2. protein targeting from the cytosol

Question 1

in the absence of a tgareting signal, where do newly shynthesised proteins remain

Answer 1

the cytosol

Question 2

what are the two types of protein made at the endoplasmic reticulum

Answer 2

water soluble secretory proteins

transmembrane proteins embedded in the lipid bilayer eg insulin receptor

Question 3

ER targeting signals

Answer 3

stetch of 8+ hydrophobic amino acids at the N terminus of protein
cleaved after targeting by signal peptidase, follows the -3, -1 rule (small hydrophobic amino acids near cleavge site)

Question 4

integral membrane proteins

Answer 4

N terminal signal sequence

+ stop transfer signal which acts as a transmembrane anchor ti stop its complete translocation into the ER lumen

Question 5

singal anchor sequence

Answer 5

acts as both an ER targeting signal and a transmembrane anchor
not cleaved from the protein after targeting

Question 6

cotranslational protein translocation

Answer 6

most proteins with an ER signal are targeted to their organelle whilst theyre still being made
the growing polypeptide chain is threaded across the membrane before synthesis is completed

Question 7

tail anchored proteins

Answer 7

have hydrophobic ER targeting signal that also acts as their membrane achor
inserted into membrane post translationally as tail anchor is at end of polypetide chain

Question 8

E.coli singal sequences

Answer 8

transport proteins to plasma membrane or periplasmic space

N terminal, cleaved like ER but instead by leader translocase

Question 9

cut and paste expeirments

Answer 9

used to udentify a signal sequence

used to see if signal is sufficient and or necessary

Question 10

Er targeting in test tube

Answer 10

no ER membrane provided then preproinsulin with signal attached, bigger size higher up in gel
ER membrane provided, proinsulin without a signal sequence. reduced size of protein

Question 11

ER targeting in the cell

Answer 11

add an ER signal sequence to GFP and analyse the location of this protein in cells

Question 12

nuclear localisation signal

Answer 12

targets proteins to nucleus
high proportion of positively charged, basic amino acids like lys and arg
often less than 12 aa, found anywhere on the protein and not cleaved

Question 13

experimental testing of NLS

Answer 13

fluorescent antibody

alter one amino acid of the NLS protein is unable to enter nucleus and remains in cytosol

Question 14

mitochondrial signal sequences

Answer 14

high content of Arg, Ser and Thr
N terminal , 20 - 80 amino acids long, cleaved after transport into mitochondrial matrix
forms amphipathic alphahelix

Question 15

chrloroplast signal sequence

Answer 15

N terminal
rich in ser and thr and hydrophobic amino acids
cleaved after import into stroma

Question 16

thylakoid targeting signal

Answer 16

is another signal sequence required

Question 17

PTS1

Answer 17

peroximal targeting signal type 1
SKL sequence, not cleaved
C terminus
short highly conserved aa sequence

Question 18

PTS2 signal

Answer 18

short less well defined motif of 9 aa
N terminus
not cleaved