Structures of protein targeting complexes

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Cotranslational translocation of proteins across or into membranes is a vital process in all kingdoms of life. It requires targeting of the translating ribosome to the membrane by the signal recognition particle (SRP), an evolutionary conserved ribonucleoprotein particle. SRP recognizes signal sequences of nascent protein chains emerging from the ribosome. Subsequent binding of SRP leads to pausing of peptide elongation and docking to the membrane-bound SRP receptor. Here, the 12 Å cryo-electron microscopy structure of a targeting complex is presented consisting of mammalian SRP bound to an active 80S ribosome carrying a signal sequence. A molecular model of SRP in this functional conformation was generated. The model reveals how the S-domain of SRP contacts the large ribosomal subunit at the nascent chain exit site to bind the signal sequence, and that the Aludomain reaches into the elongation factor binding site of the ribosome explaining its elongation arrest activity. A molecular model of the first steps of protein targeting is presented. Moreover, also the docking step has been visualized by solving a cryo-EM structure of the ribosome-SRP complex bound to the SRP receptor. This structure provides first hints regarding the mechanism of ribosome transfer to the translocon. As a side result the position of the functionally significant ribosomal protein L30e has been identified in the high resolution maps of the wheat germ ribosome.