Compartmentalization directs assembly of the signal recognition particle

Many ribonucleoprotein complexes assemble stepwise in distinct cellular compartments, a process that usually involves bidirectional transport of both RNA and proteins between the nucleus and cytoplasm. The biological rationale for such complex transport steps in RNP assembly is obscure. One important example is the eukaryotic signal recognition particle (SRP), a cytoplasmic RNP consisting of one RNA and six proteins. Prior in vivo studies support an "SRP54-late" assembly model in which all SRP proteins, except SRP54, are imported from the cytoplasm to the nucleus to bind SRP RNA. This partially assembled complex is then exported to the cytoplasm where SRP54 binds and forms the SRP holocomplex. Here we show that native SRP assembly requires segregated and ordered binding by its protein components. A native ternary complex forms in vitro when SRP19 binds the SRP RNA prior to binding by SRP54, which approximates the eukaryotic cellular pathway. In contrast, the presence of SRP54 disrupts native assembly of SRP19, such that two RNA-binding loops in SRP19 misfold. These results imply that SRP54 must be sequestered during early SRP assembly steps, as apparently occurs in vivo, for proper assembly of the SRP to occur. Our findings emphasize that spatial compartmentalization provides an additional level of regulation that prevents competition among components and can function to promote native assembly of the eukaryotic SRP.