|M.Sc Student||Harel Liat|
|Subject||Novel Role for Rpb4, A Subunit of RNA Polymerase II, in|
Regulating Translation in
|Department||Department of Medicine||Supervisor||PROF. Mordechai Choder|
Rpb4 was originally identified as the fourth largest subunit of the yeast RNA polymerase II (Pol II) which composed of twelve subunits and is responsible for the synthesis of all eukaryotic mRNA. Rpb4 exhibits some unique features that distinguish it from the other subunits. It shuttles between the nucleus and the cytoplasm, and functions at the interface of transcription and transport machineries. Here we show that Rpb4 is involved in another step of the gene expression, translation in the cytoplasm.
We found that Rpb4 interacts with components of eIF3 physically (using the TAP technique) and also by co-immunopercipitation. Moreover, we found two-hybrid interaction between Rpb4 and two subunits of eIF3. We also found that Rpb4 co-immunopercipited with other translation factors like eIF4E, the poly(A)-binding protein Pab1 and Pat1. The significance of these interactions was demonstrated by the hypersensitivity of rpb4Δ cells to drugs that inhibit translation and by genetic interaction between RPB4 and CAF20 whose protein product represses translation initiation. In addition, we found that Rpb4 is required for the appropriated response to environmental stress. For ex ample, in response to heat shock (HS), the translation of most proteins is repressed. However, cells lacking RPB4 fail to repress translation, suggesting that Rpb4 is involved in translational repression in response to HS. Interestingly, in response to HS, and in correlation with the translation repression, the interaction of Rpb4 with eIF3 subunit, Nip1, increases, whereas its interaction with eIF4E decreases. This may suggest that the interaction between Rpb4 and Nip1 competes with the interactions between eIF3 and other translation factors. Rpb4 was also found to affect the interaction between Nip1 and eIF4E in an environmental responsive manner. We conclude that Rpb4 is involved in modulating translation through its capacity to interact with components of the translation initiation complex.
Based on these results and previous work, we hypothesize that Rpb4 interacts with the nascent transcript in the nucleus; it then escorts the mRNA during its export, while controlling the mRNA transport, and than modulates its translation by binding translation initiation factor(s) in the cytoplasm. Taken together, our results and those of others indicate that Rpb4 has a role in the major stages of gene expression. Therefore, it is likely to serve as a factor that couples all these stages. Rpb4 is the first factor that seems to couples most, if not all, stages of gene expression in various cellular compartments.