Vesicular Transport Flashcards
What designates a Resident RER protein? Which way is retrograde movement between the RER and Golgi and what motor protein moves it?
- Some proteins are resident primarily in the ER. These frequently carry a KDEL, known as the ER retention signal binding sequence that ensures that they will be retrieved from the Golgi if they migrate out of the ER. Retrieval from the entire Golgi stack is possible.
- A membrane receptor in the cis Golgi network captures the ER proteins and carries them in retrograde vesicles (Golgi -> ER) back to the ER. Dyneins and microtubules are responsible for the transport.
Lysosomal resident proteins
All lysosomal enzymes and lysosomal proton pumps are marked with mannose-6-P; the mannose is added in the ER and the phosphate is added in the cis Golgi network.
- In the TGN, membrane receptors for M6P (M6P-receptor) serve to sort out the lysosomal proteins. The M6P-bound receptor complex moves to the endosome, where the lysosomal proteins are released. From the late endosome, the proteins move to the lysosome.
- the resident lyosomal proteins dissaciate from the M6P-receptor from the acidic environment in the endosomes and then get recycled back to the golgi
endosome v. lysosome
endosome is not as acidic and has less hydrolases than a lysosome (ie it is a prepubescent lysosome)
ER/Golgi Pathway
RER -> Golgi -> Vesicle -> endosome/lysosome or secreted
The major roles of the GOLGI
(i) to modify the proteins via post-translational modifications (such as glycosylation, proteolysis, etc) and then,
(ii) to sort the proteins at TGN for transporting specific proteins destined for lysosomes,
for secretion or for plasma membrane. The latter two are mediated through constitutive
and/or regulated exocytosis.
KDEL
ER retention signal binding sequence that ensures that they will be retrieved from the Golgi if they migrate out of the ER. Retrieval from the entire Golgi stack is possible.
- A membrane receptor in the cis Golgi network captures the ER proteins and carries them in retrograde vesicles (Golgi -> ER) back to the ER. Dyneins and microtubules are responsible for the transport.
H+ - ATPase
The lumen of lysosomes are maintained at an acidic pH by a H+-ATPase in the lysosomal membrane which pumps H+ into the lysosome. This acidic environment activates the lysosomal enzyme (which are active at pH~5). Also, this is how the hydrolytic enzymes of the lysosome dissociate from the M6P receptor.
Mannose-6-Phosphate
This is what is used to get lyososomal hydrolases and the proton pump to the lysosome. Mannose is attached to the hydrolases in the RER, then the phosphate is attached in the cis golgi network. In the TGN, membrane receptors for M6P (M6P-receptor) serve to sort out the lysosomal proteins. The M6P-bound receptor complex moves to the endosome, where the lysosomal proteins are released. From the late endosome, the proteins move to the lysosome.
I-Cell disease
(inclusion body) is caused by a deficiency of the enzyme (FYI: Phospho-Nacetylglucosamine transferase) that modifies the M-6-P moiety to the lysosomal proteins. As a result, lysosomal hydrolases are not marked with the target signal mannose-6-P. Instead, they are secreted to the outside of the cell. This enzyme deficiency manifests itself by the appearance of huge inclusion bodies in cells and exceptionally high levels of hydrolases in the circulation. The secretion of these hydrolases accounts for the high levels found in the body. I-cell disease primarily affects connective tissue, and patients usually die by the age of 5.
receptor mediated endocytosis
Receptor-mediated endocytosis represents a major pathway for the movement of specific molecules into cells. It is a selective uptake of extracellular macromolecules that bind to cell surface receptors.
1) The process is triggered by the binding of a ligand (a specific macromolecule) to its specific receptor.
2) Subsequently, the plasma membrane is distorted toward the interior of the cell; then a vesicle is budded off toward the interior of the cell then reaching the early endosome (pH 6.0).
3) The low pH triggers a change in receptor conformation, so that the receptor releases the bound ligand.
4) The ligand is transported to and eventually degraded in the lysosome, releasing the monomers of the original macromolecules.
5) In contrast, the receptors are typically returned to the plasma membrane via transport vesicles that bud from the tubular region of endosome.
lysosomal storage diseases
comprise a group of more than 50 different genetic diseases. The mostly involve the dysfunction of lysosomal hydrolases, which result in impaired substrate degradation (i.e. accumulation of undegraded substrates in the lysosomes). However, proteins involved in vesicular traffic and the biogenesis of lysosomes have also been shown to cause storage disorder phenotype (such as I-cell)
what are the two kinds of exocytosis?
costitutive (always on) and regulated.
constitutive exocytosis
Constitutive exocytosis: many plasma proteins (e.g. albumin and clotting factors) as well as plasma membrane proteins (e.g. many carriers and channels including Na+-K+- ATPase) and membrane lipids (e.g. glycerophospholipids and sphingolipids) are continually secreted from the cell by this pathway. It is also known as the default pathway, which operates in all cells.
regulated exocytosis
Regulated exocytosis: special cells have a regulated secretory pathway (hormones,
neurotransmitters, and digestive enzymes).
-Regulated exocytosis is ALWAYS mediated through calcium signaling. (GPCR Gaq subunit?)
-Selected proteins in the trans Golgi network are diverted into secretory vesicles, where the proteins are concentrated and stored until an extracellular signal stimulates their secretion.
-Secretory vesicles are stored near the plasma membrane until signaled to release
their contents.
