Starch Binding Domain-containing Protein 1/Genethonin 1 Is a Novel Participant in Glycogen Metabolism

Stbd1 is a protein of previously unknown function that is most prevalent in liver and muscle, the major sites for storage of the energy reserve glycogen. The protein is predicted to contain a hydrophobic N terminus and a C-terminal CBM20 glycan binding domain. Here, we show that Stbd1 binds to glycogen in vitro and that endogenous Stbd1 locates to perinuclear compartments in cultured mouse FL83B or Rat1 cells. When overexpressed in COSM9 cells, Stbd1 concentrated at enlarged perinuclear structures, co-localized with glycogen, the late endosomal/lysosomal marker LAMP1 and the autophagy protein GABARAPL1. Mutant Stbd1 lacking the N-terminal hydrophobic segment had a diffuse distribution throughout the cell. Point mutations in the CBM20 domain did not change the perinuclear localization of Stbd1, but glycogen was no longer concentrated in this compartment. Stable overexpression of glycogen synthase in Rat1WT4 cells resulted in accumulation of glycogen as massive perinuclear deposits, where a large fraction of the detectable Stbd1 co-localized. Starvation of Rat1WT4 cells for glucose resulted in dissipation of the massive glycogen stores into numerous and much smaller glycogen deposits that retained Stbd1. In vitro, in cells, and in animal models, Stbd1 consistently tracked with glycogen. We conclude that Stbd1 is involved in glycogen metabolism by binding to glycogen and anchoring it to membranes, thereby affecting its cellular localization and its intracellular trafficking to lysosomes.     

Inhibition of GATE-16 attenuates ATRA-induced neutrophil differentiation of APL cells and interferes with autophagosome formation

Autophagy is an intracellular bulk degradation process involved in cell survival upon stress induction, but also with a newly identified function in myeloid differentiation. The autophagy-related (ATG)8 protein family, including the GABARAP and LC3 subfamilies, is crucial for autophagosome biogenesis. In order to evaluate the significance of the GABARAPs in the pathogenesis of acute myeloid leukemia (AML), we compared their expression in primary AML patient samples, CD34⁺ progenitor cells and in granulocytes from healthy donors. GABARAPL1 and GABARAPL2/GATE-16, but not GABARAP, were significantly downregulated in particular AML subtypes compared to normal granulocytes. Moreover, the expression of GABARAPL1 and GATE-16 was significantly induced during ATRA-induced neutrophil differentiation of acute promyelocytic leukemia cells (APL). Lastly, knocking down GABARAPL2/GATE-16 in APL cells attenuatedneutrophil differentiation and decreased autophagic flux. In conclusion, low GABARAPL2/GATE-16 expression is associated with an immature myeloid leukemic phenotype and these proteins are necessary for neutrophil differentiation of APL cells.     

ATL3 Is a Tubular ER-Phagy Receptor for GABARAP-Mediated Selective Autophagy

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LC3B is indispensable for selective autophagy of p62 but not basal autophagy

Autophagy is a unique intracellular protein degradation system accompanied by autophagosome formation. Besides its important role through bulk degradation in supplying nutrients, this system has an ability to degrade certain proteins, organelles, and invading bacteria selectively to maintain cellular homeostasis. In yeasts, Atg8p plays key roles in both autophagosome formation and selective autophagy based on its membrane fusion property and interaction with autophagy adaptors/specific substrates. In contrast to the single Atg8p in yeast, mammals have 6 homologs of Atg8p comprising LC3 and GABARAP families. However, it is not clear these two families have different or similar functions. The aim of this study was to determine the separate roles of LC3 and GABARAP families in basal/constitutive and/or selective autophagy. While the combined knockdown of LC3 and GABARAP families caused a defect in long-lived protein degradation through lysosomes, knockdown of each had no effect on the degradation. Meanwhile, knockdown of LC3B but not GABARAPs resulted in significant accumulation of p62/Sqstm1, one of the selective substrate for autophagy. Our results suggest that while mammalian Atg8 homologs are functionally redundant with regard to autophagosome formation, selective autophagy is regulated by specific Atg8 homologs.     

The Autophagy-related Protein Kinase Atg1 Interacts with the Ubiquitin-like Protein Atg8 via the Atg8 Family Interacting Motif to Facilitate Autophagosome Formation

In autophagy, a cup-shaped membrane called the isolation membrane is formed, expanded, and sealed to complete a double membrane-bound vesicle called the autophagosome that encapsulates cellular constituents to be transported to and degraded in the lysosome/vacuole. The formation of the autophagosome requires autophagy-related (Atg) proteins. Atg8 is a ubiquitin-like protein that localizes to the isolation membrane; a subpopulation of this protein remains inside the autophagosome and is transported to the lysosome/vacuole. In the budding yeast Saccharomyces cerevisiae, Atg1 is a serine/threonine kinase that functions in the initial step of autophagosome formation and is also efficiently transported to the vacuole via autophagy. Here, we explore the mechanism and significance of this autophagic transport of Atg1. In selective types of autophagy, receptor proteins recognize degradation targets and also interact with Atg8, via the Atg8 family interacting motif (AIM), to link the targets to the isolation membrane. We find that Atg1 contains an AIM and directly interacts with Atg8. Mutations in the AIM disrupt this interaction and abolish vacuolar transport of Atg1. These results suggest that Atg1 associates with the isolation membrane by binding to Atg8, resulting in its incorporation into the autophagosome. We also show that mutations in the Atg1 AIM cause a significant defect in autophagy, without affecting the functions of Atg1 implicated in triggering autophagosome formation. We propose that in addition to its essential function in the initial stage, Atg1 also associates with the isolation membrane to promote its maturation into the autophagosome.     

Ligand binding mode of GABAA receptor-associated protein

The γ-aminobutyric acid type A (GABA_A) receptor-associated protein is a versatile adaptor protein playing an important role in intracellular vesicle trafficking, particularly in neuronal cells. We present the X-ray structure of the soluble form of human GABA_A receptor-associated protein complexed with a high-affinity synthetic peptide at 1.3 Å resolution. The data shed light on the probable binding modes of key interaction partners, including the GABA_A receptor and the cysteine protease Atg4. The resulting models provide a structural background for further investigation of the unique biological properties of this protein.     

GABARAP克隆表达及其抗体的制备与应用

GABARAP(GABAA-receptor-associated protein)是最新发现的与GABAA受体g2亚基胞内区有相互作用的蛋白, 目前的研究结果表明它可能是通过与微管相互作用辅助GABAA受体向细胞膜上运输并使受体在膜上聚集。本文中用PCR方法扩增出编码人GABARAP(hGABARAP)的cDNA片段, 然后分别克隆到真核表达载体pcDNA6/HA和谷胱甘肽转移酶(GST)融合蛋白表达载体pGEX4T2中。将后者导入大肠杆菌BL21中, 经过IPTG诱导后用谷胱甘肽偶联的Sep- harose-4B柱子纯化出融合蛋白GST-hGABARAP。以此蛋白作为抗原免疫家兔制备抗GABARAP的抗血清, 并用GST- hGABARAP耦联的NHS-activated Sepharose 4柱子纯化抗体。纯化的抗体可以用于真核细胞中过量表达hGABARAP的Western和免疫染色检测。结果表明, 过量表达的hGABARAP在真核细胞核和细胞质中都有表达, 部分集中于核周 区域。     

Nix is a selective autophagy receptor for mitochondrial clearance

Autophagy is the cellular homeostatic pathway that delivers large cytosolic materials for degradation in the lysosome. Recent evidence indicates that autophagy mediates selective removal of protein aggregates, organelles and microbes in cells. Yet, the specificity in targeting a particular substrate to the autophagy pathway remains poorly understood. Here, we show that the mitochondrial protein Nix is a selective autophagy receptor by binding to LC3/GABARAP proteins, ubiquitin‐like modifiers that are required for the growth of autophagosomal membranes. In cultured cells, Nix recruits GABARAP‐L1 to damaged mitochondria through its amino‐terminal LC3‐interacting region. Furthermore, ablation of the Nix:LC3/GABARAP interaction retards mitochondrial clearance in maturing murine reticulocytes. Thus, Nix functions as an autophagy receptor, which mediates mitochondrial clearance after mitochondrial damage and during erythrocyte differentiation.     

Structural and functional analysis of the GABARAP interaction motif (GIM)

Through the canonical LC3 interaction motif (LIR), [W/F/Y]‐X₁‐X₂‐[I/L/V], protein complexes are recruited to autophagosomes to perform their functions as either autophagy adaptors or receptors. How these adaptors/receptors selectively interact with either LC3 or GABARAP families remains unclear. Herein, we determine the range of selectivity of 30 known core LIR motifs towards individual LC3s and GABARAPs. From these, we define a I nteraction 相似文献   

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.