Peroxisomes

Question 1

which genes encode peroxins which are involved in peroxisome biogenesis

Answer 1

PEX genes

Question 2

describe the various functions of peroxisomes in different organisms

Answer 2

B oxidation of fatty acids in mammals, fungi and plants.
photorespiration in plants.
glycolysis in tripanisomes.
penicillin synthesis in fungi

Question 3

what are the 2 classes of peroxisome targeting sequences

Answer 3

PTS1 and PTS2

Question 4

where is the PTS1 targeting signal found

Answer 4

N term

Question 5

which protein recognises PTS1

Answer 5

PEX5

Question 6

describe PTS1 protein binding

Answer 6

C term of PEX5 is made up of TPR repeats through which it binds to PTS1. the central domain of PEX5 binds PEX7 and repeat motifs (WXXXF?Y binds PEX14. this is required for receptor docking onto the membrane

Question 7

describe the PTS1 signalling sequence in relation to being necessary and sufficient

Answer 7

PTS1 peptides can be both necessary and sufficient however in some cases it is sufficient but not necessary and in others necessary but not sufficient. this depends upon the tripeptide- SLK is optimal bt less optimal motifs accepted but may need other factors for stabilising the interaction or the PTS1 may not be accessible but is necessary n the native context.

Question 8

describe the PTS1 signal sequence

Answer 8

SKL- optimal

S (or neutral, small amino acid), K (or charged aa), L (or hydrophobic aa)

Question 9

where is the PTS2 signal sequence found

Answer 9

N term

Question 10

what is the receptor for PTS2

Answer 10

PEX7 although PEX5 acts as a correceptor in plants and animals

Question 11

How does PEX5 act as a coreceptor for PTS2 protiens binding PEX7

Answer 11

there are 2 versions of PEX5: PEX5s and PEX5l. PEX5l has an additional exon with a PEX7 binding site

Question 12

in fungi, which proteins are used as correctors for PEX7 instead of PEX5

Answer 12

pex18 and pex21 in s.cerevisiae or PEX20 in others

Question 13

describe how, in some cases, import can occur despite removal of the PTS1 signal sequence

Answer 13

S.cerevisiae Acyl CoA oxidase forms novel contacts with PEX5 outside the TPR domain.
In other cases subunits of an oligomer are able to piggy back on other subunits which have a PTS1 signal as proteins can be imported in an oligomeric form

Question 14

what is the cause of primary hyperoxaleuria type 1 (PH1)

Answer 14

AGT has 2 mutations- P11L creates a mitochondrial targetting signal by forming a amphipathic helix and G170R inhibits dimerisation and slows folding which allows the new mit targeting signal to be recognised by import machinery and thus imported into the mitochondria. this results in the non-removal of glycoxalate causing it to convert to oxalate, bind to calcium and precipitate causing kidney damage.

Question 15

how is complex assembly for protein import into peroxisomes driven

Answer 15

thermodynamically favourable binding interactions

Question 16

describe the import cycle in peroxisomes

Answer 16

PEX5 delivers protein to the membrane to bind PEX14 and to a lesser extent PEX13. PEX5 inserts itself into the membrane and forms a gated ion channel with PEX14. proteins are transported through the pore (which opens to a diameter of 9nm) and released by an unknown mechanism,

Question 17

describe how the peroxisome protein import receptors are recycled

Answer 17

first, ubiquitin is activated by an E1 ligase. PEX4, an E2 ligase hen transfers the Ub onto PEX5 with the assistance of E3 ligase PEX12. The Ub is linked to a Cys residue at position 11 which tags the receptor for release into the cytosol. in the event that the receptor is stuck in the membrane, PEX4 and PEX12 polyUb a lysine residue which results in removal of the receptor by PEX1 and PEX6 and then degradation by the proteasome

Question 18

why is ATP require for protein import into the peroxisome

Answer 18

because it is required for ubiquitination and therefore recycling of the receptor

Question 19

why are chaperones not generally implicated in import into the peroxisome?

Answer 19

because proteins can be imported in the folded conformation