Hierarchical assembly of the Alu domain of the mammalian signal recognition particle

The mammalian signal recognition particle (SRP) catalytically promotes cotr anslational translocation of signal sequence containing proteins across the endoplasmic reticulum membrane. While the S-domain of SRP binds the N-term inal signal sequence on the nascent polypeptide, the Alu domain of SRP temp orarily interferes with the ribosomal elongation cycle until the translocat ion pore in the membrane is correctly engaged. Here we present biochemical and biophysical evidence for a hierarchical assembly pathway of the SRP Alu domain. The proteins SRP9 and SRP14 first heterodimerize and then initiall y bind to the Alu RNA 5' domain. This creates the binding site for the Alu RNA 3' domain. Alu RNA then undergoes a large conformational change with th e flexibly linked 3' domain folding back by 180 degrees onto the 5' domain complex to form the final compact Alu ribonucleoprotein particle (Alu RNP). We discuss the possible mechanistic consequences of the likely reversibili ty of this final step with reference to translational regulation by the SRP Alu domain and with reference to the structurally similar Alu RNP retropos ition intermediates derived from Alu elements in genomic DNA.