1-35 ER & Vesicular Transport

Question 1

A single organelle consisting of interconnected flattened sacs and branching tubes of membranes

Answer 1

The ER

Question 2

describe the location of ER in the cell

Answer 2

typically continuous with the outer nuclear membrane and typically extends throughout the cell

Question 3

Describe the ER lumen

Answer 3

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

Question 4

Functions of ER

Answer 4

1. protein synthesis
2. protein modification
3. protein quality control
4. lipid synthesis
5. synthesis of steroids
6. detox of lipid soluble drugs
7. Ca+2 storage

Question 5

Describe the protein synthesis activity of the ER

Answer 5

1. proteins destined for lumens or mebranes of ER, golgi, lysosomes and endosomes
2. membrane proteins destined for PM
3. proteins destined for secretion from cell
4. once in

Question 6

The rough ER serves as the…

Answer 6

Rough ER serves as the starting point for proteins destined for other organelles, the plasma membrane, extracellular secretion, or the ER itself.

Question 7

Proteins enter the ER..

Answer 7

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.

Question 8

if a soluble protein is being transported to the ER

Answer 8

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

Question 9

Proteins with insoluble regions are….

Answer 9

single or multi pass transmembrane proteins.

Question 10

Once a protein is in the ER, how can it leave?

Answer 10

vesicular transport

Question 11

multiple modifications occur in the ER, including..

Answer 11

Signal Sequence Cleavage (cotranslational modification)

N-Linked glycosylation (costranslational)

Hydroxylation of Collagen

Protein folding and disulfide bridge formation

assembly of multi-subunit proteins

Question 12

proteins that are to be retained by the ER will have a

Answer 12

ER retention signal sequence

Question 13

ER quality control?

Answer 13

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

Question 14

Smooth ER is involved with

Answer 14

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.

Question 15

muscle cells have a specialized ER

Answer 15

the sarcoplasmic reticulum that stores and releases Ca+2 ions to regulate muscle contraction

Question 16

steps of vesicular transport

Answer 16

1. cargo molecules approach membrane bound receptors, memrabne pinches off and is surrounded by clathlin-coated vesicle
2. uncoats, naked
3. vesicles reach destination, tethering molecule on target site binds RAB protein on the vesicular surface, pulling it in.
4. v-snare on vesicle intertwines with t-SNARE on target site, casuing vesicle to fuse with membrane, deliviering cargo

Question 17

describe the function of the golgi apparatus

Answer 17

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

Question 18

5 sections of golgi

Answer 18

cis network, cis cisterna, medial cisterna, trans cisterna, trans network

Question 19

products of the ER arrive at the..

Answer 19

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)

Question 20

ribosomes are on the _____ surface

Answer 20

cytosolic

Question 21

polyribosomes

Answer 21

multiple ribosomes can jump on an mRNA. the ribosomes are held together by the mRNA itself

Question 22

as signal sequence emerges from the ribosome..

Answer 22

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.

Question 23

the structure of transmembrane portion of transmembrane proteins

Answer 23

alpha helicies

Question 24

describe the single pass protein process

Answer 24

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.

Question 25

soluble proteins vs transmembrane proteins

Answer 25

soluble proteins - completely pass ER into lumen

Transmembrane proteins only partially pass into the ER

Question 26

Which end enters the lumen of the ER?

Answer 26

the NH2 end. COOH in cytosol

Question 27

signal sequences are always

Answer 27

hydrophobic, allows them to bind to the SRP

Question 28

describe the co-translational protein modifications of the ER

Answer 28

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”

Question 29

describe the protein modification hydroxylation of collagen by the ER

Answer 29

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.

Question 30

Scurvy

Answer 30

not enough vitamin c, enzyme that builds collagen will not work properly, left with scurvy.

Question 31

protein disulfide isomerase

Answer 31

within the ER, forms/breaks disulfide bridges as protein is folding towards its final conformation

Question 32

assembly of multisubunit proteins

Answer 32

we dont want to send out a single subunit, we want to send out complete proteins (like a channel). assemble as they are made.

Question 33

example of ER resident proteins

Answer 33

protein disulfide isomerase, oligosaccharyl tranferase, signal peptidase

Question 34

most proteins that enter the ER Will be

Answer 34

transported to other destination

Question 35

cells contain ____ smooth than rough ER

Answer 35

less smooth, much more rough (in general)

Question 36

smooth ER contains what type of enzymes?

Answer 36

enzymes required to synthesize hormones form cholesterol

Question 37

T/F: Proteins enter the ER post-translationally

Answer 37

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

Question 38

Steroid-secreting cells will generally have a larger amount of which type of ER (RER or Smooth ER)?

Answer 38

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

Question 39

Liver cells have more RER or smooth ER?

Answer 39

Smooth ER…detoxification of lipid soluble drugs occur in smooth ER of liver

Question 40

In which organelle are oligosaccharides on proteins and lipids made?

Answer 40

Golgi Apparatus

Question 41

T/F: Vesicular traffic between the ER and Golgi moves in one direction from the ER to the golgi

Answer 41

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

Question 42

If proteins moving through the golgi have no targeting specific signal, where will these proteins go?

Answer 42

They will go through the default pathway and be exocytosed from the cell

Many have lysosomal signals and will go there

Question 43

function of adaptins?

Answer 43

D. To link clathrin to cargo receptors in clathrin coated vesicles.

Question 44

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

Answer 44

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.

Question 45

Which of the following mitochondrial components are actually synthesized in mitochondria?

Answer 45

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.

Question 46

What is the site of synthesis of proteins destined for the mitochondrial matrix?

Answer 46

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.

Question 47

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

Answer 47

synthesis of ATP

Question 48

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

Answer 48

B.

Question 49

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

Answer 49

B

Question 50

What is N-linked glycosylation and where does this occur?

Answer 50

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