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The mechanism of toxicity of Norspermidine in Arabidopsis

AutorSayas, E.; Pozo, J. C. del; Alabadí, David; Ferrando, Alejandro
DirectorSerrano, Ramón
Fecha de publicación2-ene-2015
ResumenPolyamines are small polycationic molecules found in all organisms where they play essential functions not well understood. On the other hand, high concentrations of polyamines are toxic by mechanisms also unknown. The uncommon polyamine norspermidine (NE) has been used as toxic cation to select resistant mutants affected in cation homeostasis (Alejandro et al, 2007, EMBO J 26: 3203) and therefore it is important to know the mechanisms of NE toxicity. This is also important to ascertain the use of NE as anti-tumoral drug (Prakash et al, 1988, Anticancer Res 8: 563). NE treatment triggers a transcriptional response similar to that observed when cytosolic misfolded proteins accumulate. Accordingly, seedlings treated with NE have increased levels of insoluble proteins. This toxic effect can be explained if NE were denaturing proteins or disturbing the Ubiquitin Proteasome System (UPS). In vivo NE treatment reduced the levels of polyubiquitinated proteins and in vitro NE inhibited the ubiquitination reaction, probably interfering with E3 enzymes. Further evidence was obtained with a DELLA protein fused with Green Fluorescence Protein (GFP-RGA), that is degraded in the presence of gibberellins (GA) via the UPS (Silverstone et al, 2001, Plant Cell 13: 1555). By using confocal microscopy and western blot we showed that after a previous NE treatment, RGA is stabilized in the presence of GAs. These results suggest that one mechanism of NE toxicity is blocking the degradation of misfolded proteins by inhibition of their ubiquitination. The structural similarity between NE and lysine residues of substrate proteins of the UPS may explain this effect.
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