Lecture 12- Organelles 2 Flashcards
Functions of the ER
Protein synthesis Protein modification Protein quality control Lipid synthesis Synthesis of steroid hormones Detoxification of lipid soluble drugs Ca 2+ storage
Protein synthesis in the ER is responsible for producing:
Proteins destined for the lumens or membranes of ER, Golgi, lysosomes, or endosomes
Proteins destined for the plasma membrane
Proteins destined for secretion to the cell exterior
Proteins DO NOT return to the cytosol–go to other locations in vesicles
Import of proteins into the ER
COTRANSLATIONALLY (ie while the protein is being synthesized)
This is the ONLY place where co-translational transport occurs
All protein synthesis begins in the cytosol–an ER targeting signal sequence will direct the ribosome to the ER membrane where synthesis will continue
ER signal sequences are directed to the ER membrane by what? And how?
SRP and SRP receptor.
Signal recognition particle in the cytosol binds to the ER signal sequence and an SRP receptor or docking protein embedded in the ER membrane
Steps:
- SRP binds to ER signal sequence thereby slowing protein synthesis
- Ribosome-SRP complex binds to the ER membrane with the SRP binding to the SRP receptor and the ribosome binding to the translocation channel
- Binding of SRP to its receptor causes the SRP to release the signal sequence thereby allowing protein synthesis to resume with the polypeptide being threaded through the translocation channel.
- SRP recycled back to the cytosol
Import of soluble proteins into the ER lumen
- Signals almost always at the N-terminus
- One the ribosome-SRP complex has bound to the ER membrane the N terminal signal sequence opens the translocation channel and remains bound to it.
- Rest of the protein threaded through the channel as a large loop
- Signal sequence is cleaved off by signal peptidase on the luminal side of the ER membrane, releasing the newly synthesized protein into the ER lumen
Import of membrane proteins into the ER membrane
- ER signal sequence may be located at the N-terminal or internally
- Some parts of the polypeptide chain are transported across the membrane
- Membrane spanning domains are released laterally from the translocation channel to become embedded in the ER membrane
- Will only be inserted in one particular topology
Signal peptidase
Cleave ER signal sequences (the N-terminal ones)
Located in the ER lumen
N-linked Glycosylation
- Most proteins imported into the ER lumen or membrane are converted to glycoproteins by the covalent additions of sugars
- A preformed oligosaccharide of 14 sugars is covalently attached to asparagine residues during translocation (cotranslationally)
- Oligosaccharyl transferase in the ER lumen transfers the oligosaccharide block from dolichol to the polypeptide as it emerges from the ribosome
- Attached to NH2 residue of Asn
Hydroxylation of collagen
Collagen molecules are hydroxylated on prolines and lysines to allow interchain hydrogen bonds to help stabilize triple stranded helix of collagen molecules
Protein folding and disulfide bond formation
Chaperone proteins help to ensure correct folding
Disulfide bonds between cysteine side chains are formed in the ER by protein disulfide isomerase (ER lumen)
Assembly of multisubunit proteins
Assembled with partner polypeptides to form the mature protein within the ER
Retention of ER resident proteins
ER retention signals indicate which proteins will reside in the ER
KDEL at C terminus
Unfolded protein response
Triggered when the quality control signal becomes overwhelmed and misfolded proteins accumulate in the ER.
- Signals ER to expand in size and increase its chaperones
- If load is still too large, will instruct apoptosis
Membrane lipid synthesis in the ER
Enzymes in the cytosolic half of the ER bilayer synthesize new phospholipids from free fatty acids and insert them exclusively into the cytosolic half of the bilayer
lipids delivered from ER to golgi, lysosomes, endosomes, and plasma membrane via vesicles
Transfer to peroxisomes and mitochondria requires cytosolic lipid carrier proteins
Scramblases
Move random phospholipids from one half of the bilayer to the other to redistribute evenly
Flippases
Located in the Golgi and plasma membrane
Move specific phospholipids from one side of the bilayer to the other to create an asymmetric distribution of phospholipids that is maintained during vesicular transport
Functions of smooth ER
Synthesis of steroid hormones
Detoxification of lipid soluble drugs
Two major pathways along which transport vesicles travel
Secretory pathway (outward)–proteins synthesized in the ER are delivered to the cell surface or lysosomes via the Golgi
Endocytic pathway (inward)–extracellular molecules are taken up at the plasma membrane and delivered to lysosomes, via endosomes, for degradation
Protein coat of vesicles
- Drives vesicle budding
- Discarded prior to fusion with target membrane
Clathrin coated vesicles
- Bud from golgi apparatus in the outward secretory pathway and from the plasma membrane in the inward endocytic pathway
- Assembles on cytosolic trans surface of golgi
- Basketlike network of hexagons and pentagons
- Introduce curvature into the membrane
Adaptins
- Second major coat protein in clathrin-coated vesicles
- Bind the clathrin coat to the vesicle membrane
- Help select cargo molecules for transport via its interaction with both clathrin and various transmembrane cargo receptors
- Vary according to the nature of the cargo they are binding
Dynamin
- Small monomeric GTP binding protein
- Assembles as a ring around the neck of each bud
- Hydrolysis of bound GTP causes the ring to constrict, pinching off the vesicle from the membrane
- After pinching the clathrin coat is quickly removed
Rabs
Large subfamily of monomeric GTPases that serve as the molecular markers identifying each membrane type
Recognized by tethering proteins on target membranes
Tethering proteins
Capture vesicles via their interaction with Rabs
