11.1 The Nuclear Envelope and Traffic between the Nucleus and the Cytoplasm

Question 1

The nuclear envelope consists of:

Answer 1

inner and outer membranes that are joined at nuclear pore complexes

Question 2

the critical function of the nuclear membranes is to

Answer 2

act as a barrier that separates the contents of the nucleus from the cytoplasm

Question 3

a meshwork of filaments beneath the inner nuclear membrane that provides structural support

Answer 3

nuclear lamina

Question 4

the nuclear lamina is composed of fibrous proteins called

Answer 4

lamins (60-80 kd)

Question 5

how are lamin/the lamin assembly associated to the inner nuclear membrane?

Answer 5

• posttranslational addition of lipid (prenylation of C-terminal cysteine residues)
• binding to specific inner nuclear membrane proteins

Question 6

give examples of the inner nuclear proteins associated to the nuclear lamina

Answer 6

emerin and lamina B receptor

Question 7

the nuclear lamina is also directly connected to the cytoskeleton through

Answer 7

LINC complexes

Question 8

LINC complexes are composed of:

Answer 8

KASH (attaches to cytoskeletal filament) and SUN (attaches to nuclear lamina) proteins

Question 9

a list of diseases caused by defects in the nuclear lamina

Answer 9

laminopathies

Question 10

what are the 2 hypotheses on why laminopathies cause tissue-specific symptoms

Answer 10

1. mechanical stress hypothesis
2. gene expression hypothesis

Question 11

explain the mechanical stress hypothesis

Answer 11

mutations affecting the lamina weaken the structural integrity of the nucleus and its connection to the cytoskeleton; thus the nucleus is more vulnerable to stress transmitted via cytoskeletal interactions

Question 12

explain the gene expression hypothesis

Answer 12

mutations cause a change in gene expression caused by defective chromatin localization (the correct interaction of lamins with the nuclear envelope is essential for the normal tissue-specific expression of certain genes)

Question 13

the only channels allowing passage of molecules across the nuclear envelope

Answer 13

Nuclear pore complexes

Question 14

each nuclear pore complex is composed of multiple copies of about 30 different

Answer 14

nucleoporins (NUPs)

Question 15

depending on their size and structure, molecules can travel through the nuclear pore complex by either:

Answer 15

1. passive diffusion - for small molecules (less than 40 kd)
2. selective transport - larger molecules (like proteins and RNA) must be recognized by specific signals that direct their transport between the nucleus and cytoplasm

Question 16

the nuclear pore complex consists of an assembly of

Answer 16

8 spokes arranged around a central channel

Question 17

• proteins that line the central channel of nuclear pore complexes
• the barrier to permeability of the pore and facilitate regulated transport between the nucleus and cytoplasm

Answer 17

FG-NUPs (phenyl-alanine and glycine-rich nucleoporins)

Question 18

specific animo acid sequences that direct protein transport through the nuclear pore complex

Answer 18

nuclear localization signals

Question 19

nuclear localization signals are recognized by:

Answer 19

nuclear transport receptors

Question 20

How did Kalderon, Roberts, Richardson, and Smith identify nuclear localization signals

Answer 20

• by studying the SV40 T antigen, they found that altering a certain amino acid sequence (126-132) prevented the antigen from entering the nucleus
• moreover, they also found that the addition of this sequence allowed normally cytoplasmic proteins to enter the nucleus

Question 21

an NLS that is composed of 2 separated elements is called

Answer 21

bipartite

Question 22

“classical” NLSs are sequences composed of

Answer 22

basic amino acids

Question 23

nuclear transport receptors that bring proteins into the nucleus

Answer 23

importins

Question 24

transport receptors that direct protein transport through the nuclear pore complex to the cytoplasm

Answer 24

exportins

Question 25

Like importins, exportins are members of a family of nuclear transport receptors known as

Answer 25

karyopherins

Question 26

Directionality of transport across the nuclear envelope is determined by the

Answer 26

GTP-binding protein Ran

Question 27

Summarize the classical cycle of nuclear import

Answer 27

1. importin binds to the NLS of a cargo protein in the cytoplasm
2. cargo/importin complex binds to proteins in the cytoplsmic filaments of the nuclear pore complex
3. transport proceeds via the interaction of importins with the FG-NUPs that line the central channel of the nuclear pore
4. a Ran/GTP complex binds to the importin once the cargo/importin complex reaches the nuclear side
   
   • this causes a change in the conformation of importin
5. cargo/importin complex is disrupted, and the cargo protein is relased into the nucleus

Question 28

how is the cycle of nuclear import maintained

Answer 28

export of importin-Ran/GTP complex back through the nuclear pore complex, and import of Ran/GDP back into nucleus (converted to Ran/GTP), maintaining a high concentration of Ran/GTP in the nucleus

Question 29

summarize the process of exporting importin-Ran/GTP complex

Answer 29

1. outside the nucleus, Ran/GTP is hydrolyzed into Ran/GDP by action of Ran GAP (GTPase-activating protein) which is associated to the cytoplasmic filaments of the nuclear pore
2. importin is released in the cytoplasm
3. Ran/GDP complex is now able to bind to NTF2 (also an import receptor) and transported back into the nucleus
4. Ran GEF (guanine-nucleotide exchange factor) stimulates the exchange of GDP and GTP, which causes the formation of Ran/GTP

Question 30

note that Ran/GTP promotes the formation of stable complexes of between (1) and their cargo, whereas it dissociates the complexes between (2) and their cargos

Answer 30

1. exportins
2. importins

Question 31

summarize the cycle of nuclear export

Answer 31

1. complexes between cargo proteins (bearing nuclear export signals), exportins, and Ran/GTP form and exit through the nuclear pore
2. Ran GAP stimulates the hydrolysis of bound GTP to GDP, releasing the cargo protein and exportin
3. Ran/GDP binds to NTF2 and exportin, allowing the complex to enter the nucleus
4. exportin and NTF2 are released, and Ran GEF allows the regeneration of Ran/GTP inside the nucleus

Question 32

overview of ribosomal RNA transport

Answer 32

1. ribosomal RNAs are first associated with ribosomal proteins in the nucleolus
2. nascent 40S and 60S ribosomal subunits are transported separately to the cytoplsm by the exportin Crm1
3. export from the nucleus is mediated by nuclear export signals present on ribosomal proteins

Question 33

overview of mRNA transport

Answer 33

1. pre-mRNAs are associated with a set of at least 20 proteins throughout their processing in the nucleus and eventual transport to the cytoplasm
   
   • these proteins include a distinct mRNA exporter complex, which is recruited to pre-mRNAs in the nucleus during the completion of splicing and polyadenylation
2. the exporter complex then transports the mRNAs through the nuclear pores
   
   • directionality of the process is established by an RNA helicase localized to the cytoplasmic face of the nuclear pore complex
3. the helicase remodels the mRNA and removes the exporter complex, releasing the mRNA into the cytoplasm

Question 34

summarize how snRNAs are transported

Answer 34

• 1. snRNAs are initially transported from the nucleus to the cytoplasm by the exportin Crm1
  2. in the cytoplasm, snRNAs associate with proteins to form snRNPs
  3. sequences present on the snRNPs are recognized by an importin called snurportin, which mediates their transport back to the nucleus

Question 35

Regulated transport of () is a mechanism for controlling gene expression

Answer 35

transcription factors

Question 36

in one mechanism of regulation, transcription factors (or other proteins) bind to cytoplasmic proteins that (), thus preventing their transport into the nucleus

Answer 36

mask their NLS