Interaction of rice and human SRP19 polypeptides with signal recognition particle RNA

The signal recognition particle (SRP) controls the transport of secretory proteins into and across lipid bilayers. SRP-like ribonucleoprotein complexes exist in all organisms, including plants. We characterized the rice SRP RNA and its primary RNA binding protein, SRP19. The secondary structure of the rice SRP RNA was similar to that found in other eukaryotes; however, as in other plant SRP RNAs, a GUUUCA hexamer sequence replaced the highly conserved GNRA-tetranucleotide loop motif at the apex of helix 8. The small domain of the rice SRP RNA was reduced considerably. Structurally, rice SRP19 lacked two small regions that can be present in other SRP19 homologues. Conservative structure prediction and site-directed mutagenesis of rice and human SRP19 polypeptides indicated that binding to the SRP RNAs occurred via a loop that is present in the N-domain of both proteins. Rice SRP19 protein was able to form a stable complex with the rice SRP RNA in vitro. Furthermore, heterologous ribonucleoprotein complexes with components of the human SRP were assembled, thus confirming a high degree of structural and functional conservation between plant and mammalian SRP components.