Mechanisms of Co-Translational Protein Targeting by Mammalian SRP

Abstract

Proper biogenesis of nascent protein is essential for cell survival. Signal recognition particle (SRP) is an essential and universally conserved factor involved in biogenesis of ~30% of the proteome through co-translational targeting of nascent proteins to Endoplasmic Reticulum (ER). Despite its importance, the mechanisms by which eukaryotic SRP ensure selective and efficient delivery of substrates to ER is poorly understood. Here, we reconstituted human SRP and SRP receptor (SR) to study the interaction between human SRP and SR, and conformational dynamics of SRP and SRP-SR targeting complex through biochemical and biophysical methods. We find that signal sequence and ribosomal components of the substrate sequentially activate human SRP. Especially, presence of signal sequence pre-organizes SRP conformation for efficient recruitment of SR, allowing specific and efficient targeting. In addition, we discover two essential roles of a conformational change in SR, where it is required not only for brining targeting complex near the ER membrane, but also for inducing subsequent conformational changes of SRP-SR complex that ensures proper delivery of substrates to Sec61 translocon on ER membrane.