Rough endoplasmic reticulum: proteins for secretion, membranes and lysosomes
The rough endoplasmic reticulum (the membrane network studded with ribosomes) is where ribosomes make proteins for secretion, the cell membrane and lysosomes (the cell's digestive compartments).
Remember
Ribosomes bound to the rough endoplasmic reticulum make proteins for secretion, lysosomes and membranes, and a signal sequence (a short amino acid tag, like a postal address) sends each of those ribosomes there.
The rough endoplasmic reticulum (RER) is a network of flattened sacs and tubes that runs through the cell. It looks rough because ribosomes (the molecular machines that build proteins) are attached to its outer surface.
A protein bound for secretion, the cell membrane or a lysosome starts with a signal sequence, a short amino acid tag at its start that works like a postal address. As the tag emerges from the ribosome, the signal recognition particle (SRP, a complex of protein and RNA) binds it and takes the whole ribosome to the RER.
The rest of the chain is then fed into the lumen (the space inside the sacs) or into the membrane while it is still being made. This is called co-translational targeting: the protein enters the RER while it is still being built. Sugar chains are added to many of the new proteins inside the lumen, and a vesicle (a small membrane bubble) then carries the protein to the Golgi apparatus, which modifies, sorts and packages it.
Free and RER-bound ribosomes have the same large and small subunits, and the location of each is decided by the protein it is building. Free ribosomes make proteins that stay in the cytosol (the fluid part of the cell) or are imported afterwards into organelles such as mitochondria or the nucleus. The smooth endoplasmic reticulum has no ribosomes and makes lipids and steroids instead. Cells that export a lot of protein have abundant RER.
Key facts
- Site
- ribosomes are attached to the outer surface of the membranes, which makes the endoplasmic reticulum rough.
- Products
- proteins for secretion, the cell membrane and lysosomes.
- Signal sequence
- a short tag at the start of the new protein that sends its ribosome to the RER.
- Co-translational
- the protein enters the RER while it is still being built.
- Next step
- vesicles carry the new proteins to the Golgi apparatus for further modification, sorting and packaging.
- Smooth versus rough
- the smooth type has no ribosomes and makes lipids and steroids, whereas the rough type builds proteins.
Exam traps
- Lipid and steroid hormone synthesis is not a job of the rough ER, because that belongs to the smooth endoplasmic reticulum; the rough type is defined by its ribosomes and so by protein synthesis.
- Modification and sorting of proteins for final delivery is one of the main jobs of the Golgi apparatus and not of the rough ER, because the Golgi apparatus receives the vesicles from the rough ER and then modifies, sorts and packages the proteins, although some sugar chains are added in the rough ER.
- A ribosome does not bind permanently to the rough ER, because when translation ends the subunits come apart and are recruited into a new round of translation.
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