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Cell Phys > Lec 9: Endoplasmic Reticulum > Flashcards

Lec 9: Endoplasmic Reticulum Flashcards

1
Q

ER is not visible with

A

light microscopy

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2
Q

ER Accounts for ~…

A

~50-90% of all membrane in a cell

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3
Q

ER Consists of

A

a series of tubular membranes and flattened sacks (cisternae)

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4
Q

ER flattened sacks =

A

= cisternae

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5
Q

Space within the ER (i.e., the ER lumen) is called

A

the cisternal space

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6
Q

2-types of ER:

A 
Rough ER (RER)
Smooth ER (SER)
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7
Q

RER has

A

ribosomes

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8
Q

RER is associated with

A

protein synthesis

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9
Q

RER makes proteins destined for: (3)

A
  1. ) Secretion
  2. ) Membrane bound proteins
  3. ) Proteins within membrane bound structures (like Golgi and Lysosome)
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10
Q

RER generally in the form of

A

flattened cisternae

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11
Q
  • In contrast, proteins destined for the cytosol are synthesized via
  • Polyribosome =
A

free ribosomes

= clustering of free ribosomes translating a single mRNA at same time

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12
Q

RER Ribosomes are attached to

A

the cytosolic side of the RER (not on the cisternal space)

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13
Q

RER Ribosomes contain

A

rRNA (nucleic acid with a negative charge)

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14
Q

In tissue sections, RER can be stained by…

like…

A

…basic dyes (dyes with a positive charge)

…hematoxylin

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15
Q

In histology, RER is said

A

to be basophilic

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16
Q

In early 1900s, histologists referred to the dark basophilic regions of cell cytoplasm as

A

ergastoplasm (ergastoplasm = RER)

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17
Q

(RER) Nissl Body (or Nissl Substance) =

A

= dense clustering of RER and free ribosomes in neurons – dark staining regions of the neuron cell body

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18
Q

Since some of the proteins made by the RER are destined for ______, they must enter into the…
for…

A

secretion
…RER cisternal space (lumen) …for packaging into ‘transition’ vesicles which bring them to the Golgi for packaging into ‘transport’ vesicles.

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19
Q

The proteins made by the RER ribosomes are generally imported into….
by a process known as…

A

….the RER cisternal space

….Cotranslational Import

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20
Q

Cotranslational Import is essentially

A

Coupling of translation and import in ER

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21
Q

(Cotranslational Import)

2 processes occur simultaneously: (3)

A
  1. ) The mRNA is translated into a polypeptide chain
  2. ) The polypeptide chain is directly imported into the ER cisternal space (lumen) via the Translocon.
  3. ) Three types of signal sequences (sequence tags).
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22
Q

(Cotranslational Import)
- 3 different protein signal sequences direct cotranslational import:

  • & involves a structure known as the… (& where is it located)
A
  1. ) ERSS – ER Signal Sequence
  2. ) Internal Stop Transfer Sequence
  3. ) Internal Start Transfer Sequence

…translocon (in the RER membrane)

23
Q

(ERSS directed import)

At the onset of translation at the ribosome:

A

On the N-terminal end of the polyeptide (the 1st portion to be synthesized) an ER Signal Sequence (ERSS) is present

24
Q 
(ERSS directed import)
The ERSS (once it is made) allows
A

the ribosome to dock to the Translocon via a riboprotein complex known as the SRP (Signal Recognition Particle). (It also temporarily blocks translation)

25
Q

(ERSS directed import)

The SRP contains

A

6 polypeptides and a 300 nucleotide rRNA

26
Q

(ERSS directed import)

The SRP directs the

A

polypeptide to the Translocon (a large pore-like structure on the RER membrane)

27
Q

(Cotranslational Import)
STEP 1:
& This will…
& Binding of…

A

The SRP binds to the ERSS (green) located on the N-terminus of the newly forming polypeptide
…direct it to the translocon in the ER membrane.
…the SRP to the ERSS temporarily blocks translation

28
Q

(Cotranslational Import)

STEP 2:

A

The SRP bound to the ERSS directs the polypeptide/ribosome unit to the translocon.
Allowing the ribosome to dock with the ER membrane.

29
Q

(Cotranslational Import/Step 2)

2 steps:

A
  1. ) The SRP-ERSS complex binds to the SRP receptor (a subunit of the translocon complex).
  2. ) The ribosome receptor of the translocon binds to the ribosome (another subunit of the translocon complex)
30
Q

Translocon =

& contains: (4)

A

= large protein pore complex in the RER membrane

  • SRP receptor
  • Pore protein
  • Ribosome receptor
  • Signal peptidase
31
Q

(Cotranslational Import)
STEP 3:
…which: (3)

A

Two GTPs then bind to the SRP and SRP receptor, which:

  • Unblocks translation
  • Opens the translocon pore
  • Forces the N-terminus (with the ERSS) into the pore.
32
Q

(Cotranslational Import)
STEP 4:
…this…
…but…

A

GTP is hydrolyzed into free GDP and Pi.

…releases the SRP
…the association between ribosome and translocon remains.

33
Q

(Cotranslational Import)
STEP 5:
…the newly synthesized…

A

As translation continues, the ERSS is cleaved off of the N-terminus by the signal peptidase (another translocon subunit)
…newly synthesized polypeptide is directed into and through the pore of the translocon.

34
Q

(Cotranslational Import)

STEP 6:

A

Once the C-terminus of the polypeptide is completed and released from the ribosome it passes through the pore of the translocon.
The polypeptide is released into the ER lumen.

35
Q

Insulin is a…
Insulin is involved in…
Insulin is secreted by…

A

…hormone/protein (endocrine system)
…the regulation of blood glucose levels
…the beta-cells of the pancreas (in the Islets of Langerhans, endocrine portion of the pancreas)

36
Q

(Insulin action)
Insulin is generally secreted into…
Acts to…
Thereby…

A

…the blood after a meal
… increase delivery of glucose into most cells of the body
…decreasing blood glucose

37
Q

What happens to the newly made polypeptide in the RER cisternal space? =
& 4 specific actions:

A

= Polypeptide is modified in the RER lumen

  1. ) Completion of folding (2* and 3* structure)
  2. ) Polypeptides may be glycoslylated
  3. ) Multimeric assembly of proteins (4* structure)
  4. ) Amino acid modification
38
Q

Polypeptide modification in ER (RER Lumen) (#1 only) =

What assists? =

Example =

What does it do? =

Also…

A

= Completion of folding (2* and 3* structure)

= molecular chaperones assist folding

= Hsp70 member (BiP, binding protein)

= Binds to hydrophobic regions preventing the protein from aggregating with other hydrophobic proteins allowing it to fold properly (ATP-dependent process).

…Disulfide bridges (Cystine) may be formed (protein disulfide isomerase)

39
Q

Polypeptide modification in ER (RER Lumen) (#2 only) =

What type? =

Addition of…

Asparagines that are…

N-linked =

O-linked =

A

= Polypeptides may be glycoslylated:

= N-linked glycosylation

…14 residue oligosaccharide chain (core oligosaccharide) to the asparagines from a lipid called dolichol (oligosaccharide protein transferase).

….glycosylated are always present in the tripeptide sequence –NXS- or –NXT-
(N= asparagine, X = any amino acid, S = Serine, T = threonine)

N-linked = linked to the amide nitrogen of asparagine (RER)

O-linked = linked to the hydroxyl group serine or threonine (happens in Golgi)

40
Q

Polypeptide modification in ER (RER Lumen) (#3 only) =

A

= Multimeric assembly of proteins (4* structure)

41
Q

Polypeptide modification in ER (RER Lumen) (#4 only) =

Example: (2)

A

= Amino acid modification

  1. ) Proline modified to hydroxyproline
  2. ) Lysine modified to hydroxylysine
42
Q

Integral membrane proteins =
…using… (2)

They may also have an…

A

Proteins with transmembrane domains are inserted into the ER membrane during protein synthesis (cotranslational import)…
…using “internal”
1.) “Stop-Transfer sequences” or
2.) “Start-Transfer sequences”

…N-terminal ERSS

43
Q

(Internal Stop-Transfer Sequence) (lowkey & ERSS)

  1. ) The stop-transfer sequence is…
  2. ) is located in…
  3. ) stops the…
  4. ) Allows for…
  5. ) Ribosome will…
  6. ) The N-terminal region will be…
A
  1. ) …hydrophobic
  2. ) …the interior of the polypeptide (not at the N-terminus)
  3. ) …process of translocation at that site
  4. ) …the transfer of that region (the hydrophobic stop-transfer sequence) of the protein laterally out of the translocon and into the lipid bilayer via a side opening of the translocon
  5. ) …disengage from translocon
  6. ) …inside the ER cisternae and the C-terminus will be in the cytosol
44
Q

(Internal Start-Transfer Sequence)

What is not required?

A

no ERSS is required

45
Q

(Internal Start-Transfer Sequence)

  1. ) aka…
  2. ) is…
  3. ) Signals the…
  4. ) Thus, after the polypeptide is partially formed, the start-transfer sequence tells…
  5. ) Start-transfer sequence enters…
  6. ) Then…
  7. ) Depending on the…
  8. ) Then, the…
  9. ) Involves the
A
  1. ) …signal anchor sequence
  2. ) …hydrophobic
  3. ) ..SRP to dock with the translocon
  4. ) …the ribosome to dock with the ER
  5. ) …the translocon
  6. ) …the remaining polypeptide is formed.
  7. ) …directionality of the start-transfer sequence, either the C-terminus or N-terminus enters the ER cisternal space
  8. ) …hydrophobic start-transfer sequence of the protein is transferred laterally out of the translocon and into the lipid bilayer via a side opening of the translocon
  9. ) SRP, SRP receptor and GTP
46
Q

Multiple Transmembrane domains =

…allows for…

A

Alternating patterns of start-transfer (signal-anchor sequence) and stop-transfer signals
…multiple transmembrane domains in a single polypeptide

47
Q

Smooth ER has

A

no ribosomes

48
Q

SER acts as

A

a series of partitions to isolate specific regions of the cytoplasm for specific functions

49
Q

SER specific functions: (5)

A
  1. ) Drug detoxification
  2. ) Carbohydrate metabolism
  3. ) Ca2+ storage
  4. ) Membrane biosynthesis
  5. ) Steroid hormone biosynthesis
50
Q

(SER Drug detoxification)

example =

which…

this makes it…

A

= Cytochrome P-450 system

…adds a hydroxyl (-OH) to the drug

…more water soluble (aids in drug removal via the renal system)

51
Q

(SER Carbohydrate metabolism)

2 examples:

A
  1. ) Glycogen breakdown

2. ) Glucose formation in liver (glucose-6-phosphatase)

52
Q

(SER Ca2+ storage)

Ca2+ is a…

Needs…

Store Ca2+ in…

In skeletal and cardiac muscle, the SER is…

A

…ubiquitous second messenger

…a mechanism to regulate the free [Ca2+]

…the ER

…modified into the sarcoplasmic reticulum (modified Smooth ER) that stores Ca2+

53
Q

(SER Membrane biosynthesis)

Many…

A

…enzymes associated with membrane biosynthesis are localized to the SER

54
Q

(Steroid hormone biosynthesis)
Many…
Endocrine cells that…

A

…enzymes associated with steroid hormone biosynthesis are localized to the SER
…produce steroid hormones have lots of SER

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