What are the functions of the endoplasmic reticulum?
- Protein synthesis/import
- Protein modification
- Protein quality control
- Lipid synth
- Synth of steroid hormones
- Detox of lipid soluble drugs
- Calcium storage
How is protein imported into the ER?
Co-translationally! Only place this happens. All protein synth starts in the cytosol.
As the newly synth polypeptide emerges from ribosome, an ER targeting signal sequence w/in the polypeptide direct the ribosome to the ER.
Single mRNA molecule can have many ribosomes, forming a polyribosome.
What components guide the ER signal sequence to the ER membrane?
A signal-recognition particle (SRP) in the cytosol binds to the ER signal sequence and an SRP receptor or docking protein embedded in the ER membrane.
1. SRP binds to ER signal sequence
2. Ribosome-SRP complex binds to ER membrane w/ SRP receptor and ribosome binding to the translocation channel
3. binding of SRP to receptor causes it to release the signal sequence thereby allowing protein synth to resume with the polypeptide being threaded through the translocation channel.
4. SRP is recycled into cytosol
Describe the import of soluble proteins into the ER lumen
ER signal sequences are almost always the N-terminus - once ribosome/SRP complex has bound to ER membrane, N-terminal signal sequence opens the translocation channel and remains bounds to it. As protein synth continues, the rest of the protein is threaded through the channel. During translocation, sig sequence is cleaved and new protein is released - signal peptide is degraded.
Describe the import of membrane proteins into the ER membrane
Signal sequence either N-terminus or internally, N-terminus typically cleaved off by signal peptidase, while internal signals are not bc they serve as transmembrane domains.
Membrane spanning domains released laterally to be embedded in ER membrane
Integral membrane proteins imported into ER will always be inserted in only one orientation (as dictated by their sequence)
Describe some protein modifications that happen in the ER
- Signal sequence cleavage (co-translational)
- N-linked glycosylation (co-translational) - covalent addition of sugars to asparagine residues by oligosaccharyl transferase
Collegen molecules are hydroxylated on prolines and lysines to help H-bonding
Protein folding (chaperone proteins) and disulfide bonds (protein disulfide isomerase)
Assembly of multisubunit proteins
Retention of ER resident proteins - have ER retention signal that returns them to the ER after going to the Golgi
Describe how protein quality control occurs in the ER
Chaperone proteins- hold the proteins in th ER until proper folding and assembly occur.
Proteins that remain misfolded and unassembled will be degraded by proteosomes.
Describe the unfolded protein response (UPR)
If quality control blows and misfolded proteins accumulate - the UPR is activated. Signals ER to expand and increase gene expression of chaperones in order to handle larger # of unfolded proteins. If the load is still too large, they instruct cell to undergo apoptosis.
Describe membrane lipid synth in the ER
Enzymes mostly in the smooth ER synth new phospholipids from free fatty acids and insert them into the membrane
Scramblases move random phospholipids from one half of the bilayer to the other, make the two halves even
Lipids are then delivered from the ER to the Golgi, lysosomes, endosomes, and plasma membrane via vesicles during vesicular transport
Flippases in the golgi and plasma membrane move specific from one side to the other to create an asymmetric distribution to contribute to the curvature
What are the fxns of the smooth ER?
Synthesis of steroid hormones - ex. leydig cells in the testes
Detox of lipid soluble drugs - lots of smooth ER in liver cells
The ER serves as a storage unit for what ion?
Calcium! In muscle cells, the specialized smooth ER (sarcoplasmic reticulum) releases Ca2+ for muscle contraction
What are the main pathways of vesicular transport?
Secretory pathway (outward)
Endocytic pathway (inward)
Vesicle budding is driven by the assembly of what?
A protein coat! Best categorized example is clathrin coat.
Forms a basket-like network consisting of hexagons and pentagons, introduced curvature to the membrane. Assembles on the cytosolic end of golgi or plasma membrane.
Adaptins - help bind clathrin coat to the vesicle membrane and select cargo molecules
Dynamin - small monomeric GTP binding protein that pinches off the vesicle from the membrane.
Shortly after, the clathrin coat falls off
Describe SNAREs and Rabs roles in vesicular transport.
Rab - serve as molecular markers identifying each membrane type, recognized by tethering proteins on target membranes
Tethering proteins - capture vesicles via interaction w/ rab
Complementary SNAREs - (v-SNAREs on vesicles and t-SNAREs on target membranes) interact to dock vesicles, wrap around each other and pull membranes until they fuse. v-SNAREs are then sent back to donor membrane to be used again.