CAS BI 315 Lecture Notes - Lecture 57: Oligosaccharide, Glucocerebrosidase, Intermembrane Space
- ER is really important for proper protein folding. That’s why you have ERAD and UPR
processes. Within 10 minutes, proteins are done with the ER.
To the Golgi Apparatus
- Once in the ER, proteins are further trafficked to their final destination via the Golgi
Apparatus
- How are proteins trafficked/transported through the cell?
- Protein transit from ER to final destination is mediated by vesicular transport (transports
from one organelle to the next ER → ERGIC(Intermediate between ER and Golgi) →
Golgi → plasma membrane)
- There are four steps:
1. Sorting of “cargo” into a vesicle bud
2. Vesicle budding from the ER exit site (ERES)
3. Vesicle “docking” to target membrane
4. Vesicle fusion with target membrane
- Proteins targeted for export have peptide and carbohydrate signals that direct their
packaging into transport vesicles.
- Unmarked proteins in the ER can also be packaged and transported to the Golgi by a
default pathway.
- Proteins that function within the ER are recognized in the ERGIC or Golgi (after being a
little bit modified) and transported back to the ER.
- These proteins, such as BiP, have an AA targeting sequence (KDEL or KKXX sequence)
at the carboxy terminus that directs retrieval back to the ER.
Golgi Apparatus
- Here, proteins from the ER are processed and sorted for transport to endosomes,
lysosomes, the plasma membrane, or secretion.
- Most glycolipids and sphingomyelin are synthesized in the Golgi.
- The Golgi is composed of flattened membrane-enclosed sacs (cisternae) and associated
vesicles.
- Has polarity, as vesicles are coming in (cis), Medial, trans, trans
-Golgi network is where
things are properly modified and sent.
- A lot of different membranes that are put together not all connected called cisternae.
- Budding → Sorting of cargo into budding vesicle (Secreted proteins and golgi proteins)
→ transmembrane protein have the same orientation in the membrane → Buds off →
Release into Golgi → Repeat process → cargo moves to plasma membrane (secreted
proteins)
- The transmembrane proteins will have a cytosol end IN the cytosol! So the Lumenal
turned “Extracellular” domain (Orientation is maintained).
1. How do cells mediate “vesicle budding”?
- Driven by “vesicle coat proteins” - The coat around the phospholipid and
cholesterol is hard protein. It’s like grabbing a ball of fat with a hard coat.
1. Coat proteins attach to the cytosolic side of membranes
2. Proteins form into “lattice” structure
3. Structure distorts and initiates vesicle budding
4. Continued assembly drives release of a
“coated vesicle”
FYI: The vesicle coat is disassembled
before docking at the target membrane.
So basically it disintegrates with the
opposite mechanism of its formation.
-COP - coat protein b/w ER and Golgi -
COPII from ER to ERGIC to Golgi and
COPI from trans to cis and to ERGIC and
to ER
-Clathrin - mediating from trans
-Golgi
network to plasma membrane and from
the membrane and golgi to the endosome
and lysosome
- Used for both going to and coming
from plasma membrane.
- Multi-subunit protein with a “triskelion” structure. Is very rigid from the
center out and made up of multiple subunits that get together and form a
larger lattice.
- Clathrin assembly drives vesicle budding.
2. How do cells package the correct cargo into a budding vesicle?
- Cells have “trafficking” proteins that recruit cargo into budding vesicle.
- These proteins interact with vesicle coat proteins via “adaptor proteins”. They
bind the cytoplasmic tail of cargo and vesicle coat proteins and that determines
what will be in each vesicle.
- Formation of coated vesicles is regulated by small GTP-binding proteins such as
ARF1, related to Ras and Ran, and Rab.
- GTP-binding proteins recruit adaptor proteins that mediate vesicle assembly by
interacting with cargo proteins and with coat proteins. (Recall the LDL receptor
and adaptor proteins in receptor-mediated endocytosis).
- Arf (with GDP) → Arf/GEF (protein that replaces GDP with GTP) ---> Activated
version attaches an adaptor protein → Receptor attachers with Cargo → clathrin
attaches to the adaptor protein → vesicle forms
- Golgi is composed of: 1) Several “cisternae” 2) Associated vesicles.
- Two key modifications within the Golgi (influences WHERE proteins end up):
1. O-linked
glycosylation (Serine and Threonine)- Sequential addition of
sugars to serine or threonine. If there’s an error here then the protein
won’t make it to the surface
2. Modification of N-linked
(Asparagine) sugars added in the ER.
− The sugars have important roles in Protein sorting and Protein
folding. (LDL gets both N-linked glycosylated and O-linked
glycosylated). (If there are mutations in the serines and threonines
Document Summary
Once in the er, proteins are further trafficked to their final destination via the golgi. Protein transit from er to final destination is mediated by vesicular transport (transports from one organelle to the next er ergic(intermediate between er and golgi) . There are four steps: sorting of cargo into a vesicle bud, vesicle budding from the er exit site (eres, vesicle docking to target membrane, vesicle fusion with target membrane. Proteins targeted for export have peptide and carbohydrate signals that direct their. Unmarked proteins in the er can also be packaged and transported to the golgi by a packaging into transport vesicles. default pathway. Proteins that function within the er are recognized in the ergic or golgi (after being a little bit modified) and transported back to the er. These proteins, such as bip, have an aa targeting sequence (kdel or kkxx sequence) at the carboxy terminus that directs retrieval back to the er.
