Guanine Nucleotide Depletion Mediates Translocation of Nucleolar Proteins, Including RNA Helicase A (DHX-9)

DHX-9, a member of the DEXH family of RNA helicases, unwinds dsRNA/dsDNA by ATP or GTP-dependent hydrolysis. We asked whether DHX-9 played a role in the GTP depletion-induced inhibition of rRNA synthesis and/or nucleolar disruption. MPA, a specific inhibitor of inosine monophosphate dehydrogenase (IMPDH), induced a rapid translocation of DHX-9 from the nucleolus to the nucleus. EGFP-tagged DHX-9 mutated at the GTP binding site also localized to the nucleus. However, knockdown of DHX-9 by siRNA did not inhibit the rRNA synthesis or cause the nucleolar disruption. Thus, DHX-9 translocation found with IMPDH inhibition does not mediate the inhibition of rRNA synthesis.