Cytoskeleton

microtubules

assembly of microtubules

subunits stack together to form a hollow cylindrical microtubule

this appears as a cylinder made up of 1.3 protofilaments

tubulin made up of a dimer of a and b tubulin

general facts

long hollow cylinders made up of tubulin monomers

typically grow from a microtubule organising centre

are more rigid and straight than intermediate or actin filaments

have polarity

approximately 25 nm in diameter

microtubules are maintained by a balance between assembly and dissassembly

a cell contains a mixture of microtubules and free tubulin

GTP hydrolysis is thought to control growth of microtubules

rapid growth with GTP-capped end

accidental loss of GTP-cap = catastrophe

rapid shrinkage

regain of GTP cap - rescue

rapid growth with GTP capped end

intermediate filaments

construction of an intermediate filaments

made up of monomers with a central rod domain and a globular region at either end

monomers dimerise

two dimers line up to form a staggered tetramer

tetramers can pack together end to end

eight tetramers are twisted into a rope of diameter approx 10 nm

cytoplasmic

neurofilaments

nerve cells

vimentin and vimentin related elements

connective tissue, muscle cells and neuroglial cells

keratins in epithelia

provide tensile strength for cells

particularly abundunt in cells that are subject to mechanical stress such as muscle cells, epithelial cells

typically form a network throughout the cytoplasm, surrounds the nucleus and extend out to the cell periphery

often anchored at the plasma membrane cell junctions

nuclear

nuclear lamins

in all nucleated cells

desmosomes

celll-cell junction which connects filaments from different cells

cadherins span the membranes and bind the 2 cells together

actin filaments

cell crawling

motor proteins

can bind and hydrolyse ATP

provides energy for movement along actin filament from minus to plus end

belong to the myosin family of proteins

caused by actin cytoskeleton rearrangement (polymerisation)

cycle

stage 3: hydrolysed ATP to ADP + Pi which moves myosin forward

stage2: ATP binds, reduction of affinity of myosin for actin: release

stage 3 : hydrolysed ATP to ADP + Pi which moves myosin forward

stage 4: weak binding of myosin to actin at new position --> release of phosphate --> thrust of protein making actin move

once again myosin is back into original position then ADP is released - back to stage 1

2 types

filopedia

needle like projection of the plasma membrane

lammelopedia

sheet like projections of plasma membrane

general information

filaments are made up of globular monomers that associate head to tail

provide mechanical strength and cell shape

actin filaments are usually concentrated in a layer just below the plasma membrane (cortex)

linked into a meshwork by actin binding proteins

found in all eukaryotic cells

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filaments are unstable without associated proteins

polymerisation of actin

actin monomer in the cytosol carry ATP

ATP is hydrolysed soon after assembly into filament

ADP bound monomer is less stable in the filament

the ADP cannot be exchanged for ATP until the monomer dissassembles

concentration of G actin is also critical

above critical concentration of G actin the molecules

G actin froms F actin in the presence of ATP, Mg and K

Motor proteins

drive intracellular transport

dyneins move towards the minus end of microtubules

have ATP including head and tails

both have ATPase activity

kinesins generally move towards the plus end of microtubules