Protein Trafficking 1

Question 1

lysosomes

Answer 1

lysosomal membrane separates digestive enzymes in the lysosome from cytopalsm- pH lower in lysosomes, optimum for enzymes

Question 2

where does proteinsynthesis take place

Answer 2

ribsomes, exception is mitochondria and chloroplasts, have their own PS machineries that synthesise mito/chloro encoded proteins

Question 3

After PS in the cytoplasm where are protein sent

Answer 3

nucleus, ER (trafficking), mito

targeted to diff areas

Question 4

Sorting signals

Answer 4

tell proteins where to go- code within the protein which is its targeting sequence
Specific stretches of AAs act as sorting signals
Sorting machineries recognise sorting signals and facilitate the sorting

Question 5

Nuclear targeting

Answer 5

proteins targeted to the nucleus contain stretches of +ve AAs
When there’s >1 stretch, the protein folds up so sequences close together

Question 6

Nuclear pores

Answer 6

Each pore composed of large number of distinct protein subunits
Long fibrils projecting into cytoplasm, basket like structure on other side

Question 7

homomeric

Answer 7

complex made up of all same type of subunit

Question 8

heteromeric

Answer 8

made up of different types of subunits

Question 9

Movement of molecules through nuclear pores

Answer 9

AAs, sugars etc small so can freely diffuse through nucelar pores
Larger molecules actively transported

Question 10

Protein import through the nuclear pore complex

Answer 10

Nuclear import receptors recognise NLS’s on prospective nuclear protein and bind, forming a complex
Complex guided to nucelar pore by fibrils
Binding of nuclear protein to pre opens it, and active transport into nucleus (with receptor), using GTP hydrolysis
Protein imported into nucleus in fully folded state

Question 11

NLS

Answer 11

nuclear localisation sequence

Question 12

Disassembling of complex to release cargo in the nucleus

Answer 12

Ran-GTP binds to nuclear import receptor, conform change, receptor loses affinity for its cargo

Question 13

Exporting receptor out

Answer 13

Receptor exported out of nucleus with RAn-GTP attached
When in cytoplasm, Ran hydrolyses GTP to GDP, causing release of energy and conform change, so Ran GDP dissociates as has different shape and less affinity for receptor.
Nuclear import receptor can bind new cargo
A nuclear RanGEF can swap GDP for GTP so Ran becomes GTP loaded

Question 14

Mitochondrial targeting

Answer 14

Mito targeting sequences different from nuclear localisation sequences
Always at N terminus
20-80 AAs, +ve charge
Makes amphipathic alpha helix- 1 polar and 1 non polar face
AAs 4 apart, all face same direction

Question 15

How does mito targeting work

Answer 15

receptors on outer mito membrane recognise mito targeting sequences

Question 16

How do proteins get into mitochondria

Answer 16

have to cross 2 membranes
via protein conducting channels- TOM (translocator of outer mem) and TIM
- chaperones in cytoplasm keep proteins to be transported in unfolded form eg Hsp70
- Translocation requires membarne potential
-energy from ATP hydrolysis
- Both cytoplasmic and matrix chaperones hydrolyse ATP to release translocating protein
- Mito targeting sequence cleaved off by proteases in mitochondria