The deubiquitinase Leon/USP5 interacts with Atg1/ULK1 and antagonizes autophagy.

Accumulating evidence has shown that the quality of proteins must be tightly monitored and controlled to maintain cellular proteostasis. Misfolded proteins and protein aggregates are targeted for degradation through the ubiquitin proteasome (UPS) and autophagy-lysosome systems. The ubiquitination and deubiquitinating enzymes (DUBs) have been reported to play pivotal roles in the regulation of the UPS system. However, the function of DUBs in the regulation of autophagy remain to be elucidated. In this study, we found that knockdown of Leon/USP5 caused a marked increase in the formation of autophagosomes and autophagic flux under well-fed conditions. Genetic analysis revealed that overexpression of Leon suppressed Atg1-induced cell death in Drosophila. Immunoblotting assays further showed a strong interaction between Leon/USP5 and the autophagy initiating kinase Atg1/ULK1. Depletion of Leon/USP5 led to increased levels of Atg1/ULK1. Our findings indicate that Leon/USP5 is an autophagic DUB that interacts with Atg1/ULK1, negatively regulating the autophagic process.

PTPN9-mediated dephosphorylation of VTI1B promotes ATG16L1 precursor fusion and autophagosome formation.

Macroautophagy/autophagy is an evolutionarily conserved intracellular pathway for the degradation of cytoplasmic materials. Under stress conditions, autophagy is upregulated and double-membrane autophagosomes are formed by the expansion of phagophores. The ATG16L1 precursor fusion contributes to development of phagophore structures and is critical for the biogenesis of autophagosomes. Here, we discovered a novel role of the protein tyrosine phosphatase PTPN9 in the regulation of homotypic ATG16L1 vesicle fusion and early autophagosome formation. Depletion of PTPN9 and its Drosophila homolog Ptpmeg2 impaired autophagosome formation and autophagic flux. PTPN9 colocalized with ATG16L1 and was essential for homotypic fusion of ATG16L1+ vesicles during starvation-induced autophagy. We further identified the Q-SNARE VTI1B as a substrate target of PTPN9 phosphatase. Like PTPN9, the VTI1B nonphosphorylatable mutant but not the phosphomimetic mutant enhanced SNARE complex assembly and autophagic flux. Our findings highlight the important role of PTPN9 in the regulation of ATG16L1+ autophagosome precursor fusion and autophagosome biogenesis through modulation of VTI1B phosphorylation status.Abbreviations: csw: corkscrew; EBSS: Earle's balanced salt solution; ERGIC: ER-Golgi intermediate compartment; ESCRT: endosomal sorting complexes required for transport; mop: myopic; NSF: N-ethylmaleimide-sensitive factor; PAS: phagophore assembly site; PolyQ: polyglutamine; PtdIns3P: phosphatidylinositol-3-phosphate; PTK: protein tyrosine kinase; PTM: posttranslational modification; PTP: protein tyrosine phosphatase; PTPN23/HD-PTP: protein tyrosine phosphatase non-receptor type 23; SNARE: soluble N-ethylmaleimide sensitive factor attachment protein receptor; STX7: syntaxin 7; STX8: syntaxin 8; STX17: syntaxin 17; VAMP3: vesicle associated membrane protein 3; VAMP7: vesicle associated membrane protein 7; VTI1B: vesicle transport through interaction with t-SNAREs 1B; YKT6: YKT6 v-SNARE homolog; ZFYVE1/DFCP1: zinc finger FYVE-type containing 1.

Staining of Lipid Droplets with Monodansylpentane.

Monodansylpentane (MDH) is a fluorophore that displays selective labeling of lipid droplets (LDs). The dye preferentially segregates into the neutral lipid cores of LDs and emits blue fluorescence, compatible with the simultaneous use of green and red fluorescent reporters in multi-color live-cell imaging. MDH can be used for visualizing LDs not only in cell cultures, but also in fixed tissues such as the fat body and ovaries from Drosophila. MDH is therefore a versatile marker for LDs in fluorescence microscopy.

ADI1, a methionine salvage pathway enzyme, is required for Drosophila fecundity.

BACKGROUND: Methionine, an essential amino acid, is required for protein synthesis and normal cell metabolism. The transmethylation pathway and methionine salvage pathway (MTA cycle) are two major pathways regulating methionine metabolism. Recently, methionine has been reported to play a key role in Drosophila fecundity. RESULTS: Here, we revealed that the MTA cycle plays a crucial role in Drosophila fecundity using the mutant of aci-reductone dioxygenase 1 (DADI1), an enzyme in the MTA cycle. In dietary restriction condition, the egg production of adi1 mutant flies was reduced compared to that of control flies. This fecundity defect in mutant flies was rescued by reintroduction of Dadi1 gene. Moreover, a functional homolog of human ADI1 also recovered the reproduction defect, in which the enzymatic activity of human ADI1 is required for normal fecundity. Importantly, methionine supply rescued the fecundity defect in Dadi1 mutant flies. The detailed analysis of Dadi1 mutant ovaries revealed a dramatic change in the levels of methionine metabolism. In addition, we found that three compounds namely, methionine, SAM and Methionine sulfoxide, respectively, may be required for normal fecundity. CONCLUSIONS: In summary, these results suggest that ADI1, an MTA cycle enzyme, affects fly fecundity through the regulation of methionine metabolism.

Effects of RING-SH2Grb², a chimeric protein containing the E3 ligase domain of Cbl, on the EGFR pathway.

The E3 ubiquitin-protein ligase Casitas B-lineage lymphoma protein (Cbl) negatively regulates epidermal growth factor receptor (EGFR) signaling pathway in many organisms, and has crucial roles in cell growth, development and human pathologies, including lung cancers. RING-SH2Grb² a chimeric protein of 215 amino acids containing the RING domain of Cbl that provides E3 ligase activity, and the SH2 domain of Grb2 that serves as an adaptor for EGFR. In this study, we demonstrated that RING-SH2Grb² could promote the ubiquitinylation and degradation of EGFR in a human non-small cell lung carcinoma cell line H1299. Moreover, we discovered that the RING-SH2Grb² chimera promoted the internalization of ligand-bound EGFR, inhibited the growth of H1299 cells, and significantly suppressed tumor growth in a xenograft mouse model. In summary, our results revealed a potential new cancer therapeutic approach for non-small cell lung cancer.

The gradient of Gurken, a long-range morphogen, is directly regulated by Cbl-mediated endocytosis.

The asymmetric localization of gurken mRNA and post-translational sorting mechanisms are responsible for the polar distribution of Gurken protein in Drosophila. However, endocytosis of Egfr, the receptor for Gurken in the follicle cells, also plays a role in shaping the extracellular gradient of the Gurken morphogen. Previously, we have found that mutation in the Cbl gene caused elevated Egfr signaling along the dorsoventral axis, and resulted in dorsalization phenotypes in embryos and egg shells. Here, we report that overexpression of the Cbl long isoform significantly changed Gurken distribution. Using an HRP-Gurken fusion protein, we demonstrate that internalization of the Gurken-Egfr complex depends on the activity of Cbl. Increased levels of CblL promote the internalization of this complex, leading to the reduction of free ligands. The Gurken-Egfr complex trafficks through the Rab5/Rab7 associated endocytic pathway to the lysosomal degradation compartment for signaling termination. We observe endocytic Gurken not only in the dorsal but also in the ventral follicle cells, which is, to our knowledge, the first visualization of Gurken on the ventral side of egg chambers. Our results show that Gurken travels towards the lateral/posterior of the egg chamber in the absence of Cbl, suggesting that Cbl actively regulates Gurken distribution through promoting endocytosis and subsequent degradation.

GSKIP is homologous to the Axin GSK3beta interaction domain and functions as a negative regulator of GSK3beta.

Although prominent FRAT/GBP exhibits a limited degree of homology to Axin, the binding sites on GSK3 for FRAT/GBP and Axin may overlap to prevent the effect of FRAT/GBP in stabilizing beta-catenin in the Wnt pathway. Using a yeast two-hybrid screen, we identified a novel protein, GSK3beta interaction protein (GSKIP), which binds to GSK3beta. We have defined a 25-amino acid region in the C-terminus of GSKIP that is highly similar to the GSK3beta interaction domain (GID) of Axin. Using an in vitro kinase assay, our results indicate that GSKIP is a good GSK3beta substrate, and both the full-length protein and a C-terminal fragment of GSKIP can block phosphorylation of primed and nonprimed substrates in different fashions. Similar to Axin GID(381-405) and FRATtide, synthesized GSKIPtide is also shown to compete with and/or block the phosphorylation of Axin and beta-catenin by GSK3beta. Furthermore, our data indicate that overexpression of GSKIP induces beta-catenin accumulation in the cytoplasm and nucleus as visualized by immunofluorescence. A functional assay also demonstrates that GSKIP-transfected cells have a significant effect on the transactivity of Tcf-4. Collectively, we define GSKIP as a naturally occurring protein that is homologous with the GSK3beta interaction domain of Axin and is able to negatively regulate GSK3beta of the Wnt signaling pathway.