compartmentalisation of cells

Question 1

different types of protein targeting

Answer 1

• gated transport
  
  • (nucleus→ cytoplasm)
• proteins imported through membranes
  
  • plastids/mitochondria
• vesicular transport
  
  • packaging

Question 2

function of nuclear pore

Answer 2

allow transport in and out

Question 3

where are nuclear pores formed

Answer 3

• pores are formed at junction of inner and outer membranes of nuclear envelope
• consists of multiple copies of around 30 different nucleoporins

Question 4

functions of nuclear pores

Answer 4

• gating material into nucleus
• histones made in cytoplasm → need transporting to nucleus
• ribosomal subunits made in nucleolus need exporting

Question 5

what size molecules can diffuse freely

Answer 5

5000 D = diffusible
60,000 = cannot enter by diffusion

Question 6

how do molecules know to actively transport

Answer 6

• with appropriate signal, pore can increase in size to allow proteins to move into pore
• nuclear localisation signals = sequence of amino acids
• polar, charged = lys, arg, pro

Question 7

where are most proteins made

Answer 7

most proteins are made in endoplasmic reticulum

Question 8

why must ribosomes be associated closely with ER membrane

Answer 8

membrane proteins undergo cotranslational translocation or post-translocational translocation

Question 9

how are membrane proteins synthesised

Answer 9

• proteins made in RER
• modifications in ER
• signal
• ribosome associated
• signal and ribosome site on translocator
• fed through pore

Question 10

role of chaperones in membrane protein synthesis

Answer 10

folds assisted by chaperones (rish in ER) as well as protein disulfide isomerases

Question 11

how are membrane proteins inserted into ER (single pass type I) ?

Answer 11

1. signal sequence stimulates transfer
2. signal is recognised by sec61 (translator) = opens
3. newly translated protein is fed through translocator
4. before going straught through, stop transfer sequence in protein (hydrophobic) allows protein to remain anchored in membrane
5. rest of protein (C-terminus) is exposed to cytoplasm

Question 12

how are membrane proteins inserted into ER (single pass type II) ?

Answer 12

• single pass type II
  
  • C terminus = outside of cell
  • N terminus = cytoplasm
  • signal sequence acts as TM domain rather than stop codon

Question 13

function of chaperones

Answer 13

chaperones ensure proper folding occurs e.g BIP stay attached on proteins until they fold or until folding correct structure

Question 14

function of glycosilation in protein folding

Answer 14

allows effective quality control of protein folding

Question 15

what happens if proteins are not correctly folded

Answer 15

• proteins are reverse translocated via eversion of same machinery
• moved out of ER lumen into cytosol and broken down

Question 16

how can protein misfolding leading to disease

Answer 16

e.g CFTRdelta508
- deletion of this amino acids causes misfolding of CFTR that is retained in ER
- excess misfolding protein → unfolded protein response

Question 17

what is the response to excess misfolding protein

Answer 17

1. close down translation = no protein synthesis
2. upregulate (via TF) synthesis of chaperones
3. activation of apoptytic pathways if severe

Question 18

types of protein targeting

Answer 18

• gated transport
  
  • (nucleus→ cytoplasm)
• proteins imported through membranes
  
  • plastids/mitochondria
• vesicular transport
  
  • packaging

Question 19

structure and function of nuclear pore

Answer 19

s
- pores are formed at junction of inner and outer membranes of nuclear envelope
- consists of multiple copies of aorund 30 different nucleoporins
f
- gating material into nucleus
- histones made in cytoplasm → need transporting to nucleus
- ribosomal subunits made in nucleolus need exporting

Question 20

how is material transported through nucleus

Answer 20

diffusion (5000kD)
active transport = adjust pore size

Question 21

what are nuclear localisation signals in transport

Answer 21

• sequence of amino acids
  
  • polar, charged = lys, arg, pro
  • mutation of amino acids = disrupts translocation of proteins

Question 22

how do proteins get into mitochondrial matrix

Answer 22

1. N terminal signal sequence recognised by TOM complex
2. protein translocates through TOM and TIM23 into matrix
3. signal is cleaved off

Question 23

structure of signal sequence peptide

Answer 23

• amphipathic alpha helices structure
  
  • polar and non polar residues = one side hydrophobic/one side hydrophilic
    hydrophobic residues bind in hydrophobic groove

Question 24

function of signal sequence peptide

Answer 24

is able to direct protein translocation through the membrane

Question 25

how are proteins translocated into bacterial membrane

Answer 25

• similar mechanism to import into mitochondria due to origin
• mitochondria and bacteria have porins in outer membranes = exchange of metab and ions
• TOM complex translocates in polypeptide chains which combine with chaperones allowing protein to assemble

Question 26

how are proteins translocated into chloroplasts

Answer 26

• receptor protein of TOC complex
• through TOC complex
• through TIC complex
• cleavage of signal sequence
• enters thylakoid space