Dynamic Membrane association of the bacterial SRP receptor in E. coli

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During co-translational protein targeting the SRP-receptor (SR) connects the soluble SRP-ribosome nascent chain complex (RNC) to the membrane-bound SecY translocon. In eukaryotes, the SR consists of two subunits: SRα recruits the SRP-RNC to the membrane whereas SRβ is responsible for sensing vacant translocons and for coordinating RNC-transfer from SRP to the translocon. Although the bacterial SR consists of only the SRα-homolog FtsY, our data indicate that FtsY is able to execute both functions of the eukaryotic SR. This raises several crucial questions: (1) How does FtsY coordinate RNC binding to the translocon? (2) What is the translocon binding site of FtsY and how is FtsY dispelled from the translocon to expose the binding sites for the RNC? (3) Are co-translational protein targeting and integration by the SRP pathway spatially coordinated as indicated by the observation that FtsY interacts with components of the bacterial cytoskeleton? These questions will be addressed by combining in vivo techniques, such as in vivo site-directed cross-linking, FRET and fluorescence microscopy, with in vitro techniques, such as our established in vitro transcription/translation systems and reconstituted proteoliposome systems.