Functional Interaction Between RNA Helicase II/Gu(alpha) and Ribosomal Protein L4

RNA helicase II/Gu(alpha) is a multifunctional nucleolar protein involved in ribosomal RNA processing in Xenopus laevis oocytes and mammalian cells. Downregulation of Gu(alpha) using small interfering RNA (siRNA) in HeLa cells resulted in 80% inhibition of both 18S and 28S rRNA production. The mechanisms underlying this effect remain unclear. Here we show that in mammalian cells, Gu(alpha) physically interacts with ribosomal protein L4 (RPL4), a component of 60S ribosome large subunit. The ATPase activity of Gu(alpha) is important for this interaction and is also necessary for the function of Gu(alpha) in the production of both 18S and 28S rRNAs. Knocking down RPL4 expression using siRNA in mouse LAP3 cells inhibits the production of 47/45S, 32S, 28S, and 18S rRNAs. This inhibition is reversed by exogenous expression of wild-type human RPL4 protein but not the mutant form lacking Gu(alpha)-interacting motif. These observations have suggested that the function of Gu(alpha) in rRNA processing is at least partially dependent on its ability to interact with RPL4.