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BIO 1090 > Endomembrane System > Flashcards

Endomembrane System Flashcards

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1
Q

Compartments of Endomembrane system

A
  • Golgi Apparatus
  • Endoplasmic Reticulum
  • Cytosol
  • Lysosome
  • Vesicles
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2
Q

What did early EM work reveal?

A
  • membrane bound organelles and vesicles within the cytoplasm
  • extensive network of membranous canals and stacks of sacs (cisternae)
  • origin of cytoplasmic membrane systems came from infolding of the plasma membrane
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3
Q

Biosynthetic pathway

A

pathway that delivers the proteins to the organelles that allows the organelles to execute their proper functions

  • journey starts at the rough ER and move proteins to where they are needed.
  • These proteins move from the ER by entering a vesicle which pinches off
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4
Q

What happens if proteins need to be secreted

A
  • proteins become part of the secretory pathway and are released to the external environment
  • constitutive secretion needs no regulation at all to merge the vesicle
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5
Q

Green Fluorescent Protein

A
  • GFP are a technique to track cell components
  • observation of the fusion protein provides information about the endogenous protein (where it is localized in the cell organism
  • this protein from jellyfish can be fused with a cellular protein
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6
Q

Vesicular transport

A
  • mechanism for exchange of proteins and lipids between membrane-bound organelles and/or the PM in eukaryotic cells.
  • utilizes transport vesicles
  • Uses cytoskeleton and motor proteins, sorting signals recognized by receptors
  • Targeted movement (directed)
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7
Q

Steps in vesicular trafficking from ER to the golgi

A
  1. donor compartment budding
  2. receptor
  3. vesicle
  4. recipient compartment fusion
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8
Q

Exocytosis and Endocytosis

A
  • mediated by the endomembrane system
    Exocytosis: the process of moving materials from within a cell to the exterior of the cell (organelle to PM)
    Endocytosis: the process of bringing substances into the cell (PM to organelle)
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9
Q

role of receptors in vesicular transport

A
  • Presence of a receptor allows proteins to concentrate in the forming vesicle
  • All of the soluble proteins will come off their receptor and enter the lumen
  • The signal for releasing soluble proteins is a change in pH
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10
Q

How is it ensured that vesicles arrive at the proper membrane?

A
  1. Movement of vesicles: uses cytoskeleton and motor proteins
  2. Tethering: via proteins called Ribs and tethering proteins, Rabs will only like rabs from certain compartments and won’t fuse with ones they don’t like
  3. Docking: uses SNARE proteins, assembles through twisting v-SNARE around t-SNARE
  4. Fusion of vesicle and target membrane
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11
Q

Secretion of a neurotransmitter

A
  • A synaptic terminal waits for the arrival of an action potential which causes voltage gated calcium channels to open
  • Calcium rushes into the cell which stimulates fusion of the vesicles with the membrane and releases neurotransmitter into the synaptic cleft
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12
Q

Rough vs Smooth ER

A

Rough: associated with ribosomes on the cytoplasmic membrane side, many proteins including those destined for secretion are synthesized by ribosomes on the rough ER
Smooth: lacks ribosomes, primary site of lipid synthesis

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13
Q

Functions of the smooth ER

A
  • lipid synthesis
  • production of steroid hormones like glucorticoids, androgens and estrogens
  • detoxification
  • storage (in muscle cells celled the sarcoplasmic reticulum)
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14
Q

Functions of rough ER

A
  • synthesis of membrane phospholipids
  • glycosilation of proteins
  • protein folding, quality control (involves activation of molecular chaperons)
  • protein synthesis, modification and transport (ribosomes sit on top of a channel that the protein is moving its way through as its being synthesized and is released when synthesis is done)
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15
Q

protein synthesis

A

DNA is copied to make RNA in transcription
RNA is folded into proteins in translation
- facilitated by ribosomes

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16
Q

Translation is completed in 1 of 2 ways

A

Free ribosomes: make cytosolic proteins, peripheral membrane proteins, or proteins targeted to nucleus, mitochondria, peroxisomes and chloroplasts
ER associated ribosomes: secreted proteins, integral membrane proteins and soluble proteins associated with the lumen of endomembrane system

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17
Q

Translation completed on free ribosomes

A

There are different signals that exist within the actual protein
No signal peptide = soluble cytoplasmic protein
Signal on amino terminal sequence = to chloroplast or mitochondria
Internal signal = protein to the nucleus

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18
Q

phagocytosis

A

process by which certain living cells called phagocytes ingest or engulf other cells or particles and eliminate them

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19
Q

True or false: once proteins enter the endomembrane system they are able tio come out?

A

false

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20
Q

How do ribosomes on the RER surface get there?

A
  • ribosomes are targeted to the ER membrane by a signal sequence
  • The protein contains a signal sequence located at its amino-terminus
  • Contains sever consecutive hydrophobic amino acids
  • Signal sequence directs synthesis to ER
  • Protein moves through channel into ER
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21
Q

Cotranslational Protein Import : step 1

A

Signal recognition particle (SRP) binds to signal sequence

  • translation stops
  • mRNA codes for a soluble protein that needs to be in the lumen
  • Signal sequence is the first part of the protein that is synthesized
  • SRP binds to the signal sequence, interacts with ribosome and pauses translation
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22
Q

Cotranslational Protein Import : step 2

A

Targeting of translation complex to ER

  • includes SRP/ribosomes + mRNA/new polypeptide
  • SRP binds to SRP receptor
  • SRP receptor is an integral membrane protein only found in rough ER
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23
Q

Cotranslational Protein Import : step 3

A

SRP is released and ribosome binds the translocon

  • SRP comes off the signal sequence and the polypeptide finds its way into the central channel of the translocon
  • Synthesis can now resume
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24
Q

Cotranslational Protein Import : step 4

A

Polypeptide enters the ER (trough that translocon) as it is translated
- signal sequence is cleaved off and chaperone folds protein

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25
Q

Exocytic pathways of protein sorting

A

a protein targeted to the ER lumen, after it is fully synthesized and properly folded has 1 of 2 options

  1. It is retained in the ER lumen if that is where it is needed
  2. It is transported from the ER to the Golgi complex for further modification and delivered to distal parts of the biosynthetic/secratopry pathway. Final destination could be secretion
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26
Q

Roll of transport vesicles

A

move soluble proteins and membrane proteins to compartments or out of the cell

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27
Q

Why is protein sorting critical?

A

Mislocalization of a protein can lead to disease

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28
Q

What causes cystic fibrosis?

A

disfunction of the CFTR chloride channel that secretes chloride out of our lungs
- If this channel is dysfunctional and the integral protein can’t reach the plasma memb or other sites and it is degraded in the ER

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29
Q

Cotransitional protein import

A

How soluble proteins get into the endomembrane system

  1. The signal recognition particle binds to a signal sequence in the amino-terminal end of the growing polypeptide and halts translation
  2. SRP binds to the SRP receptor on the ER membrane
  3. The SRP receptor brings the ribosome to a transmembrane channel, the SRP dissociates, protein synthesis resumes, and the growing polypeptide chain is threaded through the channel
  4. The protein ends up in the lumen of the ER, where it may remain, be transported to the lumen of another organelle, or be secreted out of the cell
30
Q

How do target proteins get to the mitochondria and chloroplasts?

A
  • determined by N-terminal sequences
  • other intrinsic sequences within the protein direct it to the correct compartment or membrane of the organelle
  • these proteins are synthesized completely in the cytoplasm first
31
Q

Where does synthesis of integral membrane proteins take place?

A

The rough ER

-

32
Q

How does synthesis of integral membrane proteins start?

A

Integral membrane proteins have the signal sequence
The signal sequence delivers the ribosome, nRNA and polypeptide to the translocon
Proteins with signal-anchor sequences are threaded through channel in the ER membrane until the signal-anchor sequence is encountered

33
Q

What happens once the signal-anchor sequence is encountered in integral membrane protein synthesis?

A

Channel provides access to the lumen
The anchor opens up the lateral gate which opens into the lipid bilayer in response to the hydrophobic alpha helix
The lateral gate is responsible for releasing integral membrane protein laterally into the lipid bilayer

34
Q

What happens when the integral membrane protein is completely synthesized?

A

The ER releases the protein into the membrane
Translation continues but the remainder of the protein faces the cytoplasmic side (C-terminus)
When translation is completed the protein remains embedded in the membrane

35
Q

Transport from the ER to the Golgi complex

A
  • Golgi complex receives protein and lipids from the ER and sorts them to other organelles, the plasma membrane or the cell exterior
  • Material moves from ER to Golgi and then other compartments and the plasma memb in a proximal to distal direction
  • Slowly progress from cis Golgi-network to trans Golgi-network
36
Q

Structure of the golgi-complex

A

structure: smooth, flattened disk-like cisternae
- 8 or fewer cistern per stack
- Curved like a shallow bowl
- Show polarity: cis-medial -trans-cisternae, cisternae are biologically unique
- Membrane supported by protein “skeleton” (actin and spectrin)
- Scaffold linked to motor proteins that direct movement of vesicles into and out of Golgi

37
Q

Cis-Golgi Network

A

acts as a sorting station, sorts whether proteins should continue onto the next Golgi stack or be shopped back to the ER
- sorts ingoing

38
Q

Trans-Golgi Network

A

sorts protein into different types of vesicles, vesicles go to plasma membrane or other intracellular destinations (like lysosomes)
- sorts outgoing

39
Q

How are proteins modified in the Golgi?

A

“step-wise”
- Different cisternae of the Golgi contain different enzymes that modify proteins
The differential staining of the Golgi cisternae reflects their biochemical differences
functions of the golgi

40
Q

Functions of the Golgi

A
  1. Processing factory of the cell
  2. Synthesis of complex polysaccharides
  3. Modification of proteins and lipids - glycosylation (glycoproteins and glycolipids) - proteolytic modification
  4. Transport and sorting of proteins
41
Q

Endocytotic Pathways of protein sorting

A

fully processed proteins are exported to the TGN and then sorted and delivered to their final destinations

42
Q

How are vesicles directed in the endocytic pathway?

A
  • Vesicles are formed via COAT proteins
  • Vesicles are transferred from ER to the Golgi
  • Coat proteins have 2 functions: help form the vesicle and help select “cargo” (I.e. material in vesicle lumen or inserted in membrane)
  • Vesicles forming at the ER concentrate the correct soluble and intermembrane proteins and transport them to the golgi
43
Q

Vesicular transport + COAT proteins

A

Receptors will only localize where coat proteins are
Integral membrane proteins also interact with coat proteins and are packaged into vesicle.
The COAT completely dissolves once vesicle pinches off and vesicle can interact with the cytoskeleton

44
Q

How do COP I and COP II proteins carry out functions?

A

COPI and COPII are protein complexes that assemble on the cytosolic surface of donor compartment membranes at sites where budding takes place

45
Q

COP I

A

COPI-coated vesicles move in retrograde direction
COPI vesicles are formed at the cis-golgi netowrk and travel to the ER
COPI vesicle forms, the coat protein falls off and the vesicle moves via the cytoskeleton back to the rough ER

46
Q

COP II

A

COPII-coated vesicles move in anterograde direction
COPII moves soluble cargo from the rough ER to the golgi network
SNARE proteins need to be in a COPII vesicle

47
Q

How are COP I and COP II similar?

A
  • COPI and COPII have the same roles but move different directions
  • COP proteins assemble on the cytosolic surface of donor compartment membranes at sites where budding takes place
  • COP proteins and other coat proteins interact with cytosolic parts of transmembrane cargo receptors
48
Q

What directs vesicles to other parts of the cells?

A

Clathrin coated vesicles - move from the trans golgi network to other vesicles within the cell

  • Clathrin also helps form endocytic vesicles that form at the plasma membrane
  • Once the clathrin coated vesicles form the clathrin falls off and the vesicles go to their destination
49
Q

What is the typical signal to release proteins in vesicular transport?

A

a change in pH

50
Q

What determines where a protein ends up

A

intrinsic information in the protein amino acid sequence

51
Q

Characteristics of lysosomes

A
  • digestive organelles in the cell
  • Internal pH of 4.6 (acidic), generated by H+ ATPase
  • Hydrolytic enzymes, acid hydrolyses
  • Lysosomal membrane - glycosylated proteins
  • Productive lining next to lumen
52
Q

Function of lysosomes

A
  • degradation of internalized material
  • recycle plasma membrane components
  • destroy pathogens (bacteria and viruses)
  • golgi delivers enzymes that break down macromolecules to a lysosome
  • a vesicle with macromolecules also merges with the lysosome
53
Q

What maintains the acidic pH in lysosomes?

A
  • proton pumps maintain pH, while broken down macromolecules are transported out of the lysosome by transporters
54
Q

Autophagy

A
  • organelle turnover (destruction and recycling of organelles)
  • lysosome fuses with ER-derived autophagic vesicle which forms autolysosome
  • contents enzymatically digested
  • ER-derived membrane engulfs the damaged organelle and form autophagic vesicle which fuses with a lysosome and the acid hydrolase are capable of breaking down the organelle
55
Q

What is the most common organelle that is destroyed?

A

mitochondria

56
Q

Functions of the cytoskeleton

A

structural support, spatial organization within the cell, intracellular transport, contractility and motility

57
Q

What is the cytoskeleton?

A

dynamic network of interconnected filaments and tubes that extends throughout the cytosol of eukaryotes

58
Q

Microtubules function

A

2 major types
axonemal: highly organized, stable, part of structures involved in cell movement
cytoplasmic: loosely organized, very dynamic, located in cytosol
-

59
Q

Microtubule structure

A
  • hollow tube formed from tubulin dimers
  • form a cage like structure that give the plant its shape
  • Have negative and positive end that migrate away
  • 13 protofilaments form a longitudinal array creating a hollow cylinder
  • Heterodimers are aligned from head to tail creating structural polarity, important for growth, movement and direction of material
60
Q

Dynamic assembly and disassembly of microtubules

A
  • Dynamic instability is rapid turnover of microtubules within the cell (half-life is minutes)
  • Shrinkage can occur rapidly at the plus end called catastrophe
  • Formation of MTs is regulated/controlled
  • Microtubule organization centre (MTOC) = central cite of mictrotubule assembly (aka centrosome)
61
Q

What are Microtubule associated proteins

A

MAPS are several different proteins that bind MTs to modulate assembly/function, mediate interactions with other cellular structures (ex, vesicles/organelles) and stabilize MTs of stimulate assembly

62
Q

2 classes of MAPs

A
  1. Non-motor MAPS: control MT organization in cytosol (ex. Tau protein in neurons)
    Defective tau protein -> neurofibrillary tangles -> Alzheimers disease
  2. motor MAPs: two main types - kinesin and dynein, use ATP to generate force, can move material along MT track, can generate sliding force between MTs
63
Q

Kinesin and Dynein

A

Kinesin: powers intracellular transport negative to positive end
- Kinesin carries cargo such as vesicles toward the plus end of microtubules
Dynein: powers intracellular transport positive to negative end

64
Q

Largest to smallest cytoskeletal components

A

microtubules > intermediate filaments > micro filaments

65
Q

Microfillaments

A

double helix of actin monomers

Microfilaments are 7-9nm in diameter

66
Q

Intermediate filaments

A

strong fibre composed of intermediate filament protein subunits, are anchored to the plasma membrane

  • Not all cells have intermediate filaments
  • In epithelial cells intermediate filaments organize organelles
67
Q

Negative end of microtubules

A

Negative end of microtubules is associated with the organizing centre and the positive end is wherever the end or the microtubule is.
Negative end is more stable than the positive end because it is anchored

68
Q

Where do all microtubules originate from

A

the centrosome

69
Q

What might a mutation in the gene encoding BiP chaperone protein do

A

result in misfolded proteins in the ER lumen

70
Q

What happens to the signal sequence after a protein is synthesized in the ER lumen?

A

it is cleaved off by signal peptidase

71
Q

Order of cotranslational import

A

cytosol to translocon to RER lumen

72
Q

What is the lateral gate?

A

responsible for releasing integral membrane proteins into the lipid bilayer

BIO 1090 (7 decks)

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