Ch13 (Pt 3): Cytoskeleton and Cell Movement

What are unconventional myosins?

non-muscle myosins that don't form filaments and aren't involved in muscle contraction (function in transport of vesicles and organelles)

Describe the makeup of myosin I

-globular head groups act as molecular motors

-short tails bind to other structures

Describe the makeup of myosin V

two-headed dimer that transports vesicles and other cargo along actin filaments

What are some unconventional actins involved in?

-actin filament reorganization

-anchor actin filaments to plasma membrane

What are microtubules? What are they made up of? What are their functions?

-rigid hollow rods that are dynamic structures (undergo continual assembly/disassembly)

-made of globular protein tubulin

-function in cell movements and determining cell shape

Describe the makeup of tubulin

-tubulin dimers consist of alpha and beta tubulin (encoded by related genes)

-gamma tubulin in centromeres helps initiate microtubule assembly

Tubulin dimers polymerize to form microtubules that are made up of what?

13 protofilaments around a hollow core

What are protofilaments?

head-to-tail arrays of tubulin dimers arranged in parallel

What do microtubules have that determine direction of movement?

polarity (plus and minus ends)

To cause depolymerization and loss of tubulin bound to GDP from the minus end, ___ bound to ___ (alpha or beta) tubulin is hydrolized to ___ shortly after polymerization

GTP

beta

GDP

What is dynamic instability?

-microtubules stabilized at minus end by rapid GTP hydrolysis

-alternating between cycles of growth and shrinkage

How can a GTP cap remain at the plus end microtubule to continue growth?

new GTP-bound tubulin dimers add faster than GTP is hydrolyzed

What happens if GTP is hydrolyzed faster than new subunits are added?

GDP-bound tubulin at the plus end of the microtubule leads to disassembly and shrinkage

Rapid turnover of ___ allows for remodeling of the cytoskeleton that occurs during ___.

microtubules

mitosis

What drugs affect microtubule assembly and are useful as experimental cancer treatments?

colchicine and colcemid

What drugs are used in cancer chemotherapy because they inhibit microtubule polymerization (affect rapidly dividing cells)?

vincristine and vinblastine

What drug stabilizes microtubules and blocks cell division?

taxol

What do microtubule-associated proteins (MAPs) regulate? How does this affect the plus and minus ends?

-regulate dynamic behavior of microtubules

-Plus End: regulates growth or shrinkage

-Minus End: stabilized by proteins that prevent depolymerization

In animal cells, most microtubules extended outward from the ___

centrosome

During mitosis, microtubules extend outward from duplicated centrosomes to form the ___. What is this responsible for?

-mitotic spindle

-separation and distribution of chromosomes to daughter cells

Centrosomes are initiation sites for ___ assembly, which grow outward from ___ (plus or minus) ends anchored in the centrosome

microtubule

minus

If cells are treated with ___ microtubules disassemble. When the drug is removed, new microtubules can be seen growing outward from the centrosome

colcemid

What is the g-tubulin ring complex? What step does this complex bypass?

-gamma tubulin in the centrosome associated with other proteins in a ring-shaped structure

-bypasses the rate-limiting nucleation step

Most animal cell centrosomes have a pair of what? How are they oriented? What are they surrounded by?

-pair of centrioles

-oriented perpendicular to each other

-surrounded by pericentriolar material

What shape are centrioles? What is this shape based on?

-cylindrical

-based on nine triplets of microtubules

Centrioles also form what cells? What initiates microtubule assembly in these cells?

-form basal bodies of cilia and flagella

-pericentriolar material initiates microtubule assembly

What modifications regulate microtubule stability? How do they regulate stability?

-post-translational modification of tubulin by phosphorylation, acetylation, etc.

-provide sites for specific MAP binding (allows microtubule stabilization in specific locations and determines cell shape and polarity)

What are the two types of microtubules in nerve cells? Describe the organization of each

1. Axons: microtubule plus ends towards the tips (associated with tau)

2. Dendrites: microtubule oriented in both directions (associated with MAP2)

What two families of motor proteins are responsible for powering movements that microtubules participate in? Describe the movement of each family

1. Kinesins: move toward plus end

2. Dyneins: move toward minus end

A major role of microtubules is to transport ___ and ___ through the cytoplasm. Different members of the kinesin and dynein families are thought to transport cargo in ___ (same or opposite) directions.

vesicles

organelles

opposite

Describe the association of the endoplasmic reticulum with microtubules

ER extends to cell periphery in association with microtubules that involves kinesin I

Explain how microtubules reorganize during mitosis (interphase and prophase)

-Interphase: microtubule array disassembles, free tubulin subunits reassemble into mitotic spindle, centrosome is duplicated

-Prophase: centrosomes migrate to form 2 poles of mitotic spindle

As the cell enters mitosis, the rate of microtubule disassembly ___ (decreases or increases), resulting in shrinkage of microtubules. But the number of microtubules emanating from the two centrosomes ___ (decreases or increases).

increases

increases

What four types of microtubules make up the mitotic spindle? Describe the attachment of each

1. Kinetochore Microtubules: attach to condensed chromosomes at centromeres

2. Chromosomal Microtubules: connect to chromosome ends via chromokinesin

3. Polar Microtubules: not attached to chromosomes but stabilized by overlapping in cell center

4. Astral Microtubule: extend outward from centrosomes with plus ends anchored in cell cortex

After the centrosomes move to opposite sides of the cell, the duplicated chromosomes attach to kinetochore and chromosomal microtubules, and align on the ___ plate. Then the links between the sister chromatids are severed and ___ begins.

metaphase

anaphase

What are intermediate filaments? How are they involved in cell movements? What are they not found in?

-diameters intermediate between actin filaments and microtubules

-not directly involved, provide mechanical strength and scaffold for localization of cell processes

-not found in yeast, plants, and some insects

Type I and II filaments are ___, in epithelial cells. ___ forms a network extending out from the nucleus toward the cell periphery.

keratins

vimentin

What are neurofilament (NF) proteins? How are they expressed during embryonic development in some stem cells? What domains determine the specific functions?

-major intermediate filaments of many neurons that provide support for long axons

-nestins

-head and tail domains

What are type V NF proteins?

nuclear lamins that form a meshwork of underlying nuclear membrane

Describe the assembly of intermediate filaments

-filaments assemble with a central rod domain of 2 polypeptides forms a coiled coil

-dimers form protofilaments

-eight protofilaments wind together to form a filament

Describe desmosomes. What filament attaches here?

-junctions between adjacent cells

-keratin filament attachment mediated by desmoplakin (plakin family protein)

Describe hemidesmosomes. What filament attaches here?

-junctions between epithelial cells and underlying connective tissue

-keratin filaments attached to different plakins, transmembrane integrins link to the extracellular matrix