Chapter 9: Cytoskeletal Elements

Cytoskeleton

Complex network of interconnected filaments and tubules that extends from the nucleus to the inner surface of the plasma membrane

Functions of the cytoskeleton

1. cell shape

2. cell movement

3. cell division

4. chromosome segregation

5. movement of organelles and other substances

6. cell signaling and adhesion

Three types of eukaryotic cytoskeletal elements

1. microtubules

2. microfilaments

3. intermediate filaments

Monomer of microtubules

tubulin

Monomer of microfilaments

actin

Monomer of intermediate filaments

keratin

Bacterial homologue of microtubules

Ftsz

Bacterial homologue of microfilaments

MreB and Mbl

Bacterial homologue of intermediate filaments

crescentin

Structure of microtubule

1. hollow tube with 13 protofilaments

2. dimers of a-tubulin and b-tubulin

3. exhibits polarity

Microtubule assembly

1. Tubulin dimers

2. oligomers

3. protofilament

4. sheets of protofilaments

5. closing microtubule

6. elongating microtubule

Kinetics of microtubule assembly

1. lag phase

2. elongation phase

3. plateau phase

How are microtubules assembled initially?

13 linear protofilaments around a hollow core

Dynamic instability of microtubules

alternation of rapid growth and shrinkage

Treadmilling

Tubulin is added to the plus end and is released from the minus end

Why is GTP important?

forms a GTP cap at the plus end that stabilizes the microtubule and promotes growth

Microtubule functions

1. Cilia and flagella motility

2. cell shape

3. chromosome movement

4. organelle transport

FtsZ function

1. Forms Z-ring at future division sites

2. Forms protofilaments

3. Binds and hydrolyzes GTP

Colchicine

Inhibits microtubule assembly

Taxol

1. Binds and stabilizes microtubules

2. promotes microtubule assembly

Microfilaments

smallest cytoskeletal filaments

Microfilaments functions

1. muscle cell contraction

2. amoeboid movement

3. cytoplasmic streaming

4. cleavage furrow

5. cell shape

Microfilament assembly

G-actin bind together to form helical strands of F-actin using Arp 2/3 complex

MreB functions

1. maintains rod-shape

2. organizes cell membrane

3. chromosome segregation

4. maintains polarity

5. cell wall synthesis

Similarity between microtubules and microfilaments

1. both have polarity

2. both can be used as tracks that protein motors walk on

What is needed for actin to form branches?

Arp 2/3 complex

WASP proteins

activate Arp 2/3 complex

Listeria entry into host

1. hijacks the cell adhesion machinery for entry

2. hijacks the cytoskeleton once it enters

Listeria movement within host cells

1. attachment after binding to E-cadherin

2. internalization

3. listeria cell divides

4. tail formation

5. infection of neighboring cell

Actin bacterial homologue

ParM

Plasmid partitioning

ParM filaments indirectly bind to centromere-like regions of plasmids and push them to opposite poles of the cell through a bidirectional polymerization mechanism

Each daughter cell receives a copy of the plasmid

Bacterial chromosome partitioning

ParABS system separates bacterial chromosomes into each daughter cells

Intermediate filaments

1. no nucleotide requirement

2. new building blocks are inserted in the middle

3. no polarity

Crescentin function

cell curvature

Nucleotide requirement for microtubules

GTP

Nucleotide requirement for microfilaments

ATP

Microtubules diameter

Outer: 25 nm, inner 15 nm

Microfilaments diameter

7 nm

intermediate filaments diameter

10 nm