Alternative glycosylation controls endoplasmic reticulum dynamics and tubular extension in mammalian cells.

The endoplasmic reticulum (ER) is a central eukaryotic organelle with a tubular network made of hairpin proteins linked by hydrolysis of guanosine triphosphate nucleotides. Among posttranslational modifications initiated at the ER level, glycosylation is the most common reaction. However, our understanding of the impact of glycosylation on the ER structure remains unclear. Here, we show that exostosin-1 (EXT1) glycosyltransferase, an enzyme involved in N-glycosylation, is a key regulator of ER morphology and dynamics. We have integrated multiomics and superresolution imaging to characterize the broad effect of EXT1 inactivation, including the ER shape-dynamics-function relationships in mammalian cells. We have observed that inactivating EXT1 induces cell enlargement and enhances metabolic switches such as protein secretion. In particular, suppressing EXT1 in mouse thymocytes causes developmental dysfunctions associated with the ER network extension. Last, our data illuminate the physical and functional aspects of the ER proteome-glycome-lipidome structure axis, with implications in biotechnology and medicine.

TRAF3 can interact with GMEB1 and modulate its anti-apoptotic function.

BACKGROUND: Members of Tumor Necrosis Factor (TNF) Receptor-Associated Factors (TRAFs) family interact with the cytoplasmic tails of TNF receptor family members to mediate signal transduction processes. TRAF3 has a major immunomodulatory function and TRAF3 deficiency has been linked to malignancies, such as multiple myeloma and lymphoid defects. In order to characterize the molecular mechanisms of TRAF3 signaling, the yeast two-hybrid system was used to identify proteins that interact with TRAF3. RESULTS: The yeast two-hybrid screen of a human B cell cDNA library with TRAF3 as bait, identified Glucocorticoid Modulatory Element-Binding Protein 1 (GMEB1) as a TRAF3-interacting protein. Previous studies indicated that GMEB1 functions as a potent inhibitor of caspase activation and apoptosis. The interaction of TRAF3 and GMEB1 proteins was confirmed in mammalian cells lines, using immunoprecipitation assays. The RING and TRAF-C domains of TRAF3 were not essential for this interaction. The overexpression of TRAF3 protein enhanced the anti-apoptotic function of GMEB1 in HeLa cells. On the other hand, downregulation of TRAF3 by RNA interference decreased significantly the ability of GMEB1 to inhibit apoptosis. In addition, LMP1(1-231), a truncated form of the EBV oncoprotein LMP1, that can interact and oligomerize with TRAF3, was also able to cooperate with GMEB1, in order to inhibit apoptosis. CONCLUSIONS: Our protein-interaction experiments demonstrated that TRAF3 can interact with GMEB1, which is an inhibitor of apoptosis. In addition, cell viability assays showed that overexpression of TRAF3 enhanced the anti-apoptotic activity of GMEB1, supporting a regulatory role of TRAF3 in GMEB1-mediated inhibition of apoptosis. Better understanding of the molecular mechanism of TRAF3 function will improve diagnostics and targeted therapeutic approaches for TRAF3-associated disorders.

Systematic interactome mapping of acute lymphoblastic leukemia cancer gene products reveals EXT-1 tumor suppressor as a Notch1 and FBWX7 common interactor.

BACKGROUND: Perturbed genotypes in cancer can now be identified by whole genome sequencing of large number of diverse tumor samples, and observed gene mutations can be used for prognosis and classification of cancer subtypes. Although mutations in a few causative genes are directly linked to key signaling pathways perturbation, a global understanding of how known cancer genes drive oncogenesis in human is difficult to assess. METHODS: We collected available information about mutated genes in Acute Lymphoblastic Leukemia (ALL). Validated human protein interactions (PPI) were collected from IntAct, HPRD and BioGRID interactomics databases, or obtained using yeast two-hybrid screening assay. RESULTS: We have mapped interconnections between 116 cancer census gene products associated with ALL. Combining protein-protein interactions data and cancer-specific gene mutations information, we observed that 63 ALL-gene products are interconnected and identified 37 human proteins interacting with at least 2 ALL-gene products. We highlighted exclusive and coexistence genetic alterations in key signaling pathways including the PI3K/AKT and the NOTCH pathways. We then used different cell lines and reporter assay systems to validate the involvement of EXT1 in the Notch pathway. CONCLUSION: We propose that novel ALL-gene candidates can be identified based on their functional association with well-known cancer genes. We identified EXT1, a gene not previously linked to ALL via mutations, as a common interactor of NOTCH1 and FBXW7 regulating the NOTCH pathway in an FBXW7-dependend manner.