Fluorescence-detected assembly of the signal recognition particle: binding of the two SRP protein heterodimers to SRP RNA is noncooperative.

Protein-RNA and protein-protein interactions involved in the assembly of the signal recognition particle (SRP) were examined using fluorescence spectroscopy. Fluorescein was covalently attached to the 3'-terminal ribose of SRP RNA following periodate oxidation, and the resulting SRP RNA-Fl was reconstituted into a fluorescent SRP species that was functional in promoting translocation of secretory proteins across the membrane of the endoplasmic reticulum. Each of the two protein heterodimers purified from SRP elicited a substantial change in fluorescein emission upon association with the modified RNA. The binding of SRP9/14 to singly-labeled SRP RNA-Fl increased fluorescein emission intensity by 41% at pH 7.5 and decreased its anisotropy from 0.18 to 0.16. The binding of SRP68/72 increased the fluorescein anisotropy from 0.18 to 0.23 but did not alter the emission intensity of SRP RNA-Fl. These fluorescence changes did not result from a direct interaction between the dye and protein because the fluorescein remained accessible to both iodide ions and fluorescein-specific antibodies in the complexes. The spectral changes were elicited by specific SRP RNA-protein interactions, since (i) the SRP9/14- and SRP68/72-dependent changes were unique, (ii) an excess of unlabeled SRP RNA, but not of tRNA, blocked the fluorescence changes, and (iii) no emission changes were observed when SRP RNA-Fl was titrated with other RNA-binding proteins. Each heterodimer bound tightly to the RNA, since the Kd values determined spectroscopically and at equilibrium for the SRP9/14 and the SRP68/72 complexes with SRP RNA-Fl were less than 0.1 and 7 +/- 3 nM, respectively. The binding affinity of SRP68/72 for SRP RNA-Fl was unaffected by the presence of SRP9/14, and hence the binding of the heterodimers to SRP RNA is noncooperative in the absence of SRP54 and SRP19. The SRP protein heterodimers therefore associate randomly and independently with SRP RNA to form domains in the particle that are distinct both structurally and functionally. Any cooperativity in SRP assembly would have to be mediated by SRP54 and/or SRP19.