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 t hat mediates translocation of secretory polypeptides into the endoplas mic reticulum. We have used a Xenopus oocyte microinjection assay to d etermine how signal recognition particle (SRP) RNA is exported from th e nucleus. Following nuclear injection, SRP RNA accumulated in the cyt oplasm while cytoplasmically injected SRP RNA did not enter the nucleu s. Cytoplasmic accumulation of SRP RNA was an apparently facilitated p rocess dependent on limiting trans-acting factors, since nuclear expor t exhibited saturation kinetics and was completely blocked either at l ow temperature or by wheat germ agglutinin, a known inhibitor of nucle ar pore-mediated transport. At least one target for traits-acting fact ors that promote nuclear export of SRP RNA appears to be the Alu eleme nt of the molecule, since a transcript consisting of only the Alu sequ ence was exported from the nucleus in a temperature-dependent manner a nd the Alu transcript competed in the nucleus for transport with intac t SRP RNA. Although the identities of trans-acting factors responsible for SRP RNA transport are at present unknown, we suggest that protein s contained within the cytoplasmic form of SRP are candidates. Consist ent with this idea were the effects of a mutation in SRP RNA that prev ented binding of two known SRP proteins to the Alu sequence.