The signal recognition particle (SRP) is central to membrane protein targeting; SRP RNA is essential for SRP assembly, elongation arrest, and activation of SRP guanosine triphosphatases. In eukaryotes, SRP function relies on the SRP68-SRP72 heterodimer. We present the crystal structures of the RNA-binding domain of SRP68 (SRP68-RBD) alone and in complex with SRP RNA and SRP19. SRP68-RBD is a tetratricopeptide-like module that binds to a RNA three-way junction, bends the RNA, and inserts an α-helical arginine-rich motif (ARM) into the major groove. The ARM opens the conserved 5f RNA loop, which in ribosome-bound SRP establishes a contact to ribosomal RNA. Our data provide the structural basis for eukaryote-specific, SRP68-driven RNA remodeling required for protein translocation.

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The signal recognition particle (SRP) is a ribonucleoprotein complex that plays an essential role in co-translational targeting of membrane proteins. It is found in all three domains of life and exhibits a high diversity regarding composition and structure. In most organisms, SRP can be divided into two functional domains. The S domain mediates recognition and transport of ribosome-nascent chain complexes to the translocation channel, while the Alu domain stalls elongation of the ribosome until the complex has been faithfully delivered._x000B_Here we present the crystal structures of the complete bacterial SRP Alu domain and the ternary complex of human SRP S domain RNA, SRP19, and the SRP68-RBD. Together with previous structures, our data underline the taxon-specific evolutionary adaptation of SRP RNA that has important implications in SRP-mediated targeting.

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