Alu RNA regulates the cellular pool of active ribosomes by targeted delivery of SRP9/14 to 40S subunits

Ivanova, Elena; Berger, Audrey; Scherrer, Anne; Alkalaeva, Elena; Strub, Katharina

These results demonstrated that Alu RNPs inhibit translation initiation in mammalian translation systems consistent with the differences observed in the polysome profiles using the wheat germ translation system (4). The inhibitory effect occurs prior to formation of the 48S complex. In the reconstituted mammalian system, the inhibition was maximal at 5-fold higher particle concentration (500 nM) than in RRL (100 nM), which we attributed to absolute and relative differences in the abundances of active ribosomes and initiation factors as compared to RRL. Moreover, a fraction of the Alu particles might be inactivated by nonspecific binding to components present at elevated and imbalanced concentrations in the reconstituted system. Figure S3A , Table 1 ). Furthermore, we produced AluY RNAs representing the different subfamilies of the genomic AluY elements to generalize our findings on AluY RNPs (Table 1 , Supplementary Figure S3C ). Particles were purified and analyzed as before and their RNA and protein contents were consistent with a 1:1 stoichiometry of the complexes (Supplementary Figure S3E ). The effect of the AluY L and SA110 RNPs on translation efficiency was monitored at different concentrations in the pPL-primed RRL. The scAluY L RNP decreased the translation efficiency in a concentration-dependent way reminiscent of the results obtained for the scAluY A RNP ( Figures 1B and 2A ). In contrast, the SA110 RNP had no effect at all concentrations tested. Analysis of the activities of three additional AluY RNPs and of SA86 and SA151 RNPs showed that all AluY RNPs inhibited translation whereas SA151 and SA86 RNPs had no effect ( Figure 2B ).