Nucleolar Relocalization of mRNAs and RNA Binding Proteins Induced by Transcriptional Inhibition and Severe Heat Shock in Trypanosoma cruzi

NAZER EZEQUIEL; SANCHEZ DANIEL

Congreso; XIII International congress of Protistology; 2009

Recently, the nucleolus has also been implicated in a variety of cellular processesapart from the well-known rRNA transcription and ribosome assembly. Some ofthese additional functions are related to the regulation of mitosis, cell-cycleprogression, biogenesis of ribonucleoprotein particles and stress response to avariety of stressors. In this frame, several nucleolar proteomic analyses also supportthe notion that the nucleolus may participate in different cellular processes. Inregard with RNA metabolism, the presence of several transcription factors relatedwith RNA Pol II, splicing factors and different RNA Binding Proteins (RBPs), suchas SR proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs), suggeststhe participation of the nucleolus in many RNA processes such as transcription,pre-mRNA processing, degradation, transport and localization.In this work we show that under transcription inhibition, several RBPs involved inthe mRNA metabolism are relocalized into the nucleolus in Trypanosoma cruzi as aspecific stress response. ATP depletion as well as kinase inhibition markedlyreduced the nucleolar localization response, suggesting that an energy-dependenttransport modulated by the phosphorylation status of the parasite might be required.Deletion analyses in one of such proteins, TcSR62, showed that a domain bearingbasic amino acids located in the COOH terminal region was sufficient to promoteits nucleolar relocalization. Surprisingly, we found that the mechanism/pathwaybehind the stress-induced nucleolar accumulation of RBPs is absent in T. brucei, aclose relative of T. cruzi, thus suggesting a different degree of conservation duringthe evolution of the trypanosomatid lineage.Interestingly, we showed that in addition to RBPs, the poly(A)+ RNA is alsoaccumulated into the nucleolus in response to ActD treatment.Finally, we found out that nucleolar relocalization of both RBPs and mRNAs is alsotriggered by severe heat shock in a reversible way. On the other hand, a mRNAwhich codifies for a heat shock response protein, such as Hsp70, could bypass suchnucleolar accumulation. These results suggest that mRNAs related with heat shockivprotein may not show such nucleolar accumulation, being exported to thecytoplasm where their translation could continue.Together, our results suggest that the nucleolus of T. cruzi could participate incertain stress responses sequestering key factors related to gene expressionregulation such as mRNAs and RBPs, which could finally be interpreted as analternative way of post-transcriptional regulation.Finally, these results also suggest that the non-traditional features of the nucleoluswould be already present in early divergent organisms such as trypanosomes,supporting the notion that these characteristics were acquired early during evolutionof the eukaryotic cell.