Secretory Pathway I & II Flashcards
Proteins move through _____ across a membrane from cytosol in an organelle
translocators
Steps of transmembrane transport to the ER
- peptide synthesis is initiated on ribosome in the cytoplasm
- signal sequence is reached during synthesis that signals for the translocation of the growing peptide in the ER
- a signal recognition particle (SRP) binds the translated signal sequence and free ribosome
- SRP then binds GTP and halts elongation of peptide
- Ribosome and peptide are brought the SRP receptors on the cytosolic face of the ER
- The growing peptide is transferred to a translocon complex
- SRP dissociated from the complex as GTP is hydrolyzed
- Elongation of the peptide continues, feeding the growing peptide into the lumen of the ER
- After synthesis is complete, the signal sequence is cleaved by signal peptidase and the ribosome dissociates and is recycled
signal sequences vary, but all contain
8-15 hydrophobic residues
SRP as a
methionine rich binding pocket that allows it to be flexible in terms of what it binds
cargo proteins are released into
the lumen of the ER
Transmembrance proteins differ in that they
contain a transmembrane called the stop transfer sequence.
when the stop transfer sequence is reached during synthesis, the
growing peptide is moved from the translocon complex to the membrane, where its synthesis, as a transmembrane protein, continues
type 1 transmembrane proteins have their amino terminus
inside the lumen
type 2 transmembrane proteins have their amino terminus
outside the lumen
transmembrane proteins may have
more that one transmembrane domain
Proteins and lipids move from one compartment to another by
membrane bound transport intermediates
a vesicle forms through _____
exocytosis: a portion of the membrane of the ER or other compartment and its contents (transmembrane and cargo proteins) buds off
the vesicle is lined with a _____
a coat that is specific for the proteins it contains and the final destination of the bud
the vesicles is transported to its destination along
microtubules via motor proteins (dyenin and kinesin)
the vesicle sheds its coating and membrane fusion occurs via ____ proteins
via SNARE proteins
the contents of the vesicles are then released into the ____
lumen of the target compartment
List the major function of the ER
- synthesis of lipids
- control cholesterol homeostasis
- synthesis of proteins on membrane bound ribosomes
- Co-translational folding of proteins and early post translational modifications
- Quality control
synthesis of which lipids occurs in the ER?
phospholipid, ceramide and cholesterol
What is cotranslational translocation?
step in transmembrane transport where a peptide being made on a free ribosome is transported to a translocon in the membrane of the ER
synthesis of the growing peptide then ensues in the ______
translocon
name 3 well studied vesicle coats
- clathrin
- COPI
- COPII
Vesicle coats function is to _____
assist the membrane to change its shape in order to form a vesicle
how are different vesicular coats specific for cargo and transmembrane proteins? due to its variation in their shape and conformation,
because of its variation in their shape and conformation,
vesicular coats function to
target proteins to the correct final destination
clathrin function is t0 ____
transports proteins from the plasma membrane to lysosomes or back to the plasma membrane
COPI transports proteins from ___ to ____
transports proteins from Golgi to the ER
COPII transports proteins from ___ to ____
transports proteins from the ER to the Golgi
polyribosomes are located on
all on one mRNA
___ is the dominant organelle
ER
plasma cells function is ____
synthesizes and secretes anitbodies
Major Functions of the ER
- Synthesis of lipids (phospholipid, ceramide, and cholesterol)
- Control of cholesterol homeostasis (cholesterol sensor and synthesis)
- Ca+2 storage (rapid uptake and release)
- Synthesis of proteins on membrane bound
ribosomes - Co-translational folding of proteins and early post-translational modifications. 2. Post- translational insertion into the membrane
- Quality Control
Integral membrane proteins: insertion into the ER membrane
Two mechanisms:
- cotranslational insertion (discovered 30 years ago) and
2. post- translational insertion (recently discovered).
signal sequence binding pocket is lined by _____
methionines and is dry flexible
How to move membrane
and cargo from one compartment to another?
Vesicle budding, translocation, and fusing are critical.
ER soluble proteins
C terminal KDEL,
binds to KDEL receptor
ER membrane proteins
C terminal KKXX,
binds to COPI coats
Mannose 6-phosphate is a sorting signal for _____
sorting signal for lysosomal proteins
Mannose 6-phosphate binds to a receptor that is targeted to ______
vesicles that fuse with the late endosome
In the late endosome pH is ____ and the receptor and M6P tagged protein _____.
low
dissociate
How are membrane proteins made with multiple TMDs?
These proteins have internal stop and start sequences. Some transporters and channels have many transmembrane domains. Most amino acid transporters have 12 TMDs, the CFTR (cystic fibrosis transmembrane conductance regulator) has 12 TMDs, and the voltage- gated sodium channel responsible for the neuronal and muscle action potential has 24 TMDs.
vesicles move:
“forward” from the ER to the Golgi
and from the Golgi to the plasma membrane
lysosome move:
“backward” moving vesicles
for retrieval.
Describe the 3 mechanisms of protein transport
- gated transport
- Transmembrane transport:
- Vesicular transport: membrane-bound transport; requires adaptor and coat proteins.
Gated transport:
requires a specific receptor protein to carry a folded protein from the cytosol into the nucleus through the pore complex.
Transmembrane transport:
requires a translocator protein or protein complex (ie translocon) to move proteins across membrane.
Vesicular transport:
membrane-bound transport; requires adaptor and coat proteins.
Translocon:
Protein-conducting aqueous channel that spans the rough ER membrane.
Protein sorting signals:
most proteins are “tagged” with a region (primary sequence or tertiary structure) that forms a “recognition patch”: this tells the cell how to package the protein and where to send it.
there is a specific 5’ sequence that signals that the transcript is going to be pulled into the ER; if it’s absent,
the protein remains soluble in the cytosol.
The 5’ sequence that signals ER translocations causes the ribosome translating that protein to
attach to the membrane of the ER (thus causing rough ER vs smooth ER).
Co-translational translocation:
as mRNA is translated, it’s moved through the translocon into the ER lumen to be cleaved by signal peptidases and folded.
The 5’ signal sequence binds to the ______; this opens the ________.
SRP (signal receptor particle) in the ER membrane
translocon and the nascent polypeptide is threaded through the translocon into the lumen as it’s translated (the 5’ signal sequence is cleaved off almost immediately so that it doesn’t make up a part of the final protein)
SRPs:
have a multi-methionine “pocket” that binds to a wide variety of 5’ signal sequences.
5’ signal sequences:
variable; often mainly nonpolar.
Notice that the translocon is regulated on the______ as well by _______: Which means that it can:
luminal surface
binding protein BiP
expel proteins as well as admit them
If the nascent protein is meant to be a membrane protein, the protein contains another region
(aside from the 5’ signal sequence) that interacts with the translocon:
a “stop-transfer sequence” which stops the transfer of the protein into the lumen– thus the protein remains “stuck” with a transmembrane domain in the ER membrane, one end of the protein in the lumen, and the other end of the protein in the cytosol. The “stuck” protein then translocates out of the translocon for further packaging.
the positively charged end of the transmembrane domain winds up in the______ and the negatively charged end of the domain winds up in the ______
cytosol (outside the ER)
ER lumen.
Asparagine-linked (N-linked) glycosylation:
as a polypeptide is threaded through the translocon, an enzyme (oligosaccharyl transferase) catalyzes the glycosylation of select asparagine residues in the protein (“core glycosylation”) with the motif N-X-S/T.
Part of N-linked glycosylation involves trimming the
attached glycosyl groups to properly package the protein for transport to the Golgi apparatus.
The_______ is what signals the ER that a protein is ready to be transported elsewhere.
trimming process
List the major functions of the Golgi:
- Synthesis of complex sphingolipids from the ceramide backbone
- Additional post-translational modifications of proteins and lipids
- Proteolytic processing (protein cleavage)
- Sorting of proteins and lipids for Golgi compartments
- Morphology:
- How the Golgi apparatus transports proteins/lipids through itself:
o Notice that N-linked glycosylation is finished in the Golgi: this, again, is a signal to move the protein to a new location
o “Proproteins”: proteins that undergo proteolytic cleavage late in processing (Golgi).
o One package sorting mechanism: by thickness of the package’s membrane (thicker -> plasma membrane; thinner stays in the ER membrane, that sort of thing).
How does the Golgi apparatus transports proteins/lipids through itself?
- Depends on the type of things transported. Sometimes it’s by moving them from cisterna to cisterna with small vesicles; sometimes it’s by actually moving or modifying the entire cisterna that contains the material (“cisternal progression”).
- Which one used depends on the nature of the material– if it won’t fit inside a vesicle, tend to use cisternal progression.
Vesicular “coats” are assembled at:
the site of vesicle formation:
at the site of vesicle formation, what happens to the vesicular coats that are being assembled?
vesicular coats “sort” the proteins or lipids to be moved (different coat molecules “select” various types of cargo using adaptor proteins), and aid in pushing the vesicle out the side of the membrane.
there are ______ in cargo:
adaptor binding motifs
Cargo attracts a certain type of ______; the adaptor proteins then attract ____
adaptor protein
a certain type of coat.
VAMP SNAREs are found on _____ and SNAP and syntaxin SNAREs are found on ______
vesicles
target membranes
Notice that _______ determine specificity of docking location with the SNAREs on the outside of the vesicle.
GTP-binding proteins (Rab proteins)
These GTP-BPs can also regulate ______
coat assembly (all kinds).
Once a vesicle’s budded off from the ER, it’s moved to the _____ via the ______.
What are the motor proteins that move them?
Golgi via the cytoskeletal network
The motor proteins that move them are dynein and kinesin.
There’s a “vesicular-tubular cluster” that functions to ____
pre-packages the proteins before they get to the Golgi.
Exocytic pathway:
getting proteins to the organelles they’re going to.
Endocytic pathway:
getting proteins, within their target organelles, to their sites of action.
Notice that the interiors of lymphocytes are almost entirely made up of what?
ER, like most cells that produce
massive amount of proteins (in this case antibodies).
GPI-linked proteins will eventually end up where?
on the outside of the cell’s plasma membrane.
Glycosyl-phosphatidyl-inositol link (GPI) links the
protein to the cell membrane
KDEL protein function?
protein for retrieving improperly transported packages.
