Targeting secretory proteins to the endoplasmic reticulum (ER, brown). As the signal sequence (green) appears from the ribosome (blue), it is linked by the signal recognition particle (SRP, grey) and driven to the ER membrane, where it binds to the SRP receptor (orange). The SRP is then released and the ribosome is now in direct contact with this membrane. The signal sequence can now insert into a membrane channel formed by a translation complex. From this moment, the growing polypeptide chain (red) is translocated across the membrane. During this process, the signal sequence is split by the signal peptidase (scissors), an enzyme linked to the luminal face of the ER membrane. The folding of the polypeptide chain into its active shape begins as soon as it penetrates the ER lumen, helped by chaperone proteins (not shown).