BI203 Lecture 17: Protein Sorting and Transport - Slides 21-36 (Transmembrane ER Proteins and Lipid Synthesis)

Question 1

Transmembrane ER proteins are stretches of 20-25 ___ amino acids. They also allow both ___ (N or C sticking out).

Answer 1

hydrophobic, orientations

Question 2

The membrane-spanning regions of integral membrane proteins are usually ___ regions with hydrophobic amino acids.

Answer 2

a-helical

Question 3

Many proteins are inserted directly into the ER membrane by ___ ___ sequences.

These sequences are recognized by SRP, but not cleaved by signal peptidase.

The transmembrane ___ ___ exit the translocon laterally and anchor proteins in the ER membrane.

Answer 3

internal transmembrane sequences, a helices

Question 4

Polypeptides can be anchored in either ___ across the membrane.

Answer 4

direction

Question 5

Some proteins have an amino terminal signal sequence, and a transmembrane α helix in the middle of the protein that halts ___ and anchors the polypeptide in the membrane.

Answer 5

translocation

Question 6

Transmembrane proteins can also have ___ membrane spanning domains.

Answer 6

multiple

Question 7

Proteins that span the membrane multiple times are inserted by a(n) ___ ___ of internal signal sequences and transmembrane stop-transfer sequences.

Answer 7

alternating series

Question 8

Many proteins undergo post-translational modification in the ER before being transported to the ___ ___.

Answer 8

Golgi bodies

Question 9

An example of post-translational modification to polypeptides in the ER:

___ ___ - The addition of pre-assembled oligosaccharides to specific asparagine (N) residues.

Answer 9

N-linked glycosylation

Question 10

Proteins are glycosylated (N-linked glycosylation) in the ER while translation is still in progress.
The oligosaccharide is synthesized on a lipid (___) carrier.

Answer 10

dolichol

Question 11

Glycosylation helps ___ protein folding (aggregation) in the ER and provides ___ for subsequent sorting.

Answer 11

prevent, signals

Question 12

An example of post-translational modification to polypeptides in the ER:

___ ___ - Addition of glycolipids to the C-terminus of proteins, which anchors them to the membrane.

Answer 12

GPI anchors

Question 13

N-linked glycosylation - The addition of pre-assembled oligosaccharides to specific ___ (N) residues.

Answer 13

asparagine (N)

Question 14

GPI anchors - Addition of glycolipids to the C-terminus of proteins, which ___ them to the membrane.

Answer 14

anchors

Question 15

GPI anchors are assembled in the ER membrane and added to the ___ terminus of some polypeptides.
GPI-anchored proteins are transported as membrane components via the secretory pathway.
Their orientation within the ER dictates they will be exposed on the outside of the cell.

Answer 15

carboxyl

Question 16

Protein folding in the ER is slow and inefficient, and many are misfolded.

They are rapidly degraded by the ___ ___ (ERAD) process:
Misfolded proteins are identified, returned to the cytosol, and degraded by the ubiquitin-proteasome system.

Answer 16

ER-associated degradation (ERAD)

Question 17

Chaperones and protein processing enzymes in the ER lumen act as “___” of misfolded proteins.

Answer 17

sensors

Question 18

Chaperones and protein processing enzymes in the ER lumen act as sensors of misfolded proteins.

One pathway involves the chaperones ___ and ___, which assist glycoproteins to fold correctly.

Answer 18

calnexin, calreticulin

Question 19

A protein folding sensor assesses the folded glycoproteins; if correctly folded they ___ the ER.

If not folded correctly, the folding sensor will add back a ___ residue, allowing it to cycle back to calnexin or calreticulin for another attempt at folding.

Answer 19

exit, glucose

Question 20

A severely misfolded glycoprotein is recognized by ___, which removes mannose residues.

Answer 20

EDEM1 (part of the ERAD process)

Question 21

A severely misfolded glycoprotein is recognized by EDEM1, which removes ___ residues.

Answer 21

mannose

Question 22

A severely misfolded glycoprotein is recognized by EDEM1, which removes mannose residues.

The protein is returned to the cytosol through a ubiquitin ligase complex where it is marked by ___ and degraded in a proteasome.

Answer 22

ubiquitin

Question 23

If an excess of unfolded proteins accumulates, a signaling pathway called the ___ ___ ___ (UPR) is activated.
It leads to expansion of the ER and production of more chaperones.
If protein folding can’t be adjusted to a normal level, the cell undergoes ___.

Answer 23

unfolded protein response (UPR), apoptosis (programmed cell death)

Question 24

Unfolded protein response activates 3 receptors in the ER membrane:

Answer 24

IRE1, ATF6, PERK

Question 25

___ cleaves pre-mRNA of a transcription factor (XBP1). XBP1 translocates to the nucleus and stimulates transcription of UPR genes.

Answer 25

IRE1

Question 26

___ is cleaved to release the active ___ transcription factor.

Answer 26

ATF6

Question 27

___ is a protein kinase that phosphorylates translation factor eIF2, which inhibits general translation and reduces the amount of protein entering the ER.

Answer 27

PERK

Question 28

IRE1 cleaves pre-mRNA of a transcription factor (___). ___ translocates to the nucleus and stimulates transcription of UPR genes.

Answer 28

XBP1

Question 29

PERK is a protein kinase that phosphorylates translation factor ___, which inhibits general translation and reduces the amount of protein entering the ER.

Answer 29

eIF2

Question 30

Because they are ___, membrane lipids are synthesized in association with already existing membranes rather than the aqueous cytosol.

Most lipids are synthesized in the smooth ER.

Answer 30

hydrophobic

Question 31

Synthesis of phospholipids on the ___ side allows the hydrophobic fatty acid chains to remain buried in the membrane.

Answer 31

cytosolic

Question 32

New phospholipids are added only to the ___ half of the ER membrane. Some must be transferred to the other half.

Answer 32

cytosolic

Question 33

___ - Phospholipids are synthesized in the ER membrane from cytosolic precursors.

Answer 33

glycerol

Question 34

Two fatty acids linked to co-enzyme A (CoA) carriers joined to glycerol-3-phosphate, yielding ___ ___ (PA), which is inserted into the membrane. A phosphatase then converts PA to ___.

Answer 34

phosphatidic acid (PA), diacylglycerol

Question 35

Different polar head groups can then be attached to ___ to generate a variety of phospholipids.

Answer 35

diacylglycerol

Question 36

Translocation of phospholipids across the ER membrane requires passage of polar head groups through the membrane, facilitated by membrane proteins called ___.

Answer 36

flippases

Question 37

Flippases ensures ___ growth of both sides of the phospholipid bilayer.

Answer 37

even

Question 38

___ is synthesized in the smooth ER and is the precursor for ___ ___, therefore cells that make large amounts of ___ ___, such as those in testes and ovaries have large amounts of smooth ER.

Answer 38

cholesterol, steroid hormones

Question 39

___ is converted to either glycolipids or sphingomyelin (the only phospholipid not derived from glycerol) in the Golgi Bodies.

Answer 39

Ceramide

Question 40

Ceramide is converted to either glycolipids or sphingomyelin (the only phospholipid not derived from glycerol) in the ___ ___.

Answer 40

Golgi Bodies