Functions Of EIF3 Different Modules In Tumor Cell Proliferation And Protein Synthesis

The translation of mRNA to protein in eukaryotes is a complex multistep process,with the participation of mRNA,ribosomes,tRNA and initiation factors.Eukaryotic Initiation 3(eIF3)is the largest and most complex translation initiation factor during the process of initiation,containing 13 subunits(eIF3a-eIF3m).The imbalance of eIF3 subunits expression can affect the expression of the entire eIF3 complex,and in some cases,the misregulation of eIF3 can be associated with pathogenesis and progression of certain diseases,including cancer,neurodegenerative diseases,and infections.Since subunits of eIF3 play different roles in different tissues,the expression of eIF3 subunits can be either up-regulated or downregulated in different types of human cancers.Therefore,the eIF3 proteins are widely considered to play important roles in human cancer.To investigate the expression level and effects of different subunits from different modules of eIF3 complex on tumor cell growth,eIF3b and eIF3k were selected for the present study.We successfully constructed stable transfection cell lines,where eIF3b and eIF3k tagged with AID respectively.EIF3b and eIF3k can be recognized by the ubiquitin-proteasome degradation system when the auxin was added.Based on this auxin-inducible conditional degradation system,we performed experiments as below:Firstly,after the degradation of eIF3b and eIF3k,we found that the loss of eIF3b had a great impact on the overall structure of eIF3 complex.Expression levels of some other subunits were affected by the degradation of eIF3b.Loss of eIF3k had little impact on the overall structure of eIF3 complex;Secondly,we performed assays in vivo and in vitro,and we found that the absence of eIF3b significantly inhibited the proliferation of tumor cells,which was blocked in G1 phase,and inhibits the tumor growth in xenograft.However,the absence of eIF3k had no inhibitory effects on tumor cell proliferation.Finally,we explored the regulatory functions of eIF3b and eIF3k on protein synthesis,including mitochondrial proteins,by polysomes profiling and in vivo translation systems.We found that the loss of eIF3b retarded the global translation rate,while the loss of eIF3k had no significant effects on that.Moreover,neither of them can affect the synthesis of mitochondrial protein or proteasome by regulating the short 5’UTR of mitochondrial protein or proteasome gene.In summary,our study showed that eIF3b and eIF3k,as subunits in different structural modules of eIF3 complex,have different biological functions and have functions in global translation of cells.