1-27 ER and Vesicular Transport Flashcards
What are the structure and functions of the rough and smooth ER?
The rough ER has ribosomes bound on the cytosolic side of the membrane; the smooth ER lacks ribosomes. The ER has several functions:
- Protein synthesis/import (rough ER)
- Protein modification (rough ER)
- Protein quality control (rough ER)
- Lipid synthesis
- Synthesis of steroid hormones (smooth ER)
- Detoxification of lipid soluble drugs (smooth ER)
- Calcium storage (smooth ER; sarcoplasmic reticulum)
What does protein synthesis in the ER produce?
- Proteins destined for lumens/membranes of the ER, Golgi, lysosomes, and endosomes
- Membrane proteins destined for the plasma membrane
- Proteins destined for secretion to the cell exterior
What is the mechanism by which proteins are imported into the ER co-translationally?
When protein synthesis begins in the cytosol, ribosomal subunits bind to the mRNA. As the new polypeptide emerges from ribosome, an ER targeting signal sequence directs it to the ER membrane as protein synthesis simultaneously continues.
The ER targeting signal sequence is guided by the cytosolic signal-recognition particle (SRP), which binds to the polypeptide as it emerges from the ribosome and slows protein synthesis. When the ribosome-SRP complex reaches the ER membrane, the SRP binds to the SRP receptor, releasing the signal sequence, and the ribosome binds to the translocation channel. Protein synthesis resumes, with the polypeptide being threaded through the translocation channel and the SRP recycled back to the cytosol.
How does protein import into the lumen differ from protein import into the ER membrane?
Protein import into the lumen: The ER signal sequence is typically located at the N terminus, where it opens and binds to the translocation channel. The rest of the polypeptide is threaded through the channel in a loop. Once this is done, luminal signal peptidase cleaves the signal sequence, releasing the polypeptide into the lumen and allowing itself to be degraded.
Protein import into the ER membrane: The ER signal sequence can be located at the N terminus and/or internally (this is the case for multiple transmembrane domains). Transmembrane domains are released laterally.
What are some protein modifications that take place in the ER?
- Signal sequence cleavage (co-translational)
- N-linked glycosylation (co-translational)
- Hydroxylation of collagen
- Protein folding and disulfide bond formation
- Assembly of multisubunit proteins
- Retention of ER resident proteins
How is the ER involved in protein quality control and the unfolded protein response?
Chaperone proteins will detain wrongly folded/assembled proteins in the ER until they can be corrected. Proteasomes degrade any proteins that cannot be corrected.
The unfolded protein response (UPR) kicks in when the chaperone system becomes overloaded, signaling the ER to increase its size and # of chaperones. Should this measure fail, the UPR will signal the cell to initiate apoptosis.
How is the ER involved in the synthesis of steroid hormones?
Steroid hormone synthesizing cells (e.g., testosterone secreting Leydig cells in the testes) contain large areas of smooth ER to carry out steroid hormone synthesis.
What are the two pathways of vesicular transport?
- Secretory pathway (outward): proteins synthesized in the ER are delivered to the cell surface or lysosomes via the Golgi apparatus
- Endolytic pathway (inward): extracellular molecules are taken up at the plasma membrane and delivered to lysosomes, via endosomes, for degradation
How would you describe the structure, assembly, and disassembly of clathrin coated vesicles?
Clathrin coated vesicles bud from the Golgi apparatus in the outward secretory pathway and from the plasma membrane in the inward endocytic pathway. Clathrin forms a basket-like network of hexagons and pentagons that shapes the membrane into a transportable bud.
Adaptins, another coat protein, bind the clathrin to the vesicle membrane and can interact with various transmembrane cargo receptors. Dynamins ring the neck of each bud, pinching the vesicles off of the membrane and allowing the clathrin coat to disassemble.
What are the general roles of SNAREs, Rabs, and tethering protein in vesicular transport?
- Rabs are a very large subfamily of monomeric GTPases that serve as the molecular markers identifying each membrane type
- Tethering proteins recognize and capture vesicles via interaction with Rabs
- Complementary SNAREs (v-SNAREs, vesicles; t-SNAREs, target membranes) interact to dock vesicles
How does the fusion step in vesicular transport occur?
Through close interactions between the lipid bilayers, as SNAREs wrap around each other and pull the membranes close.
NOTES: This process is energetically unfavorable because water must be displaced. Additionally, docking and fusion often do not occur simultaneously.
What is the structure of the Golgi apparatus?
- Flattened, membrane-bounded cisternae stacked together (3-20/Golgi stack)
- Two distinct faces in each Golgi stack: cis face (entry face), adjacent to the ER; trans face (exit face), pointing toward the plasma membrane
- Network of interconnected tubular and cisternal structures form the cis and trans Golgi
- Networks at either end of the stack.
- Usu. located near the cell nucleus
- # Golgi stacks per cell varies from one to many depending on cell type
What are the functions of the Golgi apparatus?
- Sorting and dispatching station for proteins and lipids made in ER
- Modification of N-linked oligosaccharide chains on glycoproteins made in ER
- Synthesis of O-linked oligosaccharides on proteins and lipids made in ER
- Synthesis of glycosaminoglycan chains on core proteins of proteoglycans
How does the Golgi apparatus participate in protein sorting?
Transport vesicles containing proteins and lipids enter the cis Golgi network from the ER and exit from the trans Golgi network to the cell surface or other organelle. Protein sorting occurs on both sides of the Golgi appartus:
- cis Golgi network: sorts proteins that need to be returned to the ER (ER resident proteins) from those that will continue through the Golgi
- trans Golgi network: sorts proteins destined for lysosomes or regulated secretion from those that will continue to the plasma membrane via the default pathway (constitutive exocytosis)
