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

Expression levels of Alu RNA are kept low to minimize the detrimental effects of frequent retrotransposition events in primate cells (6) . In HeLa cells, Alu RNAs are about 500fold less abundant than ribosomes and it is therefore unlikely that Alu RNPs inactivate an important fraction of 40S subunits. However, expression of specific Alu RNAs can be increased in certain tissues, like NDM29 RNA and BC200 RNA in the nervous tissue (6) , and a transient increase in Alu RNA expression has been observed in response to viral infection and to various stresses (32, 33, 47) . In addition, Alu RNA can act in several rounds of targeted delivery of the protein ( Figure 5F ) thereby multiplying the effect of its upregulation. The SRP9/14 is already present and Alu RNA levels increase within one hour and decline to normal levels within ∼7 h after heat shock (47) . Like other regulatory mechanisms in translation, the inhibitory action of the Alu RNP is therefore rapid and transient. It is therefore conceivable that lowering the pool of active 40S subunits might contribute to shaping the translational response in these adaptive processes and thereby also participate in cellular defense mechanisms used against viral infection. A recent study, which shows that HCV translation was more strongly affected by reduced cellular levels of 40S than cellular translation, supports this hypothesis (48) .