Provides structural support

Positions organelles

directs vesicular transport

involved in locomotion

1. Microfilaments (actin)

2. Intermeidate filaments (intermediate filament proteins)

3. Microtubules (tubulin)

- determines the location of the cell

- cells are fixed so it is a static experiment

resolution limit: blurry

diffraction: bending of light waves

based on wavelength of light (250 nm where microtubules have a diameter of 2.5 nm) 

electrons have a shorter wavelength

resolution is muh better

- functions in locomotion

- could be in eukaryotic and/or prokaryotic cells

electron microscope> light microscope because you can follow it in real time

- they are very dynamic

- involved in cell crawling ( assembly and disassembly) 

- composed of small subunits which form large filament polymers

1. In present form, actin filaments are located in a specific part of the cell.

2. When a stimulus is present, it promotes the disassembly of these filaments

3. Subunits are spread throughout the cell

4. Reform into filaments where the stimulus was presented

Process known as cell crawling

They need to be strong, flexible and easy to assemble/disassemble 

- you want a high thermal stability (do not want filaments to break in the middle)

- single chains do not work well

- multiple subunits are assembled into bundles of protofilaments; each is associated laterally

- strength is due to many weak bonds

1. In nucletion phase, actin starts coming together, but it takes a while to synthesize to make a oligomer, which is the template

2. during the elongation phase, monomers are added to the ends of the growing actin filament

3. in the steady state, this reaches equilibrium in which the actin monomers coming in replace the actin monomers that are leaving and this leads to the fact that it will NEVER reach 100% 

- ATP IS NECESSARY FOR THIS TO OCCUR

- Salt changes the conformation (kick starts) and the time it takes for the filaments to form is dependent on this

Intracellular transport

Cell organization

Intracellular support

Mitosis

Cell motility

- It is a dimer

- composed of alpha and beta tubulin 

- rearrangement of these tublins create polarity where the beta end is + and the alpha is - 

- they are linked noncovalently

- minus end organised to the cetnre of the cell attached to the centrosome 

- all dimers come together to create a protofilament

13 parallel protofilaments make up the hollow tube

- all the bonds between the individual subunits are non covalent

- bonds between protofilaments arew weaker than bonds within each protofilament

- growth and disassembly of microtubules occur at the ends

 - inside is filled with lumen

- growth faster at the plus end

 incubated microtubules are incubated with a high concentration of tubulin and GTP 

- beta tubulin dimer is where GTP is cut to GDP (alpha sucks at cutting it into GTP)

- since they are stuck to GTP, they bind to each other and form protofilaments

- when it is in protofilament form for a relatively long while, then it will ut GTP to GDP 

1. polymerization followed by nucleoside hydrolysis 

2. When heterodimers are bound to GTP for a long time, it will be cut to GDP. Addition of GTP to plus end is much faster than the GTP to the minus end

- Dimer addition at the plus end is faster than GTP hydrolysis, therefore hydrolysis will NEVER catch up, so it will always have a GTP cap 

- always add in the T form but are cut in the D form 

- can have growing/shrinking 

- if you run out of forms, GTP cap is lost and everything peels off

- if you don't have a centrosome (ie. in a test tube, the - end can get shrunk since it is not ancored, but typically in a call, the - ends are anchored and do not shrink) 

T forms like to polymerase and are strong

- GTP cap at plus end; faster growth

0 stabalizes the plus end, which favours tubule growth 

- dimers in T form and more strongly to other dimers in the tubule

- hydrolysis of bound GTP reduces binding activity of the subunit

- microtubles nucleated at MTOC

- microtubles radiate from MTOC

- joined at their minus ends

- plus ends radiate out towards PM

- centrosomes is MTOC

- nucleates the formation of microtubles

- minus end is stabalized

- plus end is dynamic

- microtubule motor

- minus end directed: moves towards the minus end

dyenin is a large protein complex that is associated with other protein complexes that together transport the cargo along microtubules

- ATP required

- dynactin complex

Dyenin

- Arp1 complex

spectrin

cargo 

membrane glycoprotein 

Axon Vesicular Movement:

Dynein movement vs Kinesin movement

Dynein movement: towards cell body, microtubule minus end

Kinesin: towards axon terminus, microtubule plus end

cell motility

contractile activity

cytokinesis

Composition

Myosin

actin monomers

flexible inextensible

helical filaments

- free monomers bound to ATP, which is bound to the center of protein

- actin is an ATPase

- hydrolyzes ATP

ADP remains bound

- ATP hydrolysis occurs more rapidly after actin monomers have been incorporated into the filament

- growth of the filaments faster at plus ends and have ATP cap 

- actin filament not stable because there is continual exchange of monomers at the ends

- treadmilling concentration: net addition at +end and net loss at -end

as this happens over time, all monomers are eventually replaced 

(remember diagram) 

Proteins involved in treadmilling:

Profilin

Cofilin

Profilin: inhibits nucleation; speeds elongation

Cofilin: accelerates dissasembly of actin monomers

- tails of the two heavy chains organised in: coiled coil

- heads of ehavy chains: associated with four light chains (2 at each end) 

- ATP hydrolyzed by myison head

- skeletal muscle

- motor domains are conserved within myosin family

sliding myosin II on filaments causes muscles to contract 

1. Myosin attaches to an actin filament (by default)

2. ATP binding releaes myosin from actin

3. ATP hydrolysis occurs along with a conformational change that displaces myosin head

4. myosin binding to acting releaes Pi, triggering the force generating shape change

5. at the end of the cycle, myosin is in the initial conformation but has moved to a new position in the actin filament

invloved in structural support

coiled dimer forms staggered antiparallel tetramer

- no filament polarity

- no known motor functions

- packed into rope like filaments

-prominent in cells subjected to stress

- extensible, flexible

- filaments in each cell are anchored:

at sites of cell-cell contact

by desmosomes

- undergo mechanical stress