1-35 ER & Vesicular Transport Flashcards
A single organelle consisting of interconnected flattened sacs and branching tubes of membranes
The ER
describe the location of ER in the cell
typically continuous with the outer nuclear membrane and typically extends throughout the cell
Describe the ER lumen
internal space enclosed by the ER membrane and the organelle is subdivided into regions called the RER, where ribosomes are bound on the cytosolic side of the membrane and the smooth ER lacking bound ribosomes
Functions of ER
- protein synthesis
- protein modification
- protein quality control
- lipid synthesis
- synthesis of steroids
- detox of lipid soluble drugs
- Ca+2 storage
Describe the protein synthesis activity of the ER
- proteins destined for lumens or mebranes of ER, golgi, lysosomes and endosomes
- membrane proteins destined for PM
- proteins destined for secretion from cell
- once in
The rough ER serves as the…
Rough ER serves as the starting point for proteins destined for other organelles, the plasma membrane, extracellular secretion, or the ER itself.
Proteins enter the ER..
cotranslationally, once an ER targeting signal sequence is recognized.
Translation is slowed down or halted, and the mid-translation polypeptide is directed towards the ER by an SRP.
Translation continues once the SRP is recognized by an SRP receptor and the ribosome is directed into a translocation channel.
if a soluble protein is being transported to the ER
it is simply translated into the ER, and the signal sequence is cleaved at the end of translation, allowing for the protein to move into the ER lumen
Proteins with insoluble regions are….
single or multi pass transmembrane proteins.
Once a protein is in the ER, how can it leave?
vesicular transport
multiple modifications occur in the ER, including..
Signal Sequence Cleavage (cotranslational modification)
N-Linked glycosylation (costranslational)
Hydroxylation of Collagen
Protein folding and disulfide bridge formation
assembly of multi-subunit proteins
proteins that are to be retained by the ER will have a
ER retention signal sequence
ER quality control?
chaperone proteins are there to create and destroy disulfide bridges as the protein folds
protein misfolded -chaperones recruited to refold it before allowed to leave the ER
overwhelmed - the UPR (unfolded protein response) is initiated - causing protein synthesis to be inhibited and recruits more chaperone proteins
if this doesn’t work, apoptosie
Smooth ER is involved with
synthesis of steroid hormones and detox of lipid soluble drugs.
any organ that creates hormones from altering cholesterol has an extensive smooth ER
the liver, when inundated with toxins, can increase the size of it’s ER for as long as needed.
muscle cells have a specialized ER
the sarcoplasmic reticulum that stores and releases Ca+2 ions to regulate muscle contraction
steps of vesicular transport
- cargo molecules approach membrane bound receptors, memrabne pinches off and is surrounded by clathlin-coated vesicle
- uncoats, naked
- vesicles reach destination, tethering molecule on target site binds RAB protein on the vesicular surface, pulling it in.
- v-snare on vesicle intertwines with t-SNARE on target site, casuing vesicle to fuse with membrane, deliviering cargo
describe the function of the golgi apparatus
sorting/dispatch station for proteins and lipids made in the ER.
- n-linked oligosaccharides on glycoproteins modified
- o linked oligosaccharides are synthesized onto proteins and lipids
- glycosaminoglycan chains are synthesized on proteoglycan core proteisn
5 sections of golgi
cis network, cis cisterna, medial cisterna, trans cisterna, trans network
products of the ER arrive at the..
cis golgi network, are modified, and then secreted out of thet rans golgi network to their final destination (lysosome, plasma membrane, or outside of cell via secretory vesicle)
ribosomes are on the _____ surface
cytosolic
polyribosomes
multiple ribosomes can jump on an mRNA. the ribosomes are held together by the mRNA itself
as signal sequence emerges from the ribosome..
As signal sequence emerges from ribosome, SRP binds to it (it’s the receptor) that reconizies the targeting signal. Protein syntehsis slows down, the large complex ribosome/srp moves to SRP receptor on the membrane. The srp puts the syntehzising protein onto the SRP receptor, SRP is released. SRP receptor takes it to a protein translocator, and it is translated into the lumen with the signal sequence still bound to the protein translocator.
the structure of transmembrane portion of transmembrane proteins
alpha helicies
describe the single pass protein process
the hydrophobic stop transfer sequence within the protein sequence serves as the terminal signal sequence. Binds to protein translocator. Done. Signal peptidase cleaves signal, leaving the membrane released into the membrane laterally.
soluble proteins vs transmembrane proteins
soluble proteins - completely pass ER into lumen
Transmembrane proteins only partially pass into the ER
Which end enters the lumen of the ER?
the NH2 end. COOH in cytosol
signal sequences are always
hydrophobic, allows them to bind to the SRP
describe the co-translational protein modifications of the ER
signal sequence cleavage happens co-translationally
N-linked glycosylation - addition of preformed 14 sugar residues get transferred to asparagine as it is transported through channel from “Oligosaccharyltransferase”
describe the protein modification hydroxylation of collagen by the ER
lysine and proline will have OH groups added to them so they can form a complex with three crosslinked collagen. OH groups keep them linked.
Enzyme in ER is vitamin C-dependant.
Scurvy
not enough vitamin c, enzyme that builds collagen will not work properly, left with scurvy.
protein disulfide isomerase
within the ER, forms/breaks disulfide bridges as protein is folding towards its final conformation
assembly of multisubunit proteins
we dont want to send out a single subunit, we want to send out complete proteins (like a channel). assemble as they are made.
example of ER resident proteins
protein disulfide isomerase, oligosaccharyl tranferase, signal peptidase
most proteins that enter the ER Will be
transported to other destination
cells contain ____ smooth than rough ER
less smooth, much more rough (in general)
smooth ER contains what type of enzymes?
enzymes required to synthesize hormones form cholesterol
T/F: Proteins enter the ER post-translationally
False. Proteins enter the ER co-translationally…where the driving force for this transport is provided by the protein synthesis process itself
THIS IS THE ONLY PLACE WHERE CO-TRANSLATIONAL TRANSPORT OCCURS
Proteins enter both the mitochondria (TIM/TOM complex) and peroxisomes post-translationally
Steroid-secreting cells will generally have a larger amount of which type of ER (RER or Smooth ER)?
Smooth ER…the synthesis of steroid hormomes occurs in the smooth ER
Leydig cells in testes secrete testosterone and contain a lot of smooth ER
Liver cells have more RER or smooth ER?
Smooth ER…detoxification of lipid soluble drugs occur in smooth ER of liver
In which organelle are oligosaccharides on proteins and lipids made?
Golgi Apparatus
T/F: Vesicular traffic between the ER and Golgi moves in one direction from the ER to the golgi
False
It is bidirectional, where the cis Golgi recognizes signals on proteins that may actually be ER retention signals and therefore need to be returned to the ER
If proteins moving through the golgi have no targeting specific signal, where will these proteins go?
They will go through the default pathway and be exocytosed from the cell
Many have lysosomal signals and will go there
function of adaptins?
D. To link clathrin to cargo receptors in clathrin coated vesicles.
The synthesis and secretion of a protein such as insulin from a cell requires which transport mechanism(s)?
A. transport via nuclear pores
B. transport via vesicles
C. transport via protein translocators
D. transport via both nuclear pores and vesicles
E. transport via both vesicles and protein translocators
E - Because insulin is destined for secretion from the cell, it is synthesized by ribosomes attached to the ER and undergoes co-translational transport into the ER lumen via a protein translocator. It then travels to the Golgi apparatus via vesicles and is secreted from the cell via vesicles.
Which of the following mitochondrial components are actually synthesized in mitochondria?
The mitochondrial genome only encodes rRNAs (answer C), tRNAs and a small number of mRNAs that are translated into protein subunits involved in electron transport and oxidative phosphorylation. The other proteins and enzymes on the list are encoded in the nuclear genome, synthesized in the cytosol, and then transported into the mitochondria.
What is the site of synthesis of proteins destined for the mitochondrial matrix?
cytosol - All proteins destined for the mitochondrial matrix are synthesized in the cytosol and then imported post-translationally via transmembrane transport. The few proteins that are encoded by the mitochondrial genome and synthesized in the mitochondrial matrix are all destined for the inner mitochondrial membrane, not the matrix, so C is incorrect.
Which of the following is NOT a function of peroxisomes?
A. beta oxidation
B. synthesis of ATP
C. synthesis of phospholipids
D. synthesis of hydrogen peroxide
E. degradation of hydrogen peroxide
synthesis of ATP
Which of the following does NOT occur in the normal life cycle of an LDL receptor?
A. Insertion into the ER membrane with the LDL binding domain oriented toward the inside (lumen) of the ER
B. Transport from the Golgi apparatus directly to the lysosome
C. Vesicular transport in clathrin-coated vesicles
D. Recycling from endosomes to the plasma membrane
B.
Which of the following does NOT happen at multiple steps of vesicular transport?
A. Assembly of protein coats driving vesicle formation
B. Release of lumenal proteins from organelles into the cytosol
C. Binding of v-SNARES to t-SNARES
D. Fusion of vesicles with target organelle membranes assisted by a fusion complex
B
What is N-linked glycosylation and where does this occur?
N-linked glycosylation is a protein modification that occurs in the ER. A preformed oligosaccharide (sugar) of 14 sugars is attached to a.a. residues during translocation (co-translationally).
N-linked glycosylation modification begins in the ER, but continues in the golgi
