Protein Targeting and the Signal Sequence
A short stretch of the growing chain decides where the protein ends up: the endoplasmic reticulum, a mitochondrion, the nucleus or the cytosol. Removing or moving that stretch sends the protein to the wrong compartment.
What a learner can do afterwards
- Predicts the destination of a protein from the presence and type of its targeting sequence.
- Explains why the cell needs sorting at all, given every protein starts on a cytosolic ribosome.
- Says what happens to a secreted protein whose signal sequence is deleted.
1 · Read
Every protein is born in the same place, on a ribosome floating in the cytosol, yet jobs wait all over the cell. Sorting is needed because home and workplace differ. The fix is a signal sequence: a short stretch of the growing chain that flags the protein for its station. A scout particle grabs the tag, parks the whole ribosome on the ER surface, and the growing chain threads inside.
The rough ER, studded with docked ribosomes, builds proteins bound for export or for membranes, while the smooth ER makes lipids. New cargo sails in vesicles to the Golgi, which tags, trims, and sorts each shipment onward. Ribosomes working free in the cytosol supply the home market instead: enzymes for metabolism plus proteins for the nucleus and mitochondria.
Presence and type of tag predict destination, so removing or swapping the tag reroutes the protein. Delete the signal from a secreted protein and none of the docking happens: the chain finishes in the cytosol, stranded far from the exit route, and is usually chewed up as junk. Mistargeted proteins malfunction or clump, which is why correct targeting is vital.
For a secreted protein the pause comes early: the signal emerges first, docking happens, and the rest of translation finishes on the ER wall with the chain feeding through. Cytosolic proteins never pause at all. They translate start to finish on free ribosomes and fold right where they stand.
A short signal tag docks the ribosome on the ER, and without it cargo stays home.
2 · Watch
Take it off screen
Where it sits
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.