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Biol 331 > Intro To The Endoplasmic Reticulum > Flashcards

Intro To The Endoplasmic Reticulum Flashcards

1
Q

What is the ER

A

A system of membranes and vesicles that forms the ER and encloses the ER lumen

The lumen is separated from the cytosol

It’s divided into the rough and smooth ER

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2
Q

What is the RER

A

Has ribosomes bound on the cytosolic membrane side of the er (outside organelle)

It’s made of a network of cisternae and it’s connected and stems from the outer membrane of the nuclear (nucleuses) envelope

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3
Q

What is the SER

A

No ribosomes

Made of interconnected curved tube like membranes

Stems from (continued from) the rough ER

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4
Q

What is the function of the rough ER

A

Role in protien synthesis and addition of surfaces to protiens

The RER is more present in Cells that have role in protien secretion

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5
Q

What is the function of the smooth ER

A

Make steroid hormones and membrane lipids, Detox organic compounds in the liver

Concentrates calcium ion in skeletal and cardiac muscles (playing a role in muscle contraction)

It’s more present in cell types of skeletal muscles, kidney tubules and steroid producing endocrine glands

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6
Q

What are examples of the RER in use

A

Acinar cells in the pancreas that secrete hydrolytic enzymes

Intestinal cells that secrete mucoprotiens

Endocrine cells that secrete polypeptide (amino acid based) hormones

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7
Q

What are the two types of ribosomes

A

RER

FREE

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8
Q

what are RER ribosomes

A

Make 1/3 of protiens

The protiens made are secreted protiens, integral membrane protiens and soluble protiens that are in parts of the Endomembrane system (in golgi, lysosomes, etc.)

They implement cotranslational translocation where peptides of the protien move into the lumen of the ER while the ribosome on the ER is making them

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9
Q

What are free ribosomes

What do they make

A

The make 2/3 of protiens

Not attached to ER, the protiens made from the ribosomes are released to the cytosol and are directed to go else where if needed

They make:
Protiens that stay in the cytosol

Peripheral (weakly bound) protiens on the cytosolic side of membranes

Proteins that are moved to the nucleus, mitochondria and chloroplasts

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10
Q

The FREE and RER ribosomes are ____

A

Structurally and functionally identical

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11
Q

Where does all protiens synthesis begin

What else is synthesized first

A

On a free ribosome (not at the ER)

A signal sequence at the N terminal end of the peptide gets synthesized first, it’s made of 6-15 hydrophobic amino acids

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12
Q

What is step 1 in co trans location for secreted protien and soluble protien in the Endomembrane system

A

A free floating signal recognition particle (SRP) binds to the signal sequence of the peptide and to the ribosome making the peptide

This stops the poly peptides synthesis temporarily

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13
Q

What is step 2 in co trans location for secreted protien and soluble protien in the Endomembrane system

A

The SRP directs the ribosome/peptide complex to the ER by binding to a SRP receptor on the ER membrane

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14
Q

What is step 3 in co trans location for secreted protien and soluble protien in the Endomembrane system

A

The SRP on the receptor tranferrse the ribosome/peptide complex to the translocon (protien pore) in the ER membrane

The SRP leaves

The plug inside the translocon reacts with the hydrophobic signal sequence on the peptide and gets displaced (opening up the pore)

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15
Q

What is step 4 in co trans location for secreted protien and soluble protien in the Endomembrane system

A

The polypeptide continues to be synthesized and enters the ER lumen (inside of Er)

after translation is terminated The ribosome is released from the membrane and synthesizes elsewhere

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16
Q

What happens to the protien after the ribosome is done translating the peptide in cotranslocation

A

Signal peptidase removes the signal sequence from the peptide

Protein chaperone BiP helps fold the protien properly

17
Q

What is the first step of cotranlosacton for the synthesis of integral membrane protiens

A

The protien is made and in the ER the same way as before

But the SRP recognizes the 20AA LONG transmembrane domain of the protien as the signal sequence and brings that to the ER

The polypeptide passes through the translocon pore but not fully, it inserts itself into translocon pore and decides where to go from there

18
Q

For translocation of integral membrane protiens how does the protien decide where to go once in the translocon

A

The protiens can decide to move itself out of the pore and into the ER membrane using the gates at the side of the pore

If it’s hydrophobic enough to to partition into the lipid membrane, the protien will laterally exit the pore

If not it’s will stay in the pore

19
Q

For translocation of integral membrane protiens how does the protien exit the pore and orient into the ER bilayer

A

The direction that the peptide (N or C) is inserted depends on the placement of the positively charged amino acids

The region of the protien that will be flanking (sticking into) the cytosolic end will be the region with the positively charged amino acids POSTIVE END IN CYTOSOL

Since PS and PI phospholipids have negatively charged head groups, they would be facing the lumen NEGATIVE END IN LUMEN

20
Q

what amino acids are positively charged

A

Arg

Lys

His

21
Q

What do majority of the protiens made at the RER undergo

A

Glycosylation (sugars are added to them)

They turn into glycoproteins

22
Q

What role do carbohydrates play in Glycosylation

A

They play a role as binding sites and help in proper folding/stability of the protien

They sort and direct protiens to diff cellular components

23
Q

What is Nlinked Glycosylation

A

Common type of Glycosylation

A sugar linkage to asparagine (which is part of the larger protien) that is initiated in the RER

24
Q

What is O linked Glycosylation

A

The sugars links to a serine or threonine in the larger peptide.

This happens in the golgi complex

25
Q

What is the first step to Nlinked Glycosylation

A

Seven sugars are first transferred one at a time to lipid molecule in the ER membrane called dolichol pyrophosphate

The assembly of the sugars are on the cytosolic side (outside the ER)

The sugars are added by glycosyltranferases in the ER membrane

26
Q

What is the second step to Nlinked Glycosylation

A

Dolichol pyrophosphate with its now attached oligosaccharide (7 sugars) is flipped across the membrane

Now the sugars are in the lumen of the RER

Remaining sugars and 3 glucose from the cytosolic region add to dolichol and dolichol flips and these get added onto the growing chain inside the lumen

27
Q

What is the third step to Nlinked Glycosylation

A

The completed oligosaccharide in the lumen is transferred over to an ASPARAGINE (asn) residue of the polypeptide that’s being translated and undergoing cotanslocation

28
Q

In the third step of n linked Glycosylation, how does the sugars get transferred to the peptide

A

It gets transferred by the enzyme oligosaccharyltransferase to an asn with this sequence:

Asn-x- ser/thr

ASN can be in middle or start of peptide

X is any AA expect proline since it causes kinks and won’t be recognized by enzymes

29
Q

What are the first and second steps of quality control for misfolded proteins during co translocation

A
  1. This sugars are already on it, so glucosidase 1 and 2 remove two glucoses from the protien, leaving only one
  2. The remaining glycoprotein with 1 glucose is recognized by Calnexin (a chaperone protien inside the ER that helps protiens fold properly
30
Q

What are the third and fourth and fitfth steps of quality control for misfolded proteins during co translocation

A
  1. Calnexin removes the last glucose from the glycoprotein , if correctly folded the protien exits the membrane
  2. If still misfolded, protiens are recognized by UGGT (conformation sensing enzyme) which detects exposed hydrophobic residues (show of improperly folded) and adds a glucose molecule
  3. Calnexin process happens again and tries to refold the protien
31
Q

What are steps 6-8 of quality control for misfolded proteins during co translocation

A
  1. If properly folded the protiens exit

7-8. If still not properly folded after many attempts the protiens are degraded through a proteosome in the cytosol

Biol 331 (18 decks)

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