Nucleus, Nuclear Import, Nucleolus (Lecture 23)

Question 1

Are intermediate filaments polar or non-polar?

Are microfilaments polar or non-polar?

What does the growth of branched MF do?

Myosin is a motor protein that interacts with?

Answer 1

• Intermediate Filaments (IF) are non-polar and provide structural and mechanical support.
• Microfilaments (MF) (or F-actin) are polar cytoskeleton components with important roles in cell structure and cell motility.
• Growth of branched MF “pushes” against the plasma membrane to form lamellipodia and power cell movement.
• Myosin is a motor protein that interacts with F-actin to facilitate cell movement (“pulling”) and vesicle transport (esp. in plant cells).

Question 2

Nucleus: Structure and Function

Answer 2

Question 3

What is the nucleus structure?

Answer 3

Structure

• Nuclear Envelople
  
  • nuclear membrane
  • nuclear pores
  • nuclear lamina
• Nuclear contents
  
  • chromatin (DNA + histones)
  • nucleoplasm
  • nucleolus

Question 4

What is the Nuclear Envelope (NE)?

Answer 4

2 parallel phospholipid bilayers

• separated by 10-50 nm
• Outer nuclear membrane (ONM) binds ribosomes and is continuous with the rough endoplasmic reticulum
• Inner nuclear membrane (INM)
  
  • bears integral proteins
  • connects to the nuclear lamina

Question 5

intermembrane space is continuous with what?

Answer 5

intermembrane space is continuous with ER lumen

Question 6

What are the functions of the nuclear envelope?

Answer 6

• separates the nuclear content from the cytoplasm
  
  • separates transcription & translation
• selective barrier
  
  • allows limited movement of molecules between the nucleus and cytoplasm
• supported by the nuclear lamina

Question 7

What is the nuclear lamina?

Answer 7

• thin meshwork of filamentous proteins
  
  • lamins (intermediate filaments) (in animal cells)
  • (plants have nuclear lamina, but made of different types of proteins)
• bound to the inner membrane of NE by integral membrane proteins
• provides structural support for NE a
• attachment sites for chromatin

Question 8

What does the nuclear lamina form a meshwork next to?

Answer 8

The nuclear lamina forms a meshwork next to the nucleoplasmic leaflet of the inner nuclear membrane

Question 9

What are nuclear pores?

Answer 9

• gateways between cytoplasm & nucleoplasm
• 3,000 to 4,000 pores/nucleus
• pores occur where inner and outer membranes fuse
• pores have a complex protein structure

‣ Nuclear Pore Complex (NPC)

Question 10

What is the Nuclear Pore Complex?

Answer 10

• *NPC**
• composed of nucleoporins (NUPs)
• octagonal symmetry
• projects into cytoplasm and nucleoplasm

Question 11

Nuclear pore is a huge _______ _______

How big is it compared to a ribosome?

Answer 11

Nuclear Pore is a huge supramolecular complex

15 - 30 times the size of a ribosome

Question 12

What is the function of the nuclear pore complex?

Answer 12

• passive diffusion of molecules smaller than 40 kDa
  
  • rapid (100/min./pore)
• regulated movement of larger molecules
  
  • slow (6/min./pore)
• Regulated movement of proteins into the nucleus requires a Nuclear Localization Signal (NLS)
• NLS = several positively charged amino acids within the protein sequence

Question 13

How does the NLS target proteins to the nucleus?

Answer 13

1. Protein with NLS (cargo) interacts with Importin protein in the cytoplasm
2. Cargo/Importin complex interacts with FG-NUPs at the Nuclear Pore (NPC) and enters the nucleoplasm
3. Ran-GTP interacts with Importin; cargo dissociates and stays in the nucleoplasm
4. Ran-GTP/Importin complex exits nucleus through NPC
5. GTP hydrolyzed to GDP. Importin released in the cytoplasm to find new cargo.

Question 14

Nuclear import and export are critical for cellular function

What are the points under nucleocytoplasmic trafficking?

Answer 14

Nucleocytoplasmic trafficking

• Nucleotides
• Structural proteins
• DNA packaging proteins
  
  • Histones
• Proteins for DNA replication, repair, transcription
• Proteins for RNA processing & export
• Proteins for ribosome synthesis & export

Question 15

Where does ribosome synthesis happen?

Answer 15

Nucleolus

Question 16

What is the function of the nucleolus?

Answer 16

Ribosome Biogenesis

• synthesis of rRNA
• rRNA processing
• assembly of subunits (rRNA + proteins)
• The 40S and 60S subunits are exported to cytoplasm

Question 17

Where do the following go…

a. No signal peptide
b. Amino-terminal signal
c. Internal signal

Answer 17

a. No signal peptide → to cytosol
b. Amino-terminal signal

→ to the chloroplast

→ To mitochrondrion

c. Internal signal

→ To nucleus

Question 18

OVERVIEW: Where Proteins Go? (Part 1)

Answer 18

Question 19

OVERVIEW: Where Proteins Go? (Part 2)

Answer 19