Table of Contents
The proteins of different organelles are synthesized either in the cytosol on the free ribosomes or on the ribosomes linked to the surface of endoplasmic reticulum (ER). The signals encoded in the amino acid sequence of proteins target them to the different organelles. These so-called signal sequences are recognized and bound by specific proteins, thus making possible the protein targeting to the required places.
Protein targeting to the endoplasmic reticulum
The protein translation starts in the cytosol. The proteins targeted to ER have a signal sequence at their N-terminal part and this signal sequence is recognized by a signal recognition particle (SRP) targeting the ribosome to the SPR receptor in the ER membrane (Figure III-1). As soon as the signal sequence enters the ER lumen the SRP is released and it is able to participate in a new cycle. The synthesized protein enters the ER lumen, where the signal peptidase enzyme cleaves off the signal sequence while the ribosome will be linked with special linker proteins to the ER membrane forming the rough endoplasmic reticulum (RER) (Figure III-2). When the translation is finished, the ribosome complex is dissociated (Figure III-3).