Cell Structure Lecture 2 Sep 4 Flashcards
How are lipids and proteins meant for the plasma membrane and organelles other than the ER delivered?
What is the overall direction of membrane flow?
Membrane from the ER flows to the golgi.
Membrane from the golgi can flow to multiple places:
- back to the ER
- Out to the plasma membrane in vesicles
- To the endosomes to form lysosomes
- To other areas of the cell in secretory vesicles
What overal mechanism does membrane flow happen through?
vescicles budding off and fusing with new membranes.
This requires fusion of lipid bilayers, and thus maintains the topological orientation of membrane proteins and lipids.
Familiar hypercholesterolemia (FH) is an example of a specific deficit in an endocytic pathway.
Describe it.
Patients have a defect in the cell’s ability to endocytose cholesterol as LDL particles, so the cholesterol builds up in the blood and you get the deposition of atherosclerotic placques.
Which face of the golgi receives vesicles? which face of the golgi release vesicles?
The cis face (closest to the ER) receives vesicles.
The contents of those vesicles are modified as they pass through the golgi. Some of them will stay and be luminal golgi proteins. Others are meant for transport elswhere.
These proteins will leave in vesicles being released from the trans face.
What sorts of modifications do proteins undergo as they pass through the golgi?
Any N-linked glycosylation they have will be trimmed or added to.
O-linkage glycosylation will occur in the golgi
What are the 7 functions of the golgi?
- Receives lipid and protein products from the ER
- Returns “escaped” proteins that should be resident in the ER
- Modifies glycoproteins (both trimming and addition)
- Solfation and other post-translational modifications
- Glycolipid and sphingomyelin production
- Add O-linked oligosaccharides to proteoglycans
- Directs materials to be transporte dfurther along 3 primary routes: lysosomes, secretory vesicles, plasma membrane
What protein coat is used in transport from the golgi out to the plasma membrane or to the lysosomes?
What protein coat is used in transport from the ER to the golgi?
What protein coat is used in transport rom the Golgi back to the ER and intra-golgi transport?
Golgi to plasma membrane or lysoeoms uses clatherin
ER to golgi uses COP2
Golgi to ER or intra-golgi uses COP1
What is the series of steps for COP1 and COP2 vesicle formation?
This utilizes small G proteins: Sar1 for COP2 and Arf1 for COP1
The small G proteins will be turned off when they’re attached to GTP and turned off if they’re attached to GDP.
GEFs catalyze the switch from GDP to GTP, activating the small G proteins. GAPs hydrolyze the GTP to GDP, inactivating the small G proteins.
So in COP2:
- a GEF catalyzes the GDP to GTP switch, activating Sar1.
- Sar1 then extends a ydrophobic tail that inserts into the lipid bilayer
- Sar1 then recruits other proteins including Sec23 and Sec24. Sec 24 binds transmembrane receptors for specific cargo proteins–these are the protein taht will be in the vesicle for transport
- THe binding of these proteins results in a conformational change that begins to curve the membrane
- Sec13/31 is then recruited, which forms a cage-like structure, helping the besicle to further form and pinch off.
How does the golgi know which proteins to send back to the ER?
Resident ER proteins will all have a specific amino acid sequence called the KDEL.
KDEL receptors in the golgi membranes bind these proteins, which are then packaged into vesicles for return to the ER using COP1.
What three proteins play critical roles in the accurate delivery and fusion of vesicles?
Rabs
Rab effectors
SNAREs
How does vesicle docking occur with selectivity?
The selectivity comes from the Rabs. Different Rabs bind to different membranes, thus serving as markers of membrane types.
Vesicles with a specific Rab will only dock on membranes that have the same Rab.
SNARES come in pairs as well, with v-SNARES only interacting with their favorite t-SNARES
What protein actually mediates the fusion of a vesicle to the membrane?
SNARES
the vSNARE is on the vesicle and the tSNARE is on the target membrane.
Once the vesicle gets close enough, the v and t SNARES will wrap around each other and pull the vesicle in.
Besides Rabs and SNARES, what is another way membrane targets are identified for vesicle formation and fusion?
Throguh the lipid concent of the membrane
Specifically, the content of specific phosphotidylinositol species can serve to identify different membranes and membrane domains.
Why are proteins modified in different ways at diferent locations in the Golgi as they pass through?
Because enzymes are arranged differently from the cis faces through to the trans face.
How does the golgi know to send lysosomal hydrolytic enzymes to the lysosome?
The hydrolytic enzymes are all tagged with a mannose 6-phosphate on the oligosaccharide chain.
There are receptors in the golgi membrane trans face that will bind to the mannose 6-phosphate, concentrate the hydrolytic enzymes together in a vesicle, and use the coating protein clatherin to bud off a secretory vesicle to go to the lysosome.
What are lysosomes? WHat is their function?
They are membrane-enclosed compartments that are filled with hydrolytic enzymes; they carry out the intracellular digestion of numerous cellular materials.
They have proteases, nucleases, glycosidases, lipases, phospholipasea, etc.
All are ACID HYDROLASES, meaning they’re only active at low pH.
What safety mechanism does the body use to protect itself just in case the hydrolytic enzymes get spilled out form the lysosomes?
The hydrolytic enzymes are active only at low pH, and when they’re being transported in vesicles from the golgi tot he lysosome, the vesicle packages them at neutral pH so they’re not active until they reach the lysosome.
What are some forms of specialization that the lysosomal membrane has?
- the membrane is highly glycosylated for protection
- it contains proton pumps (H+ ATPases) to create an acidic lumen
- contains numerous transporters to shuttle breakdown products (sugars amino acids and nucleotides) to be used again in the cytoplasm