The signal recognition particle and its interactions during protein targeting

The synthesis of secretory or integral membrane proteins can be directly coupled to their translocation across or insertion into membranes. In co-translational targeting, the translation machine, the ribosome, is transferred to the respective membrane by the signal recognition particle (SIRP) and its receptor (SIR) as soon as a signal sequence emerges. Protein synthesis can continue at the membrane, with the nascent peptide chain directly inserting into the ribosome-bound protein-conducting channel, the Sec61 complex. During the past two years, several structures have been solved by crystallography and cryo-electron microscopy that represent distinct functional states of the SIRP cycle. On this basis, the first structure-based models can be suggested that explain important aspects of protein targeting, such as the SRP-ribosome and SIRP-SR interactions.