No abstract

No abstract

The mammalian signal recognition particle (SRP) catalytically promotes cotranslational translocation of signal sequence containing proteins across the endoplasmic reticulum membrane. While the S-domain of SRP binds the N-terminal signal sequence on the nascent polypeptide, the Alu domain of SRP temporarily interferes with the ribosomal elongation cycle until the translocation 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 initially bind to the Alu RNA 59 domain. This creates the binding site for the Alu RNA 39 domain. Alu RNA then undergoes a large conformational change with the flexibly linked 39 domain folding back by 1808 onto the 59 domain complex to form the final compact Alu ribonucleoprotein particle (Alu RNP). We discuss the possible mechanistic consequences of the likely reversibility of this final step with reference to translational regulation by the SRP Alu domain and with reference to the structurally similar Alu RNP retroposition intermediates derived from Alu elements in genomic DNA.

The mammalian signal recognition particle (SRP) is an 11S cytoplasmic ribonucleoprotein that plays an essential role in protein sorting. SRP recognizes the signal sequence of the nascent polypeptide chain emerging from the ribosome, and targets the ribosome–nascent chain–SRP complex to the rough endoplasmic reticulum. The SRP consists of six polypeptides (SRP9, SRP14, SRP19, SRP54, SRP68 and SRP72) and a single 300 nucleotide RNA molecule. SRP9 and SRP14 proteins form a heterodimer that binds to the Alu domain of SRP RNA which is responsible for translation arrest. We report the first crystal structure of a mammalian SRP protein, that of the mouse SRP9/14 heterodimer, determined at 2.5 Å resolution. SRP9 and SRP14 are found to be structurally homologous, containing the same α‐β‐β‐β‐α fold. This we designate the Alu binding module (Alu bm), an additional member of the family of small α/β RNA binding domains. The heterodimer has pseudo 2‐fold symmetry and is saddle like, comprising a strongly curved six‐stranded amphipathic β‐sheet with the four helices packed on the convex side and the exposed concave surface being lined with positively charged residues.

No abstract