Chloroplasts

Question 1

describe the similarities between the Mit and chloroplast protein import system

Answer 1

protein is translated in the cytosol and contains an Nterm presequence. the proportion binds the TOC/TIC machinery which is analogous to TOM/TIM. Proteins import in the unfolded conformation. it requires chaperones

Question 2

what is the main difference between the mist and chloroplast protein import system

Answer 2

requires GTP and ATP whereas mit just needs ATP. delta psi only needed for ATP synthesis and not for protein import

Question 3

what is the presequence on proteins targeted to the stroma

Answer 3

lots of hydroxylated amino acids at the N term with overall positive charge

Question 4

what is the presequence on proteins targeted to the thylakoid

Answer 4

helical section with an N term positive charge and C term small side chain amino acid

Question 5

why do uncouplers inhibit transport in the light but not in the dark

Answer 5

because in the light they prevent ATP synthesis but in the dark, if ATP is added import can occur showing membrane potential is not necessary in chloroplasts

Question 6

which proteins comprise the cytosol guidance complex

Answer 6

hsp70 and 14-3-3 protein

Question 7

describe the process of import protein docking at TOC complex

Answer 7

hydroxyl containing presequence is phosphorylated co-translationally by protein kinase which is specific for transit peptides. It then can bind the hsp70/14-3-3 cytosol guidance complex. this then docks at TOC complex in the outer envelope. the precursor is then dephosphorylated before import

Question 8

what is the role of TOC75

Answer 8

it is analogous to TOM44 and forms the voltage gated cation selective channel

Question 9

what is the role of Toc34

Answer 9

receptor protein for protein docking in GTP dependent fashion. may have a role in regulating import

Question 10

what is the role of toc64

Answer 10

it is the receptor for proteins which use hsp90 and proposed binding site for the guidance complex

Question 11

what is the role of toc159

Answer 11

this is the major receptor which binds ATP and GTP. its N term is highly negative for binding positive presequence. this works with toc34

Question 12

how does toc34/159 work

Answer 12

preprotein binds to phosphorylated toc34 which activates GTP hydrolysis to GDP. this drives conformational changes allowing toc159 to help deliver the proportion though the pore, acting as a motor

Question 13

what are the components of the tic machinery

Answer 13

tic110,40,20,22,32,55 and 62

Question 14

what is the role of tic110

Answer 14

associates with stroll chaperones and recruits them to drive translocation and folding

Question 15

what is the role of tic40

Answer 15

it associates with tic110. the majority of it faces the storm with an n term membrane anchor has homology to hsp70 at the cterm

Question 16

what is the role of of tic22

Answer 16

it is a peripheral membrane protein facing the IM space which may be a receptor or mediate association of tic and toc complexes

Question 17

what is the role of tic20

Answer 17

could possible form the channel

Question 18

what is the role of of tic 32, 55 and 62

Answer 18

they all have potential to regulate via redox potential. 32 and 55 bind NADPH and 62 binds FNR, linking ETC with translocation machinery

Question 19

which chaperones are thought to be involved in outer envelope insertion of proteins

Answer 19

AKR2 and tok75

Question 20

give an example of a protein which is dual targetted to the the chloroplast and mitochondira

Answer 20

tRNA synthetases

Question 21

how can dual targeted proteins be sorted to 2 places

Answer 21

potentially as a result of different transcription start sites, alternative splicing or an ambiguous presequence which is recognised by both machinery

Question 22

why is it thought that some chloroplast proteins go first to the ER

Answer 22

they have an ER signal sequence

Question 23

how is carbonic anhydrase delivered to the chloroplast

Answer 23

it goes first to the ER, travels to the golgi then is glycosylated then reaches the chloroplast b vesicular trafficking