Two protein components help guide ER signal sequences to the ER membrane: (1) a signal recognition particle (SRP), present in the cytosol, binds to both the ribosome and the ER signal sequence as it emerges from the ribosome; and (2) an SRP receptor, embedded in the ER membrane, recognizes the SRP.The cleaved signalsequence is then released from the protein translocator into the lipid bilayer and rapidly degraded.Once the C-terminus of a soluble protein has passed through the translocator, the protein is released into the ER lumen (Figure 15-15).In addition to directing proteins to the ER, the signal sequence--which for soluble proteins is almost always at the N-terminus, the end synthesized first--functions to open the protein translocator.