Nuclear export of signal recognition particle RNA is a facilitated process that involves the Alu sequence domain

The signal recognition particle is a cytoplasmic RNA-protein complex that mediates translocation of secretory polypeptides into the endoplasmic reticulum. We have used a Xenopus oocyte microinjection assay to determine how signal recognition particle (SRP) RNA is exported from the nucleus. Following nuclear injection, SRP RNA accumulated in the cytoplasm while cytoplasmically injected SRP RNA did not enter the nucleus. Cytoplasmic accumulation of SRP RNA was an apparently facilitated process dependent on limiting trans-acting factors, since nuclear export exhibited saturation kinetics and was completely blocked either at low temperature or by wheat germ agglutinin, a known inhibitor of nuclear pore-mediated transport. At least one target for trans-acting factors that promote nuclear export of SRP RNA appears to be the Alu element of the molecule, since a transcript consisting of only the Alu sequence was exported from the nucleus in a temperature-dependent manner and the Alu transcript competed in the nucleus for transport with intact SRP RNA. Although the identities of trans-acting factors responsible for SRP RNA transport are at present unknown, we suggest that proteins contained within the cytoplasmic form of SRP are candidates. Consistent with this idea were the effects of a mutation in SRP RNA that prevented binding of two known SRP proteins to the Alu sequence.