Secretory Pathways Flashcards
- List the major functions of the Golgi.
5. Name three well-studied vesicle coats and describe how these coats function in vesicular transport.
x
Describe the three mechanisms of protein transport.
(1) gated transport between the cytosol and the nucleus (nuclear transport)
(2) transmembrane transport across a membrane from the cytosol into an organelle through translocators (e.g., protein synthesis and mitochondrial import)
(3) vesicular transport in which membrane bound transport intermediates move proteins and lipids from one compartment to another
What are the 6 major functions of the ER? Which one is most specific to the RER? To the SER?
(1) synthesis of lipids (phospholipid, ceramide, and cholesterol; primarily in the smooth ER)
(2) control of cholesterol homeostasis (cholesterol sensor)
(3) storage of Ca+2 (rapid uptake and release)
(4) synthesis of proteins on membrane bound ribosomes (rough ER)
(5) co-translational folding of proteins and early posttranslational modifications
(6) quality control
What is the deal with co-translational translocation?
Same pool of ribosomes. When an mRNA codes for an ER signal sequence, this signal sequence “directs” the engaged ribosome to the ER membrane.
The signal sequence is recognized by a signal recognition particle (SRP), which is a complex of six proteins bound to one RNA molecule. The binding pocket of the SRP that recognizes the signal sequence is flexible, allowing it to bind a variety of signal sequences. The SRP is multifunctional – it binds the nascent polypeptide chain, the ribosome, and a receptor on the ER membrane. When the SRP binds the ribosome and nascent chain, it induces a pause in translation until it binds also binds the SRP receptor (located next to the translocon in the ER membrane). Once the ribosome is attached to the translocon, which is a protein channel allowing the polypeptide chain to enter the ER, the SRP detaches and can bind another ribosome and nascent polypeptide.
Are the ER and the Golgi fuse? How does movement between them occur?
A fundamental feature is that the ER and Golgi never fuse – the lumens of these organelles have different membrane proteins and different lumenal environments.
The movement of cargo and membrane proteins occurs by budding of vesicles, fusion of some of these vesicles into tubules, and fusion of these vesicles/tubules with the next compartment. This process requires the formation of coated vesicles and adapter proteins that recognize (1) the cargo, (2) the coat, and (3) the membrane proteins/lipids destined to move.
What are the four functions of the Golgi?
(1) Synthesis of complex sphingolipids from the ceramide backbone
(2) Additional post-translational modifications of proteins and lipids
(3) Proteolytic processing
(4) Sorting of proteins and lipids for post-Golgi compartments.
What are the three well-studied coat structures?
COPI, COPII, Clathrin
COPII: origin—->destination
COPII: ER—->Golgi (forward direction)
COPI: origin—->destination
COPI: Golgi—->ER (reverse direction)
Clathrin: origin—->destination
Clathrin: Golgi—->plasma membrane
How do coats work?
Coats are assembled from soluble cytoplasmic proteins and lipids at sites of vesicle formation. They disassemble almost immediately after budding, as they would interfere with targeting of the vesicle. They are physically responsible for deforming the membrane into a bud.
How is a protein with transmembrane domains (TMD) synthesized?
For membrane proteins with a single TMD and with the amino terminal in the ER lumen (designated a Type I membrane protein), the mRNA contains a sequence recognized by the translocon as a “stop transfer” signal. This stop transfer domain is released by the translocon and the remainder of the protein, the C-terminal end, is synthesized on the cytosolic face (Figure 4).
A single TMD transmembrane protein can also have the opposite orientation (designated a Type II membrane protein).
What is N-linked glycosylation? Where does it occur (on the protein and in the cell)?
N = AsparagiNe = Asp. Glycosylation occurs at
Asp-X-Ser/Thr (X = any aa but proline)
Occurs in the ER (may be processed further in Golgi).
[N-linked glycosylation has two important purposes. It helps to keep proteins from aggregating when hydrophobic domains are exposed. In addition, glucose residues act as tags to monitor unfolded proteins]
In simple terms, what is a translocon?
A pore in the ER membrane, usually shut, unless an mRNA is being transcribed. Ribosomes sit on pm and squirt protein into the lumen of the ER.
What enzyme cleaves the signal peptide from the nascent polypeptide?
Signal peptidase.
