Stress Sensing Of Nucleolar Proteins:Mechanisms And Implications

The nucleolus is a sub-nuclear structure of cell,where ribosome biogenesis,including rRNA transcription,processing and assembly of ribosomes takes place.Different from other subcellular structures,there is no membrane to separate nucleolus from nucleoplasm,facilitating highly dynamic shuttling of nucleolar proteins between nucleolus and nucleoplasm.Recent years,the researches revealed that various cellular stresses including hypoxia,UV exposure,drug treatment and heat shock can all result in nucleolar disruption,in which many nucleolar proteins translocate to nucleoplasm,and may be resulte to ribosome biogenesis fault and p53 accumulation.This phenomenon is named as“nucleolar stress response”.However,why the nucleolar proteins behavior in the same way upon various cellular stresses and whether there are any common sensing mechanisms involved are unanswered.We utilized nucleus-localized redox biosensor NLS-roGFP1 to monitor the nucleolar redox changes in individual live-cell.We showed that the nucleolus was a compartment in a relative reductive status,and upon various cellular stresses including hydrogen peroxide?H₂O₂?,hypoxia,UV,heat shock exposures and Actinomycin D treatment,the nucleolus tended to be oxidized,indicating that oxidative stress in the nucleoli is a universal stress sensing mechanism.To understand the nucleolar proteins sensing mechnisms,we investigated the mechanisms of nucleoplsmic translocation of B23 and SENP3,since they translocate frequently in response to cellular stresses.We utilized time-lamps realitme confocal microscope,Mass spectrometry,Grx1fusion,ChIP and RIP analyses revealed that the glutathionylation on Cys²7575 of B23 dissociates with nucleolar nuclear acids,lost the parking position and translocates to nucleoplasm;and we also found nucleoplasmic translocation of SENP3 was induced by oxidative stress caused upon nucelolar stresses.Furthermore,we found the Cys⁵³² was partly responsible for sensing oxidative signal and therefore induced SENP3 translocation.Both of the B23 and SENP3 translocation mechanisms indicated the role of nucleolus as a cellular stress sensor.Finally,we investigated the changes of nucleolar and nucleoplasmic functions of two nucleolar proteins and found that the nucleoplasmic translocation of B23 was essential for p53 stabilization upon nucleolar stresses,and the nucleoplasmic translocation of SENP3 might resulted in SUMOylation and translocation of PELP1,therefore causing 32S rRNA processing fault.In summary,we investigated the mechanisms of B23 and SENP3 translocation,and the results indicated the roles and implications of nucleolus in nucleolar stress sensing.