Signal Sequences

Question 1

Import into the ER

Answer 1

8 or more consecutive nonpolar (hydrophobic) amino acids
near N-terminus (usually)
could be away too

Question 2

Import into mitochondria/chloroplasts

Answer 2

heptamer
every 4th and then 3rd amino acids are positively charged (R/K/L)
always near N-terminus
creates an asymmetrically-charged alpha helix

Question 3

Import into the nucleus

Answer 3

Pro-Pro- 5 basic (positively charged) amino acids - at least 1 NPAA
can be anywhere in the polypeptide primary sequence
exposure of the signal can depend on phosphorylation

Question 4

Retention in the ER

Answer 4

KDEL sequence at C-terminus
and
initial ERSS

Question 5

Import into peroxisomes

Answer 5

SKL sequence, often near the C-terminus

Question 6

ER transmembrane protein

Answer 6

either
first ERSS in the middle of the polypeptide
or
first ERSS at N-terminus and second stop sequence found somewhere in the middle of the polypeptide

Question 7

ER transmembrane protein orientation

Answer 7

(+) faces cytosol
(-) faces ER lumen
if (+) is near N-terminus, (-) is near C-terminus, N-terminus would be in the cytosol, C-terminus would be in the ER lumen
if (-) is near N-terminus, (+) is near C-terminus, N-terminus would be in the ER lumen, C-terminus would be in the cytosol

Question 8

Internal NPAA sequence (8 or more AAs)

Answer 8

ER transmembrane protein

Question 9

How are signal sequences removed?

Answer 9

cleaved off by the action of signal peptidases