Signal peptides

Question 1

What are the characteristics of an ER import signal?

Answer 1

ER signals are transmembrane and have a hydrophobic core.

A signal recognition protien (SRP) binds to the signal peptide, and arrests translation until the ribosome is translocated to the rought ER.

An SRP receptor binds to the SRP and determines whether or not the N terminal side should face the cytosol or lumen.

A stop transfer sequence ends translantion into the membrane. Multiple start and stop trasfer seuqnces facilitate the formation of proteins with multiple transmembrane motifs.

Leu-Leu-Leu-Val-Gly-Ile-Leu-Phe-Trp-Ala

Question 2

What is the KDEL sequence?

Answer 2

KDEL is a signal peptide directing protiens to the lumen of the ER, the signal is located at the C-terminal end.

KDEL is involved in retrograde transport from the golgi to the ER when mistakenly taken up. COP 1 facilitates the transport from the golgi to the ER.

Model sequence

Lys-Asp-Glu-Leu-COO-

Question 3

Which import sequence is characterized by very basic amino acids?

Answer 3

The nuclear import sequence is characterized by basic amino acids.

example sequence

Lys-Lys-Lys-Arg-Lys

Question 4

Which import sequence is characterized by an amphateric alpha helix?

Answer 4

The mitochondrial petide import sequence has a amphateric alpha helix.

Question 5

What does the SKL seuquence stand for and which organel does it direct proteins to?

Answer 5

Peroxisomes always have Serine, Lysine, Leucine at the C-termial.

-Ser-Lys-Leu-COO- (SKL)

Question 6

Which peptide transfer signal is characterized by a leucine rich sequence?

Answer 6

Nuclear export sequence.

Question 7

What is the role of Ran in nuclear import/export?

Question 8

What is the difference between RanGAP and RanGEF?

Answer 8

RanGAP(GTpase activating protein) is found in the cytosol and converts RanGTP to RanGDP.

RanGEF(Guanine nucliotide exchange factor) is found in the nuclius and converts RanGDP to RanGTP.

Question 9

How does the removal/addition of GTP facilitate nuclear import?

Answer 9

Nuclear Import

importin, RanGAP, RanGEF, GTP, protien with basic signal sequence

RanGEF adds GTP to Ran in the nuclius releases the cargo. Ran–importin are then free to exit the nuclius.

RanGAP removes GTP which releases Ran from importin freeing importin to bind to an import signal and Ran to bind to the import complex

Question 10

How does the removal/addition of GTP facilitate nuclear export?

Answer 10

Nuclear export

importin, RanGAP, RanGEF, GTP, protien with leucine rich signal sequence

RanGAP removes GTP which releases Ran-exportin from the exported protien. GDP, Ran-exportin enter the nuclius.

RanGEF adds GTP to a Ran-exportin ternary complex bound to a protien with an export signal. This allows the protien to exit through the nuclear pores.

Question 11

Which common chaperone is used to move a protein through the two mitochondrial membranes?

What is is mechansim?

What needs to be done to the protien once it enters the inner mitochondrial matrix?

Answer 11

Hsp70 uses a ratched mechanism to bind to the hydrophobic residues of the protien and pulls it into the mitochondria in an ATP-dependent process.

Hsp70 and Hsp60 in the mitochondrial matrix help refold proteins that pass through the two membranes.

Question 12

What are some characteristics of the peroxisome?

How do protiens enter the peroxisome?

Answer 12

Peroxisomes have oxidative environments

• Oxidizes ethanol to acetyladehyde
• Oxidation of branched chain fatty acids
• Oxidation of cholesterol to bile acids (liver)

peroxisome membrane transport

• proteins are inserted into the peroxisome pos-tranlationally, with the C-terminus containing the signal entering the peroxisome first