Studv On Stablit Of Sas10 And Mpp10 And Their Biological Function Of Zebrafish Liver Development

Ribosomes are absolutely essential for life,generating all cellular proteins required for growth.In eukaryotes,ribosome biogenesis consumes more than 60%of the energy of a cell and this process includes transcription of the pre-rRNA,translation of ribosomal proteins and non-ribosomal proteins for maturation of rRNAs,maturation of 18S,5.8S and 28S rRNA,assembly of the small and large ribosome subunits.The biogenesis of ribosome small subunit starts from the processing and maturation of 18S rRNA from the 47S pre-rRNA transcript and is a precisely controlled stepwise process.The SSU processome mediates 18S rRNA maturation by cleavage at AO.A1 and A2 sties of 47S pre-rRNA.Mpp10(M Phase Phosphoprotein 10)interacts with Imp3(Interacting with Mpp10 protein 3),Imp4(Interacting with Mpp10 protein 4)and Sas10(Something About Silencing 10)in the nucleolus.Structural analysis has shown that the Mpp10-Imp3-Imp4 complex is a core component of the SSU processome,but whether Mpp10 has other function except this remains unknown.Interestingly,the structural analysis fails to localize Sas10 which leaves us a big question what is the biological function of the Mpp10-Sas10 complex.Previous work in our lab reported Def(Digestive-organ expansion factor)recruited cysteine protease Calpain3(CAPN3)to the nucleolus to cleave target proteins including the tumor-suppressor p53.However,whether the Def-CAPN3 pathway targets other proteins in nucleolus is unknown.Here we report that zebrafish Mpp10 and Sas10 are conserved nucleolar proteins.To further investigate the function of these two proteins,we generate sas10 and mpp10 mutants,respectively,using the CRISPR-Cas9 technique.We find both two mutants have obvious phenotypes,such as small eyes,pericardium edema and no swimming bladder.The dysplasia of the digestive organ is much severe and obvious.And the depletion of either Sas10 or Mpp10 is embryonic lethal.We also find both of these two proteins participate in rRNA processing,consistent with the function in yeast and HeLa cells.Mpp10 and Sas10 form a stable complex and stability of Sas10 and Mpp10 are mutually dependent on each other.Mpp10,but not Sas10/Utp3,is a target of the nucleolar-localized Def-CAPN3 protein degradation pathway.Def interacts with Sas10 to form the Def-Sas10-Mpp10 complex that likely facilitates the cleavage of Mpp10 by CAPN3.Sas10 protects Mpp10 from CAPN3-mediated cleavage by masking the CAPN3-recognintion site in Mpp10.Therefore,the Mpp10-Imp3-Imp4,Mpp10-Sas10 and Mpp10-Sas10-Def protein complexes are evolved to regulate ribosome biogenesis by fine-tuning the Mpp10 turnover in higher eukaryotes.