Topic 5, Lecture 1

Question 1

What is the pathway from the endoplasmic reticulum to the cell exterior?

Answer 1

Secretory Pathway

Question 2

Will membrane proteins be secreted?

Answer 2

No, they are bound to the membrane.

Question 3

What is the most important aspect to the mechanism by which proteins are inserted into the membrane?

Answer 3

Orientation

Question 4

What are the four examples of transmembrane proteins?

Answer 4

Enzymes, Receptors, Anchors, and Transporters

Question 5

Describe receptors

Answer 5

Function at the surface of the cell to bring things inside and function at the surface of the cell as part of a signaling pathway, ligand-binding site must be on the outside of the cell,

Question 6

Describe anchors

Answer 6

Function to hold things in place

Question 7

Describe transporters

Answer 7

Function to move small molecules across the membrane

Question 8

Define signal anchor

Answer 8

Transmembrane segment of a protein that anchors it into the membrane thereby creating a transmembrane protein

Question 9

Describe a transmembrane domain

Answer 9

A transmembrane domain is the portion of a protein that sits directly in the membrane

Question 10

Describe Type I transmembrane proteins

Answer 10

A type I transmembrane protein had one transmembrane domain whose amino terminus is in the lumen of the endoplasmic reticulum and carboxy terminus is in the cytosol

Question 11

Describe Type II transmembrane proteins

Answer 11

A type II transmembrane protein has one transmembrane domain whose amino terminus is in the cytosol and carboxy terminus is in the lumen of the endoplasmic reticulum

Question 12

Describe Type III transmembrane proteins

Answer 12

A Type III transmembrane protein has a topology identical to that of Type I, but the mechanism for inserting it into the membrane is different. The amino terminal end on the side of the ER lumen is typically very short.

Question 13

Why must the amino-terminal end of the type III transmembrane protein be short?

Answer 13

There is no energy source for pushing the amino-terminal end into the ER lumen.

Question 14

Describe type IV transmembrane proteins.

Answer 14

A type IV transmembrane protein, also called a multi-pass transmembrane proteins, is “stitched” into the membrane.

Question 15

Describe the conservation of membrane faces.

Answer 15

The cytosolic leaflet will always be nearest to the cytosol. The exoplasmic leaflet must be on the inside of the vesicle so the exoplasmic leaflet will be on the outside once it is absorbed by the plasma membrane.

Question 16

Describe the mechanism by which type I transmembrane proteins are inserted into the membrane.

Answer 16

The type I has a typical signal sequence that functions as a start-transfer sequence and is recognized by the SRP. There is an internal signal sequence that functions as a stop-transfer sequence (signal anchor) that causes translocation to stop, and leave the translocon through a lateral exit gate. The ribosome will continue translating the protein, but the rest of the protein will be left in the cytosol.

Question 17

Describe the characteristics of the signal anchor.

Answer 17

There are about twenty-five amino acids that are attracted to the nonpolar region of the membrane. There are twenty-five amino acids because it is equivalent to the width of the membrane.

Question 18

Why are type II and type III transmembrane proteins different from type I?

Answer 18

Type I transmembrane proteins have a start and stop-transfer sequence, while type II and type III only have one internal signal sequence that is recognized by the SRP.

Question 19

How is the orientation of a type II or type III transmembrane protein determined?

Answer 19

It protein will enter based on which end of the signal sequence is flanked by positively charged amino acids.

Question 20

Describe the mechanism of a Type II transmembrane protein.

Answer 20

The signal sequence functions as a start-transfer sequence, so the amino-terminal end will be oriented on the cytosolic side of the membrane because the positive charges are located on the amino-terminal end of the signaling sequence. Once the SRP releases, the carboxy-terminal end will continue to be translated and pushed into the lumen.

Question 21

Describe the mechanism of a Type III transmembrane protein.

Answer 21

The signal sequence functions as a stop-transfer sequence, so the carboxy-terminal end will be oriented on the cytosolic side of the membrane because the positive charges are located on the carboxy-terminal end of the signaling sequence. Once the SRP releases, the carboxy-terminal end will continue to be translated and the amino-terminal end will be pushed into the ER lumen by an unknown energy source.

Question 22

Describe the mechanism for a double pass transmembrane protein.

Answer 22

A double-pass transmembrane protein must have two signal anchors: first a start-transfer sequence and then a stop-transfer sequence. The start-transfer sequence will be flanked with positive charges so everything after will be translated into the lumen. Once the stop-transfer sequence is translated, the rest of the protein will be in the cytosol.

Question 23

What are the two types of Type IV transmembrane proteins?

Answer 23

Type IV-A and Type IV-B

Question 24

Describe a Type IV-A transmembrane protein.

Answer 24

Type IV-A will always have its amino terminus in the cytosol.

Question 25

Describe a Type IV-B transmembrane protein.

Answer 25

Type IV-B will always have its amino terminus in the ER lumen.

Question 26

Why are SNARE proteins important?

Answer 26

Vesicular Trafficking

Question 27

Are SNARE proteins transmembrane proteins?

Answer 27

No, they do not span the entirety of the membrane. They are tail-anchored.

Question 28

Will SNARE proteins be recognized by the SRP?

Answer 28

No, the signal sequence is on the tail-end of the protein so it will be recognized by the pre-targeting complex.

Question 29

Describe the mechanism for SNARE protein insertion.

Answer 29

The pre-targeting complex will bind to the newly formed protein and then bind to a protein complex called Get3-ATPase. Get3 has a high affinity for a transmembrane protein complex. Get3 is then hydrolyzed and recycled.

Question 30

List the ways proteins can be embedded into the membrane.

Answer 30

Type I-III with hydrocarbon anchor, Type IV, Beta Barrels or Alpha Helices, Amphipathic Alpha Helix Signal Anchor, Anchored by a hydrocarbon tail, Anchored with a GPI anchor, Bound to the membrane indirectly via another transmembrane protein

Question 31

Describe the mechanism of a GPI anchor.

Answer 31

GPI anchor proteins have a preformed anchor that is added to the type I transmembrane protein via the transamidase enzyme (translocon). Transamidase cleaves the C-terminal group, and the carbohydrate core is added to a phospholipid.

Question 32

Can proteins move in the membrane?

Answer 32

Yes, think green/red mouse cell and FRAP experiments