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

The unexpected feature in the Alu RNP-mediated translation inhibition is the role of the Alu RNA in the targeted delivery of SRP9/14 to a functional site in the 40S subunit. The Alu RNA is not part of the inactive 40S-SRP9/14 complexes and is therefore able to function in several rounds of SRP9/14 targeting to the 40S subunit ( Figure 5F ). This conclusion is supported by a parallel study from our laboratory (24) . In this study, we show that upon addition of the 40S subunit to an Alu RNP immobilized on beads via its RNA moiety, a fraction of SRP9/14 dissociates within minutes from the RNA to bind to the 40S subunit. Indeed, SRP9/14 appears to equilibrate between the two binding partners ( Figure 5A , C, D and (24)) suggesting that the functional binding site of SRP9/14 in 40S has a similar affinity for SRP9/14 as Alu RNA. The capacity of SRP9/14 to exchange rapidly between binding partners has already previously been observed (25) and is consistent with a high off rate of the SRP14-9 fusion protein from a 7SL-Alu domain RNA (36) .