Cell compartmentalization and transport (Lec 14)

Question 1

Why do eukaryotic cells contain membrane-enclosed organelles?

Answer 1

Membrane-enclosed organelles increase the surface area of the plasma membrane and provide optimal conditions for specific reactions to occur on the surface of lipid membranes.

Question 2

How do membrane-enclosed organelles promote optimal conditions for specific reactions?

Answer 2

Membrane-enclosed organelles promote optimal conditions for specific reactions occurring on the surface of lipid membranes by providing:

1) high [substrate] for specific enzymes
2) optimal biochemical environment (pH, salt, oxidative, penitential)
3) High [used enzymes]

Question 3

What are the basic features of membrane-enclosed organelles?

Answer 3

Membrane-enclosed organelles are differentially abundant and serve the same basic function in different cells.

Membrane-organelles have additional properties in specialized cells

Question 4

Topological relationship of organelles may be explained by?

Answer 4

evolutionary origins

Question 5

Based on topological classification, how are the cytoplasmic content divided?

Answer 5

Cytoplasmic division classified into different spaces.

Inside (acronym- CNC): nucleus, cytoplasm, cilium

Outside: secretory/endocytic system, cell exterior; ER, golgi, endo, lyso, peroxisomes

Endosymbiont: mitochondria and plastids

Question 6

How can proteins move between different compartments?

Answer 6

Protein synthesis starts at the cytosol (except proteins translated in mitochondria or plastids) and a signal sequence determines the movement of proteins. Through signal sequences, proteins can remain in the cytosol or move to different compartments through gated, transmembrane, or vesicular transport.

Question 7

Where does protein synthesis start? What are the exceptions?

Answer 7

Protein synthesis starts at the cytosol (except proteins translated in mitochondria or plastids)

Question 8

What are the three types of transport that allow proteins to move between cell compartments?

Answer 8

1) gated
2) transmembrane
3) vesicular

Question 9

Can organelles be constructed de novo? Why or why not?

Answer 9

No, information is required in the organelle itself.

Question 10

During protein synthesis, what is the general role of signal sequences and sorting receptors?

Answer 10

Directs protein movement between different cell compartments

Signal sequence (targeting)
Signal receptors (shuffling)

Question 11

What do signal sequences do?

Answer 11

Determines fate of protein movement-Targeting, function remains unaffected even if transplanted to other proteins

Question 12

What do signal receptors do?

Answer 12

Signal receptors can be reused for shuttling similar protein classes

Question 13

Inner nuclear membrane

Answer 13

Involved in transport between nucleus and cytosol, and makes contact with nuclear lamina and chromosomes

Question 14

Outer nuclear membrane

Answer 14

Continuous with ER filled with ribosomes and is involved in transport between nucleus and cytosol

Question 15

Role of nuclear pores during transport

Answer 15

Mediate import/export from nucleus

Question 16

Nuclear pores are formed by?

Answer 16

Nuclear pore complex

Question 17

Nuclear pore complex formed by?

Answer 17

nuclear porins (4 major classes)

Question 18

Nuclear pore complex

Answer 18

Formed by nuclear porins, the nuclear pore complex is composed of ~30 distinct proteins forming the nuclear pores in the nuclear envelope
Has an 8-fold rotational symmetry with reflection symmetry at the center
125X 106 Da molecular weight (vertebrates)

Question 19

What are the four major classes of nuclear porins and what are their general characteristics or functions?

Answer 19

Membrane ring: interact with nuclear envelope
Scaffold: have domains related to clathrin or COPII vesicle coats
Channel: FG-repeat domains, forming a gel that blocks the channel
Fibrils and nuclear basket

Question 20

NPC contributes to ______ transport.

Answer 20

gated

Question 21

Describe transport of molecules with the size of >5 kDA, >60 kDa, <60 kDa through nuclear pore complex.

Answer 21

5 kDA: free passage, aqueous channel
<60 kDA: size-dependent ungated entry- bigger the slower
>60 kDA: bound ‘entry receptor’ through karyopherin needed for entry

Question 22

Nuclear localization signal (NLS)

Answer 22

directs nuclear proteins to nucleus

Question 23

Nuclear localization signals are localized….?

Answer 23

somewhere on the surface of a protein in loops or patches

Question 24

How are NLS regulated?

Answer 24

masked or displayed

Question 25

The NLS sequence consists of..?

Answer 25

1 or 2 short sequences, rich in + charged amino acids (K and R)

Question 26

Transport of large protein complexes

Answer 26

fully folded and assembled across the NPC (slide 29/51)

Question 27

Nuclear import receptors bind to ..?

Answer 27

NLS and NPC

Question 28

What are nuclear import receptors ? Give an example.

Answer 28

Example: Importins

soluble proteins that are thought to dissolve the gel formed by F-G repeats locally

Question 29

Nuclear import receptors like importin interact with?

Answer 29

NLS of cargo protein and FG repeats of nuclear porins (which form the cytoplasmic fibrils, channel, and nuclear basket)

Question 30

Nuclear export receptors (exportins)

Answer 30

recognize nuclear export signal (NES) on cargo proteins

Question 31

The transport of most nuclear cargo through the NPC via karyopherins happens by simple diffusion. What mechanism could cells use to impart directionality to this process?

Answer 31

GTPase Ran

Question 32

During protein transport through NPC, GTPase Ran contributes to what?

Answer 32

directionality

Question 33

GTPase Ran mechanism

Answer 33

d

Question 34

Regulation of NPC transport

Answer 34

The early D. melanogaster synccytium has a transcriptional regulator called “Dorsal”. Dorsal is imported only into some nuclei for transcriptional regulation.

Question 35

How is nuclear transport regulated?

Answer 35

controlling signal sequences that are regulated by phosphorylation

Question 36

How are signal sequences (NLS/NES) regulated ?

Answer 36

1. phosphorylation
2. hiding/ displaying them
3. tethering cargo proteins in cytosol

Question 37

Give a real example of how signal sequences (NLS/NES) are regulated by phosphorylation.

Answer 37

During T cell activation, calcium influx is controlled by the nuclear factor of activated T cells (NF-TA)

Question 38

Give a real example of how signal sequences (NLS/NES) are regulated by tethering cargo proteins in cytosol

Answer 38

Regulation of cholesterol biosynthesis through feedback regulation of the sterol response element binding protein (SREBP)

Question 39

During mitosis, what happens to the nuclear envelope?

Answer 39

nuclear envelope disassembles

Question 40

What is the nuclear lamina made of?

Answer 40

Class of intermediate filaments called nuclear lamins

Question 41

Describe events that lead to nuclear lamina disassembly during mitosis.

Answer 41

1. Mitosis onset: cyclin-dependent kinase (cdk) phosphorylates NUPs and lamins
2. Disassembly of NPCs give rise to soluble NUPs, some bound to importins
3. nuclear lamina disassembly