Lecture 2: The Endoplasmic Reticulum Flashcards
Learning objectives
- What is the structure and the functions of the endoplasmic reticulum (ER), rough and smooth regions (RER and SER)?
- What happens during transmembrane protein transport (co- and post-translational)?
- What are the posttranslational modifications that occur to proteins in the ER?
What are the functions of the ER?
- produces cellular lipids, most membrane proteins, all secreted proteins, almost all proteins destined for the lumen of the ER, Golgi and lysosomes.
- proteins are folded and modified here (quality control) - many diseases are caused if proteins are folded wrong, e.g. cystic fibrosis.
- adding a GPI anchor
- synthesising cell membrane lipids
What is the structure of the ER?
The ER, which is present in all eukaryotic cells, is continuous with the other nuclear membrane. It is a large organelle, so has to be supported by microtubules (cytoskeleton). Without these, the ER would collapse. The ER is dynamic, so if a cell suddenly needs more protein made, the ER expands.
What is special about the rough ER?
It is covered in ribosomes, so allows co-translational protein import.
What are protein signal sequences?
Amino acid sequences within a protein which direct proteins to the rough ER. They are then cleaved once they have reached to the RER. These signal sequences are often at the N terminus, but can be internal. They can form a signal patch, which needs the protein to fold up in order for the patch to be activated. The exact sequence is not that important: +ve charge and 6-12 hydrophobic aa residues.
What is the mechanism for co-translational protein transport?
1) Signal recognition particles (SRP) move between the ER membrane and the cytosol, binding to any signal sequences (stays bound only to correct one).The ER signal sequence is called a start transfer sequence. When the SRP binds the protein and ribosome in a cleft, translation is paused.
2) Then the SRP docks with its receptor. This hydrolyses 2 GTPs. The SRP and the receptor come apart and the polypeptide and ribosome dock on a protein translocator on the ER.
3) The protein translocator moves the growing polypeptide chain across the ER membrane.
Describe the structure of the protein translocator.
The protein translocator (aka. translocon) is a water-filled pore. It is mostly made up of the protein Sec61. It is kept closed when it is not needed (it is gated). The polypeptide is fed through unfolded through the core’s hydrophobic lining.
The core of the translocon cans also open along its side to allow the cleaved signal out (this is then degraded) and the integrate membrane proteins into the membrane). The part of the membrane containing the new protein can be pinched off and fused to the membrane of the destination organelle.
What is the mechanism of post-translational protein import?
Post-translational protein import into the ER very rarely happens in humans, but does occur in yeast. It doesn’t use a SRP or a SRP-receptor like co-translational protein import. Bip is a chaperone protein, which helps with folding and transport around a cell. Bip.ATP binds to the Sec61 pore. The ATP is hydrolysed and then Bip.ADP can bind to the polypeptide and stop it sliding out whilst it ‘ratchets’ it into the ER through the middle of the translocon.
Which signal will keep proteins in the ER?
The KDEL sequence (Lysine, Aspartic acid, Glutamic acid, Leucine). Proteins involved in helping other proteins to fold are often retained in the ER, such as protein disulfide isomerase, which forms disulphide bonds.
What happens to soluble proteins once they have been cotranslationally imported into the ER?
They are secreted from the cell or transported via vesicles to other organelles.
What post-translational modifications occur in the ER?
- Disulphide bridges are formed in the RER (protein disulfide isomerase)
- fold and assemble multi-subunit (multimeric) proteins
- may cleave at some sites
- Glycosylation (covalently add and process carbohydrates)
Describe glycosylation.
Glycosylation is the covalent addition of sugar to protein to make a glycoprotein. N-linked glycosylation occurs in the ER (the sugar is added to the N-terminus of the protein). The 14 residue oligosaccharide is built up on the dolichol lipid molecule (embedded in the ER membrane), with the first few residues being added on the cytosolic side of the ER membrane and the rest on the ER side. The oligosaccharide in then transferred onto the NH2 group of Asn (asparagine) in the growing polypeptide.
Monosaccharides are added/removed in the ER and Golgi (always keep 5 of the 14).
Why are some proteins glycosylated?
- Aids folding (prevents wrong loops happening by being a physical block)
- Quality control (which sugars are exposed/hidden tells cell if protein is folding properly)
- Stabilise secreted proteins
- protect proteins
- cell to cell adhesion
- immune response
- signalling
What happens to misfolded proteins in the ER?
They are translocated out of the ER into the cytosol, where they are deglycosylated and tagged by ubiquitinate (tag shows that protein should be degraded) and the degraded by proteosome.
If there is a build up of misfolded proteins a kinase in the ER membrane is activated. There is up-regulation of transcription of chaperones and proteins involved in translocation out via an ER-associated-degradation-complex (ERAD). Then the misfolded proteins are degraded.
What is the SER?
The smooth ER. This region of the ER does not have any associated ribosomes as it is specialised to deal with lipids, not proteins.
Most cells only have a little, but some cells, such as lipid metabolisers (e.g. liver), have lots.
