The Alu domain of the mammalian signal recognition particle (SRP) comprises the heterodimer of proteins SRP9 and SRP14 bound to the 5′ and 3′ terminal sequences of SRP RNA. It retards the ribosomal elongation of signal-peptide-containing proteins before their engagement with the translocation machinery in the endoplasmic reticulum. Here we report two crystal structures of the heterodimer SRP9/14 bound either to the 5′ domain or to a construct containing both 5′ and 3′ domains. We present a model of the complete Alu domain that is consistent with extensive biochemical data. SRP9/14 binds strongly to the conserved core of the 5′ domain, which forms a U-turn connecting two helical stacks. Reversible docking of the more weakly bound 3′ domain might be functionally important in the mechanism of translational regulation. The Alu domain structure is probably conserved in other cytoplasmic ribonucleoprotein particles and retroposition intermediates containing SRP9/14-bound RNAs transcribed from Alu repeats or related elements in genomic DNA.
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O.W. received a pre-doctoral fellowship from the Boehringer-Ingelheim-Fonds. K.S. is supported by a START subsidy and a grant from the Swiss National Science Foundation and the Canton de Genève. K.S. and S.C. are both principal investigators belonging to SRPNET, a research network of the European Union Training and Mobility in Research program, which also supported K.W. We thank members of the EMBL/ESRF Joint Structural Biology Group (JSBG) and the staff of BM30 and ID13 for support on ESRF beamlines; in particular, A. Thompson for the europium MAD experiment and A. Perrakis for the ID13 measurements and help with wARP. C. Petosa commented critically on the manuscript.
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