An excerpt from Figure 17-16 from Lodish, et al. (2000). The figure shows the role of the signal recognition particle (SRP) in binding to the nascent, N-terminal signal sequence of a protein destined for insertion into the plasma membrane or for secretion in a eukaryotic cell. After binding the signal sequence, the SRP docks the actively synthesizing ribosome on the Translocon by first binding to a nearby SRP receptor embedded in the endoplasmic reticular membrane. In the course of targeting the ribosome to the Translocon, the SRP and the SRP receptor both hydrolyze bound GTP to GDP. The SRP exchanges GDP for GTP when it engages a new signal peptide.