Cell Bio lec 9

The cytoskeleton is a network of
interconnected _____and_____
extending through the cytosol

filaments and tubules

the cytoskleoton plays a role in cell ______and______

movement and divison

essential components of muscle fibrils

microfilaments

structural elements of cilia and flagella

microtubules

Research has shown that cytoskeleton is dynamically _______and_______

assembled and disassembled

tubulin-like_____protein determines where bacterial cells will divide

FtsZ protein

actin-like, involved in DNA segregation

MreB

: IF-like, an important regulator of cell shape
-3-D structure similar
- not very similar at amino acid level

crescetin

Two basic classes of microtubules

• dynamic, loosely organized
• Maintain and alter cell shape
• Formation of mitotic/meiotic spindles
• highway for vesicle trafficking

Cytoplasmic microtubules

• stable, highly organized
• cilia, flagella, basal bodies

Axonemal microtubules

1. Cytoplasmic MTs are simple tubes, or_____ MTs, with 13 protofilaments

singlet

Some axonemal MTs form ____ or ____ MTs

____and t_____ contain
one 13-protofilament tubule
(the A tubule) and one or two
additional incomplete rings
(B and C tubules) of 10 or 11
protofilaments

doublets and triplets

Model for microtubule assembly in vitro

1. Tubulin dimers aggregate into oligomers nucleation
2. Oligomers form linear chains called protofilaments
3. Protofilaments associate to form sheets
4. Sheets with 13 protofilaments can close into tube
5. Elongation via addition of tubulin heterodimers

• MT formation is slow at first because the process of nucleation is slow; this period is known as the ____ _____

lag phase

The ____ phase is much faster than the lag phase

elomgation

When the mass of MTs
reaches a point where
the amount of free
tubulin is diminished, the assembly is balanced by disassembly; this is known as the ______ ______

plateau phase

The tubulin concentration at which MT assembly is exactly balanced by disassembly is called the ________ _________

critical concentration

The rapidly growing MT end is the ____ ___, and the other is the _____ ___

plus end; minus end

disrupts mitotic spindle of dividing cells  used as anticancer drugs
- Colchicine, Taxol (Treatment of lung, ovarian, breast cancer),
Nocadazole, Vinblastine, vincristine

Antimitotic drugs

___________ _______in muscle cells- interact with thicker filaments of myosin to cause contractions

contractile fibrils

Cell movements involve

cell migration via lamellipodia & filopodia, amoeboid movement & cytoplasmic streaming
• Produce cleavage furrow during cytokinesis

cell cortex (network of Mfs) beneath plasma membrane
• structural core of microvilli- finger-like extensions in cells
• attachment of cells to each other and ECM

arre involved in

developing andf maintaining cell shape

: globular actin, individual actin molecules

G-actin

: G acting polymerize to form F (filamentous) actin

F-actin

– regulate/modify function of actin
– are regulated/modified by actin

Actin-binding proteins

_____ iss a very abundant protein in all eukaryotic cell

actin

Actins can be broadly divided into:

1. muscle-specific actins (α-actins)
2. nonmuscle actins (β- and γ-actins)

G-actin monomers reversibly polymerize into filaments through a lag phase followed by an elongation phase, similar to tubulin assembly is how _____ ________ formed

actin filaments

What is the structure of an F-actin filament?

F-actin consists of two linear strands of polymerized G-actin twisted into a helix, with all actin monomers oriented in the same direction (polarity)

How does polarity affect microfilament (MF) growth?

Due to their polarity, G-actin is added more rapidly at the plus (+) end than at the minus (−) end of the microfilament.

What happens to ATP after G-actin is added to a microfilament?

After G-actin assembles into the microfilament, its ATP is slowly hydrolyzed to ADP. As a result, the growing ends contain ATP-actin, while most of the filament is made of ADP-actin.

How is the amount of free G-actin regulated in the cell?

Back: Most free G-actin is bound by thymosin β4, preventing it from polymerizing until needed.

What is the role of formins in actin polymerization?

Formins associate with the fast-growing (+) end of actin filaments and promote actin polymerization.

What is the function of gelsolin

Gelsolin severs (breaks) actin microfilaments and caps the newly exposed plus (+) ends, preventing further polymerization.

What is the function of CapZ?

CapZ is a capping protein that binds to the plus (+) end of an actin filament, preventing the addition or loss of actin subunits.

What is the function of filamin

Filamin is an actin-crosslinking protein that joins actin microfilaments where they intersect, forming loose actin networks.

What is the function of α-actinin and fimbrin?

α-Actinin and fimbrin are actin-bundling proteins that organize actin filaments into tight, parallel bundles.

The Arp2/3 complex nucleates new actin branches on existing filaments, creating dendritic (tree-like) networks. WASP proteins activate the Arp2/3 complex.

How are branched actin networks formed?

The Arp2/3 complex nucleates new actin branches on existing filaments, creating dendritic (tree-like) networks. WASP proteins activate the Arp2/3 complex.

How does Listeria monocytogenes enter a host cell?

Listeria binds E-cadherin on the host cell using its internalin A protein, allowing it to enter the cell and multiply.

How does Listeria monocytogenes spread between cells?

Listeria forms an actin "comet tail" by polymerizing host cell actin, which propels the bacterium into neighboring cells.

What are the key characteristics of intermediate filaments (IFs)?

Back: Intermediate filaments are 8–12 nm in diameter (between microtubules and microfilaments), are made of fibrous subunits, occur singly or in bundles, provide structural/tension-bearing support, and are the most stable, least soluble cytoskeletal elements.

How are intermediate filaments different from microfilaments and microtubules?

Intermediate filaments remain after detergents remove MTs, MFs, and other proteins, acting as a scaffold for the cytoskeleton. Unlike MFs and MTs, their composition varies between tissues (intermediate filament typing).

What are the fundamental subunits of intermediate filaments (IFs)?

The basic subunits of IFs are dimers made from fibrous IF proteins.

What is the structure of intermediate filament proteins?

IF proteins have a central rod-like helical domain (310–318 amino acids) with variable N- and C-terminal domains that differ between IF proteins

How is a fully assembled intermediate filament formed?

Dimers align laterally to form tetramers, tetramers join to form protofilaments, and 8 protofilaments bundle together to create a mature intermediate filament.

How can intermediate filament (IF) mutations affect cells?

Mutations in intermediate filament proteins can weaken structural support, causing cells to become fragile and easily damaged.

What is Epidermolysis bullosa simplex (EBS)?

EBS is a hereditary blistering skin disease caused by keratin mutations, which make epidermal cells fragile and prone to rupture.