Synthesis on the Rough Endoplasmic Reticulum

Synthesis of proteins destined for import into the endoplasmic reticulum starts on free polyribosomes. When the growing polypeptide chain is about 20 amino acids long, the endoplasmic reticulum signal sequence is recognized by a signal recognition particle that is made up of a small RNA molecule and several proteins (Fig. 10.7). The signal recognition particle brings the ribosome to the endoplasmic reticulum membrane where it interacts with a specific receptor—the signal recognition particle receptor (or the docking protein). This interaction directs the polypeptide chain to a protein translocator. Once this has occurred the signal recognition particle and its receptor are no longer required and are released. Protein synthesis now continues; and, as the polypeptide continues to grow, it threads its way through the membrane via the protein translocator, which acts as a channel allowing hydrophilic stretches of polypeptide chain to cross. Once the polypeptide chain has entered the lumen of the endoplasmic reticulum, the signal sequences may be cleaved off by an enzyme called signal peptidase. Some proteins do not undergo this step but instead retain their signal sequences.

The platelet-derived growth factor receptor is an example of an integral membrane protein. It contains a stretch of 22 hydrophobic amino acids that spans the plasma membrane (Fig. 9.15 on page 198). The first part of the polypeptide to be synthesized is an endoplasmic reticulum signal sequence, so the polypeptide begins to be threaded into the lumen of the endoplasmic reticulum. This section will become the extracellular domain of the receptor.

mRNA

aribosome

ER signal — sequence signal recognition particle signal recognition particle receptor lumen ) ) cytosol signal recognition particle and its receptor dissociate

free polypeptide

ER signal — sequence signal recognition particle free polypeptide

signal recognition particle receptor lumen ) ) cytosol signal recognition particle and its receptor dissociate growing polypeptide chain

ER signal sequence oligosaccharide

Figure 10.7. Transport of a growing protein across the membrane of the endoplasmic reticulum.

growing polypeptide chain

ER signal sequence oligosaccharide

Figure 10.7. Transport of a growing protein across the membrane of the endoplasmic reticulum.

When the stretch of hydrophobic residues is synthesized, it is threaded into the translocator in the normal way but cannot leave at the other end because the amino acid residues do not associate with water. As synthesis continues, therefore, the newest length of polypeptide bulges into the cytosol. Once synthesis stops, this section is left as the cytosolic domain.

If a protein contains more than one hydrophobic stretch, then synthesis of the second stretch reinitiates translocation across the membrane, so that the protein ends up crossing the membrane more than once.

We can identify key amino acid sequences that play a role in protein targeting by making use of protein engineering. If we join the stretch of nucleotides that codes for the endoplasmic reticulum signal sequence to the cDNA that codes for a cytosolic protein, we produce a chimeric DNA molecule (page 150). When this cDNA is transfected (page 150) into cells, it will be transcribed into mRNA and then translated into a protein that is unchanged except that it now has, at its N terminus, the endoplasmic reticulum import sequence. The protein is targeted to the endoplasmic reticulum. However, because the protein has no other sorting signal, it is not recognized by any other receptor protein. It gets secreted from the cell by the constitutive route. This result shows that constitutive exocytosis is the default route for proteins synthesized on the rough endoplasmic reticulum.

Was this article helpful?

0 0

Post a comment