Abstract

The signal recognition particle (SRP) of eukaryotic cells is a cytoplasmic ribonucleoprotein machine that arrests the translational elongation of nascent secretory and membrane proteins and facilitates their transport into the endoplasmic reticulum. The spatial pathway of SRP RNA processing and ribonucleoprotein assembly in the cell is not known. In the present investigation, microinjection of fluorescently tagged SRP RNA into the nucleus of mammalian cells was used to examine its intranuclear sites of localization. Microinjection of SRP RNA into the nuclei of normal rat kidney (NRK) epithelial cells maintained at 37 degreesC on the microscope stage resulted in a very rapid initial localization in nucleoli, followed by a progressive decline of nucleolar signal and an increase of fluorescence at discrete sites in the cytoplasm. Nuclear microinjection of a molecule corresponding to a major portion of the Alu domain of SRP RNA revealed a pattern of rapid nucleolar localization followed by cytoplasmic appearance of signal that was similar to the results obtained with full-length SRP RNA. In contrast, a molecule corresponding to the S domain of SRP RNA did not display nucleolar localization to the extent observed with full-length SRP RNA. An SRP RNA molecule lacking helix 6 of the S domain displayed normal nucleolar localization, whereas one lacking helix 8 of the S domain did not. These results, obtained by direct, real-time observation of fluorescent RNA molecules inside the nucleus of living mammalian cells, suggest that the processing of SRP RNA or its ribonucleoprotein assembly into the SRP involves a nucleolar phase.