The signal recognition particle (SRP) is a ribonucleoprotein com- posed of an Alu domain and an S domain. The S domain contains unique sequence SRP RNA and four SRP proteins: SRP19, SRP54, SRP68, and SRP72. SRP interacts with ribosomes to bring translat- ing membrane and secreted proteins to the endoplasmic reticulum (ER) for proper processing. Additionally, SRP RNA is a member of a family of small nonribosomal RNAs found recently in the nucle- olus, suggesting that the nucleolus is more plurifunctional than previously realized. It was therefore of interest to determine whether other SRP components localize to this intranuclear site. In transfected rat fibroblasts. green fluorescent protein fusions of SRP19, SRP68, and SRP72 localized to the nucleolus, as well as to the cytoplasm, as expected. SRP68 also accumulated in the ER, consis- tent with its affinity for the ER-bound SRP receptor. SRP54 was detected in the cytoplasm as a green fluorescent protein fusion and in immunofluorescence studies, but was not detected in the nu- cleolus. In situ hybridization experiments also revealed endoge- nous SRP RNA in the nucleolus. These results demonstrate that SRP RNA and three SRP proteins visit the nucleolus, suggesting that partial SRP assembly, or another unidentified activity of the SRP components, occurs at the nucleolus. SRP54 apparently interacts with nascent SRP beyond the nucleolus. consistent with in vitro reconstitution experiments showing that SRP19 must bind to SRP RNA before SRP54 binds. Our findings support the notion that the nucleolus is the site of assembly and/or interaction between the family of ribonucleoproteins involved in protein synthesis, in addition to ribosomes themselves.
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