Lecture 24 – Distribution of Proteins, Transfer to the GolgiApparatus and Lysosome
Distribution of Proteins
• Certain Proteins are synthesized on either ER or free ribosomes.
• All ribosomes are of the same construction.
How does this occur?
• Proteins constructed in the ER have either a lipid component or were secreted, while proteins
constructed by free ribosomes don't
• Blobel proposed that there was a “signal” on membrane bound proteins, and that the signal was
• Because the signal was on membrane bound proteins, he proposed this signal was composed of
hydrophobic amino acids, he said that initially the protein starts to be synthesized in the cytosol,
but the hydrophobic component, which was constructed first, halted synthesis until it was in a
favorable environment (the RER membrane).
• He showed that artificially constructed ER proteins were longer than expected and suggested
that this extra portion was the hydrophobic sequence, and that it would be normally cleaved by
an enzyme he named Signal Peptidase.
• Protein Synthesis does begin in the cytosol, but if a hydrophobic sequence (signal peptide) is
first produced, it binds to a Signal Recognition Particle (SRP, a group of proteins)
• SRP holds the nascent protein until it diffuses into the ER membrane, and interacts with a
docking protein, this docking protein interacts with a protein channel in the ER membrane
which allows the protein to pass through the membrane.
1 | S y d n e s Sheffield
• Now that SRP is released and ribosome is docked to ER membrane, translation continues and
the growing peptide is deposited in the ER lumen. The signal peptide is cleaved by a signal
peptidase, and there is a chaperone protein inside the ER that helps the new peptide to assume
its tertiary structure.
• Proteins that will be bound in the ER membrane have a second hydrophobic portion about
midway through the chain that, with the aid of a gating protein (hypothetical) causes the protein
to leave the channel and become bound in the membrane, where translation is completed.
• Two t