EXO5-DNA structure and BLM interactions direct DNA resection critical for ATR-dependent replication restart

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Ambient Light Promotes Selective Subcellular Proteotoxicity after Endogenous and Exogenous Porphyrinogenic Stress

Hepatic accumulation of protoporphyrin-IX (PP-IX) in erythropoietic protoporphyria (EPP) or X-linked-dominant protoporphyria (XLP) cause liver damage. Hepatocyte nuclear lamin aggregation is a sensitive marker for PP-IX-mediated liver injury. We tested the hypothesis that extracellular or intracellular protoporphyria cause damage to different subcellular compartments, in a light-triggered manner. Three hepatoma cell lines (HepG2, Hepa-1, and Huh-7) were treated with exogenous PP-IX (mimicking XLP extrahepatic protoporphyria) or with the iron chelator deferoxamine and the porphyrin precursor 5-aminolevulinic acid (ALA) (mimicking intracellular protoporphyrin accumulation in EPP). Exogenous PP-IX accumulated predominantly in the nuclear fraction and caused nuclear shape deformation and cytoplasmic vacuoles containing electron-dense particles, whereas ALA+deferoxamine treatment resulted in higher PP-IX in the cytoplasmic fraction. Protein aggregation in the nuclear and cytoplasmic fractions paralleled PP-IX levels and, in cell culture, the effects were exclusively ambient light-mediated. PP-IX and ALA caused proteasomal inhibition, whereas endoplasmic reticulum protein aggregation was more prominent in ALA-treated cells. The enhanced ALA-related toxicity is likely due to generation of additional porphyrin intermediates including uroporphyrin and coproporphyrin, based on HPLC analysis of cell lysates and the culture medium, as well as cell-free experiments with uroporphyrin/coproporphyrin. Mouse livers from drug-induced porphyria phenocopied the in vitro findings, and mass spectrometry of liver proteins isolated in light/dark conditions showed diminished (as compared with light-harvested) but detectable aggregation under dark-harvested conditions. Therefore, PP-IX leads to endoplasmic reticulum stress and proteasome inhibition in a manner that depends on the source of porphyrin buildup and light exposure. Porphyrin-mediated selective protein aggregation provides a potential mechanism for porphyria-associated tissue injury.     

Characterization of a mannan-like oligosaccharide from Mycobacterium smegmatis

A nitrogen-free neutral mannooligosaccharide, similar in structure to the polysaccharide component of yeast mannoproteins, has been isolated from Mycobacterium smegmatis ATCC-356. It has a molecular weight of 3200 and is terminated at the reducing end by mannose. nuclear magentic resonance spectroscopy, methylation analysis, selective enzymic degradation and acetolysis indicates that the molecule consists of an alpha1 --> 6-linked backbone to which single mannose units are attached in alpha1 --> 2 linkage as sidechains.