Cytoskeleton

Question 1

What is the cytoskeleton

Answer 1

Network of protein fibers that extend throughout the cytoplasm
*Nomenclature is based on size

Question 2

What are the functions of the cytoskeleton?

Answer 2

-Cell adhesion and movement (migration)
-Cell shape and structure
-Endocytosis/ exocytosis (secretion + recycling)
-Organelle/ protein transport
-Mitosis/ Cytokinesis
-Cilia and Flagella
-Muscle contraction
Etc.

Question 3

What are the three types of cytoskeleton?

Answer 3

Microtubules, Actin (microfilaments), and intermediate filaments

Question 4

What is the structure and diameter of microtubule (MT)?

Answer 4

Protofilaments
25 nm

Question 5

What is the structure and diameter of actin?

Answer 5

Double helix
6-7 nm

Question 6

What is the structure and diameter of intermediate filaments?

Answer 6

Two anti-parallel helices, form tetramers
10 nm

Question 7

Describe microtubules
-subunit, etc

Answer 7

Polymer of alpha and beta tubulin dimer (basic subunit).
GTP-binding subunits come together in protofilaments (lines) to form hollow tubes

Question 8

Explain the dynamic instability of microtubules

Answer 8

Balance between assembly (polymerization) and disassembly (catastrophe) and reassembly (rescue).
Dependent on subunit critical concentration and temperature (lower temp = more disassembly)
- also dependent on GTP-b-tubulin cap

Question 9

What is MTOC? What is a major MTOC in cells?

Answer 9

Microtubule Organizing Center.
Centrosome is a major MTOC in cells (have multiple).
Microtubules radiate outward from the MTOC (negative end at MTOC), positive end grows away

Question 10

What are MAPs?

Answer 10

Microtubule Associated Proteins
-can alter microtubule stability
-bundle microtubules
-can be regulated - phosphorylation promotes disassembly
*one way to stabilize microtubules

Question 11

What are + TIPs? What is an example of a + TIP?

Answer 11

Possibly stabilize MTs to reduce catastrophe
e.g. EB1 (end binding protein 1) - shooting star proteins - seen in cell imaging falling-off then coming back

Question 12

What are MT severing proteins?

Answer 12

Destabilize MTs
e.g. Katanin (like a sword that cut microtubules)

Question 13

What is Kinesin?
What is it made up of?

Answer 13

MT + end motor protein (move out of cell toward + end) - Anterograde movement
Many types (> 14 known classes)
Made up of heavy chains (ATPase activity and bind MTs) and light chains (recognize cargo)
*ATP hydrolysis causes conformational changes which result in movement.

Question 14

What is Dynein? How does it work?

Answer 14

MT - end motor protein - Retrograde movement
Interacts with another accessory protein, Dynactin which binds cargo
*ATP hydrolysis causes conformational changes which result in movement (ATPase activity)

Question 15

Describe Microfilaments

Answer 15

Actin polymers - monomeric Globular (G-actin) into Filamentous (F-actin)
Double helix
Grows at - end and + end (but faster at + end)
ATP dependent
Can bundle Actin into diff cell structures - very complex.

Question 16

What is Actin (Microfilament) dependent on for polymerization/ depolymerization?

Answer 16

Dependent on Profilin (polymerization) and Cofilin (depolymerization).
Also dependent on capping proteins at - and + ends.

Question 17

What is Actin organized by?

Answer 17

Actin-binding proteins

Question 18

What cell structures / bundles (e.g. Fimbrin, a-actinin, etc.) can actin form?

Answer 18

Microvilli
Filopodia
Focal adhesion

Question 19

What is lamellapodium?

Answer 19

Branched actin
-Mesh/ lattice
-Branching is stimulated by adding laterally to existing actin
-Nucleation and branching occurs with Actin Related Protein (Arp) 2/3 Complex

Question 20

What is the motor protein that works with the Actin cytoskeleton?

Answer 20

Myosin
-Myosin II most abundant
-Heavy and light chains
-ATPase activity
-Ca+2

Question 21

What is the function of Myosin?

Answer 21

Actin motor protein
-Membrane association, endocytosis
-Contraction (muscle)
-Organelle transport
Movement towards (+) end

Question 22

Describe intermediate filaments

Answer 22

Provide structural support for cell shape
Basic subunit: Tetramer
2 dimers come together (4 polypeptides), to form a tetramer - forms protofilament, 4 protofilaments in a protofibril, 4 protofibrils in intermediate filament.
Mostly cytoplasmic
Do not have: polarity, or motor proteins
Lamins are nuclear

Question 23

What is the function of intermediate filaments?

Answer 23

IFs provide structural support necessary for cell shape

Question 24

What are Keratins?

Answer 24

Intermediate filaments
Link to Cadherins (desmosomes) and stabilize epithelial cells

Question 25

What are Lamins?

Answer 25

Intermediate filaments
Support nuclear membrane
Network - linked to nuclear lamina

Question 26

What is vimentin?

Answer 26

Intermediate filaments
Found in mesenchymal cells
Provides stability to mesenchymal - migrating cells (e.g. through uneven terrain)

Question 27

How are intermediate filaments regulated?

Answer 27

Unlike MTs and Actin cytoskeleton (which are mainly regulated by binding and capping proteins) the regulation of Intermediate filament stability is dependent on the opposing action of kinases and phosphatases.
*Need to dissasemble during mitosis - phosphorylation of serine residues induces disassembly (and prevents reassembly)

Question 28

How is protein exchange different in intermediate filaments than actin and microtubules?

Answer 28

Segments in an existing intermediate filament can be taken out and replaced with new segments. E.g. if want to change direction can just replace parts in the middle unlike MT and actin where you need to completely rebuild