Stepwise dissociation of yeast 60S ribosomal subunits by LiCl and identification of L25 as a primary 26S rRNA binding protein

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Abstract
Treatment of yeast 60S ribosomal subunits with 0.5 M LiCl removed all but 6 of the ribosomal proteins. The proteins remaining associated with the (26S + 5.8S) rRNA complex were identified as L4, L8, L10, L12, L16 and L25. These core proteins were split off sequentially in the order (L16 + L12), L10, (L4 + L8), L25 by further increasing the LiCl concentration. At 1.0 M LiCl only ribosomal protein L25 remains bound to the rRNA. Upon lowering the LiCl concentration the core proteins reassociate with the rRNA in the reverse order of their removal. The susceptibility of the ribosomal proteins to removal by LiCl corresponds quite well with their order of assembly into the 60S subunit in vivo as determined earlier. Binding studies in vitro using partially purified L25 showed that this protein binds specifically to 26S rRNA. Therefore, for the first time a eukaryotic ribosomal protein capable of binding to high-molecular-mass rRNA had been identified. Binding studies in vitro using a blot technique demonstrated that core proteins L8 and L16 as well as protein L21, though not present in any of the core particles, are also capable of binding to 26S rRNA to approximately the same extent as L25. About 9 additional 60S proteins appeared to interact with the 26S rRNA, though to a lesser extent.