Proteins enter the ER while they're being made
- The idea
- Predict
- See it
- Why
- Check
The idea
“A protein gets made first. Then the cell sends it into the ER if it belongs there.”
It's easy to picture a cell building a protein first and choosing where to ship it after. For the ER, the choice is made while the chain grows.
The short version
The mistake: A protein gets made first. Then the cell sends it into the ER if it belongs there.
What's actually true: A protein enters the ER while it is being translated, led there by a signal sequence at its start.
- Free and ER-bound ribosomes are the same. A ribosome goes to the rough ER only when its chain starts with an ER signal sequence.
- The signal is made first. It brings the ribosome to the ER, and the chain is threaded into the ER as it grows.
- The Golgi later sorts ER proteins to lysosomes, the membrane or secretion. Proteins finished in the cytosol stay there or go to places like mitochondria.
On the exam: A protein in the ER went in during translation, led by its signal sequence.