A TRUNCATION IN THE 14 KDA PROTEIN OF THE SIGNAL RECOGNITION PARTICLELEADS TO TERTIARY STRUCTURE CHANGES IN THE RNA AND ABOLISHES THE ELONGATION ARREST ACTIVITY OF THE PARTICLE

The signal recognition particle (SRP) provides the molecular link betw een synthesis of polypeptides and their concomitant translocation into the endoplasmic reticulum. During targeting, SRP arrests or delays el ongation of the nascent chain, thereby presumably ensuring a high tran slocation efficiency. Components of the Alu domain, SRP9/14 and the Al u sequences of SRP RNA, have been suggested to play a role in the elon gation arrest function of SRP. We generated a truncated SRP14 protein, SRP14-20C, which forms, together with SRP9, a stable complex with SRP RNA. However, particles reconstituted with SRP9/14-20C, RC(9/14-20C), completely lack elongation arrest activity. RC(9/14-20C) particles ha ve intact signal recognition, targeting and ribosome binding activitie s. SRP9/14-20C therefore only impairs interactions with the ribosome t hat are required to effect elongation arrest. This result provides evi dence that direct interactions between the Alu domain components and t he ribosome are required for this function. Furthermore, SRP9/14-20C b inding to SRP RNA results in tertiary structure changes in the RNA. Ou r results strongly indicate that these changes account for the negativ e effect of SRP14 truncation on elongation arrest, thus revealing a cr itical role of the RNA in this function.