Chapter 1 : Co- and Posttranslational Protein Targeting to the SecYEG Translocon in

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In vitro and in vivo studies have shown that the two targeting pathways converge at the SecYEG translocon, through which the proteins are either inserted into the membrane or translocated to the periplasm. This chapter focuses more on the cotranslational pathway that is mediated by the signal recognition particle (SRP) system of as a model for gram-negative bacteria. This pathway can be divided into three main steps: (i) the targeting step, through which ribosomes translating the relevant proteins are targeted to the membrane; (ii) the intermediate step, through which the ribosomes are transferred from the targeting system to the translocon; and (iii) the insertion/translocation step, through which the translating ribosomes extrude the nascent polypeptide chain into or across the cytoplasmic membrane. SecA is the membrane peripheral ATPase subunit of the translocon. Cotranslational targeting of proteins to the membrane is mediated by the SRP system, which includes two essential proteins and an essential RNA molecule. The GTP hydrolysis step is crucial for the dissociation of SRP from its receptor after the release of the ribosome nascent chain from the targeting complex. Ribosomes translating SRP substrates are targeted to and assembled on the translocon, such that the elongating polypeptide chain is transferred directly from the tunnel in the ribosome into the translocation channel.

Citation: Bibi E. 2007. Co- and Posttranslational Protein Targeting to the SecYEG Translocon in , p 3-15. In Ehrmann M (ed), The Periplasm. ASM Press, Washington, DC. doi: 10.1128/9781555815806.ch1
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