- cytokinesis

- cell motility

- cell adhesion

1. lag phase - G-actin aggregates into trimers

2. elongation phase - rapid growth on both ends

3. steady state - filament retains the same net length because the rate of gain and loss stays equal

cytochalasians: bind to the barbed end of an actin filament and prevent elongation

phalloiden: bind to the pointed end of an actin filament and prevent dissociation; these can be labelled with colored dye to help visualize actin filaments

both direct the growth of actin filaments by controlling where nucleation happens. 

Formins: make unbranched filaments

Arp2/3: make BRANCHED filaments

1. actin bundles - filaments are cross-linked into closely packed parallel bunches.

2. actin networks - cross-linked into networks with gel-like behavior

actin filaments in a mesh lining the inner membrane of the cell

- also associated proteins!

- determines cell shape

- helps with cell movement

- transmembrane INTEGRIN proteins bind to whatever the cell needs to bind to - these places are called FOCAL ADHESIONS

- these are also attachment sites for actin bundles called STRESS FIBERS, which are cross-linked with alpha actinin.

cell-cell contact (in epithelial cells all touch all and form adhesion belt)

- transmembrane proteins called CADHERINS bond to the other cell on the outside and CATENINS on the inside, which in turn bind to actin filaments

cross-linking ATP-powered motor protein that has two regions; the GLOBAL HEAD region interacts with actin fibers.

- these actin muscle fibers are attached on the barbed end to Z-DISCS; myosin uses ATP hydrolysis to contract and pull the Z-discs toward each other.

- they let go to relax the muscle.

SWINGING CROSS-BRIDGE MODEL

normally, the troponin complex binds to actin filaments; this keeps myosin from binding.

however, when nerve impulses release Ca²⁺ from the sacroplasmic reticulum, it shifts the complex to reveal the myosin-binding sites, and contraction can proceed.

- ends are not structurally polar

- they stay the same length (are very stable) - good at providing structure

- composed of many different filamentous proteins (actin is only composed of actin dimers-->trimers); in general, its compositional proteins have long a-helical regions that twist together and then join with other dimers, etc. 

- two of these dimers wind to make a tetramer; eight of these wind to make a filament

adherens junctions: joins ACTIN bundles to more actin bundles in the adjacent cell, joining them

desmosomes: same thing, but with KERATIN (intermediate filaments) joined to protein plaques on the outside of the cell

- rigid hollow rods; DYNAMIC (assembly/dissembly)

- made of atubulin and btubulin HETERODIMERS

- atubulin binds ATP always; btubulin can HYDROLIZE it to ADP

- 13 protofilaments (head-to-tail heterodimers) combine to make one microtubule; therefore it is POLARIZED - btubulin is the plus unit and atubulin is the minus subunit

necessary for:

1. flagella

2. cillia 

3. basal bodies

NOT found in:

1. most meiotic cells

2. plant cells

3. many unicellular eukaryotes

microtubule-associated proteins

- can promote dissociation or growth

- some attach to the plus end and direct the growth

motor proteins that carry things along the microtubules; kynesin goes toward the plus end, dynein toward the minus end.

- both have polar head domains that grab on to the microtubles

- their tail domains grab organelles, etc. and carry them where they have to be

cilia and flagella: one dimer of tubulin in the middle with nine around it (9+2)

kinetochore: the same, but with TRIPLETS instead of doublets

in cilia and flagella: 

- nexin links the dimers to each other

- two arms of dynein go from the center dimer to each atubulin

1. microtubule array dissembles; free dimers are made into the MITOTIC SPINDLE, an array of microtubules that pushes the centrosomes apart, stretches the cell, and grabs the chromosomes.

1. kinetochore: attach to the chromosomes via the centromeres

2. chromosomal: attach to the chromosomes via the arms (chromoskin)

3. polar: push on each other in the middle

4. astral: expand out freely from the centrosomes - these have EXPOSED PLUS ENDS

anaphase A: kinetochore microtubules dissemble on their plus end, drawing the chromosomes toward the centrosomes; also, motor proteins carry the chromosomes closer (minus-end directed)

anaphase B: polar microtubules are seperated by plus-end directed kinesins (sliding past each other)

1. they slide past each other, and kinesins move toward the plus ends, pushing them apart

2. the astral microtubules have dyneins moving from plus to minus, anchored to the cell cortex, and depolymerizing the plus end at the same time, dragging the polar microtubules back towards their centrosomes

- polar microtubules grow, and align with minus-directed dyneins

- then are pushed apart with plus-directed kinesins

- astral microtubules are pulled towards the cell walls by dynein minus-directed microtubules

cells are held tightly together with no gaps in between

- composed mainly of CLAUDINS and OCCLUDINS (transmembrane proteins)

- desmosomes: intermediate filaments; cell to cell

- hemidesmosomes: intermediate filaments; cell to underlying tissue

- adherens junctions: actin filaments (cadherin proteins)