Due to the high energetic expenditure for the cell, the protein biosynthesis in eukaryotes is an extensively controlled process predominantly regulated at the ribosomal biogenesis and translation initiation steps. The ribosomal biogenesis defines the global translational aptitude of the cell. It is a mainly nucleolar process which is regulated at multiple steps (e.g. transcription, rRNA processing and modification, ribosomal protein translation etc). However, the most extensively regulated and the rate limiting step of translation is the initiation. Multiple eukaryotic translation initiation factors (eIFs) function to facilitate this priming step of translation. The initial recognition of the mRNA molecule happens through the 5' cap structure found in all mRNAs of nuclear origin. This event is mediated through the recruitment of heterotrimeric complex eIF4F consisting of cap-binding protein eIF4E, scaffolding protein eIF4G and the RNA helicase eIF4A unwinding secondary structures found in 5'UTR of mRNA and thus thought to facilitate the scanning process. The helicase activity of elF4F complex or of eIF4A alone is further potentiated by eIF4B in vitro. The latter protein is at the focus of present thesis. / Signal transduction regulates multiple cellular processes including mitogenesis, differentiation, apoptosis, chemotaxis etc. Signaling pathways also regulate ribosomal biogenesis to coordinate mitogenic cues, nutrient and energy availability with the translational capacity of the cells. Mounting evidence links PI3K-Akt-mTOR and Ras-MAPK cascades to the translational control. In this thesis, I show that PI3K/mTOR and MAP kinase cascades converge to phosphorylate eIF4B on Ser422. This phosphorylation results in an increased interaction with eIF3, an essential factor bridging between eIF4F and the small ribosomal subunit. Physiological significance of eIF4B phosphorylation on Ser422 has been demonstrated by the stimulatory effect of eIF4B Ser422Asp phosphomimetic mutant on cap-dependent translation. Taken together, this represents a new paradigm of translational control mechanism regulated by signaling crosstalk. The function of eIF4B in vitro is well characterized but its in vivoeffects are disputed in literature. To address this I established HeLa cell line stably expressing shRNA targeting eIF4B. eIF4B silencing inhibits proliferation rates and anchorage-independent growth. Expression of luciferase reporter gene containing 5' terminal oligopyrimidine tract (TOP) is selectively repressed in eIF4B-silenced cells and can be rescued by exogenous eIF4B regardless of Ser422 phosphorylation status. Moreover, the de novo synthesis rates of endogenous ribosomal proteins in serum starved cultures recapitulate the luciferase reporter assay data. Utilizing polysomal analysis, I was able to show more significant inhibition of translation initiation in serum starved eIF4B-silenced cells. Our attempt to discover novel eIF4B-interacting proteins by Mass Spectrometry approach led to the identification of nucleolar RNA helicase DDX21. Confocal microscopy has shown partial co-localization of tagged eIF4B and DDX21 in nucleolar periphery. Pulse chase experiments metabolically labeling rRNA show an attenuated 28S rRNA production and concomitant accumulation of 36S intermediates in eIF4B-silenced cells. Since ribosomal biogenesis is highly coordinated process and requires strict stoichiometry maintenance of ribosomal components the observed inhibition of rRNA processing could be consequential to the decreased ribosomal protein expression. However, given the fact that eIF4B is associated with the nucleolar pre-ribosomal particle complexes its direct effect on rRNA processing cannot be ruled out. Regulation of ribosomal biogenesis by translation initiation factor may represent an important control mechanism allowing cells to co-ordinate these two processes.

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