macroautophagy/autophagy mediates the selective delivery of cytoplasmic cargo material via autophagosomes into the lytic compartment for degradation. This selectivity is mediated by cargo receptor molecules that link the cargo to the phagophore (the precursor of the autophagosome) membrane via their simultaneous interaction with the cargo and Atg8 proteins on the membrane. Atg8 proteins are attached to membrane in a conjugation reaction and the cargo receptors bind them via short peptide motifs called Atg8-interacting motifs/LC3-interacting regions (AIMs/LIRs). We have recently shown for the yeast Atg19 cargo receptor that the AIM/LIR motifs also serve to recruit the Atg12–Atg5-Atg16 complex, which stimulates Atg8 conjugation, to the cargo. We could further show in a reconstituted system that the recruitment of the Atg12–Atg5-Atg16 complex is sufficient for cargo-directed Atg8 conjugation. Our results suggest that AIM/LIR motifs could have more general roles in autophagy.