Most membrane and secretory proteins are delivered co-translationally to protein translocation channels in their destination membrane by the signal recognition particle (SRP) and its receptor. This co-translational molecular machinery is conserved across all kingdoms of life, though it varies in composition and function. Here we report recent progress towards understanding the mechanism of SRP function, focusing on findings about Escherichia coli SRP's conformational dynamics throughout the targeting process. These insights shed light on a key checkpoint in the targeting cycle: how SRP regulates engagement of an actively translating ribosome with the translocation machinery at the membrane.

The signal recognition particle (SRP) directs ribosome-nascent chain complexes (RNCs) displaying signal sequences to protein translocation channels in the plasma membrane of prokaryotes and endoplasmic reticulum of eukaryotes. It was initially proposed that SRP binds the signal sequence when it emerges from an RNC and that successful binding becomes impaired as translation extends the nascent chain, moving the signal sequence away from SRP on the ribosomal surface. Later studies drew this simple model into question, proposing that SRP binding is unaffected by nascent chain length. Here, we reinvestigate this issue using two novel and independent fluorescence resonance energy transfer assays. We show that the arrival and dissociation rates of SRP binding to RNCs vary according to nascent chain length, resulting in the highest affinity shortly after a functional signal sequence emerges from the ribosome. Moreover, we show that SRP binds RNCs in multiple and interconverting conformations, and that conversely, RNCs exist in two conformations distinguished by SRP interaction kinetics.