Cytoskeleton, cell adhesion and motility Flashcards
what is the cytoskeleton
a network of protein filaments that extend throughout the cell
what does the cytoskeleton do
determines cell shape, polarity, cell movement - extra and intracellular, adhesion and tissue structure
what does the cytoskeleton comprise of
actin (microfilaments) - 7nm
intermediate filaments - 10nm
microtubules-20nm
what are actin filaments called
F-actin (filamentous actin or microfilaments)
what are actin filaments
polymers of individual actin proteins called G proteins
how much of total protein does actin consist of
5%
it is the most abundant protein in all cells
what is the structure of an actin filament
polarised double helix
- 13 actin subunits for every complete turn of the helix -37nm
what gives the actin filament polarity
asymmetric shape if an actin monomer gives polarity to the filament + and - ends
How do actin filaments grow?
- requires ATP to be bound to the actin monomer (G actin)
- filaments are very dynamic
- monomers can be added and removed from both ends of the filament
- monomers add much more rapidly to the + end of the filament
- once incorporated into the filament the ATP is hydrolysed into ADP
where do actin filaments turn over rapidly
cell migration
- neural crest cells
- wound closure
major functions of actin
- Mechanical support
- cell shape changes and maintenance
- cell motility
actin in cochlea in the ear
- held rigid as actin fills it
- not dynamic but static
actin in RBC
RBC have concave shape as actin binds to spectrin and creates tension to pull in centre of RBC to inc SA
what type of actin proteins are there
- membrane attachment proteins
- severing proteins
- cross linking proteins
- capping proteins
- side binding proteins
- motor proteins
- actin bundling proteins
- actin sequestering proteins
structure of intermediate filaments
- rope like polymers of individual intermediate filament proteins
- 10nm diameter
- heterogenous family of proteins
what is the function of intermediate filaments
some functions rather obscure
mainly mechanical
- used to anchor cells at some cell junctions
- support nuclear structure and protects chromatin
formation of the intermediate filament polymer
- intermediate filament protein
- helical dimer
- two dimers combine and form a tetramer (the fundamental unit of the IF)
- tetramers link in a staggered formation and end - to - end to form the filament
are intermediate filaments very dynamic
- grow slowly and don’t change much
- likely to be involved in static processes e.g. maintaining shape
microtubules structure
- long, relatively stiff hollow tubes
- approx. 20-25 nm dia
what is microtubule polymer made up from
- polymer built from monomers of tubulin
- 13 monomers seen in each cross section
- tubulin monomer contains of one molecule of alpha tubulin and one of beta
why do microtubules have polarity
made from asymmetrical monomer
- one end stops with alpha (-
)
and one with beta (+)
assembly and disassembly of the microtubule
GTP rich end (+ve end) can extend and collapse within seconds
- most growth occurs at + end and also most monomers are lost from this end
- end much less dynamic
- Gtp bound monomers assemble onto filament
- GDP bound monomers dissociate rapidly
how are microtubules polymerised in centrosomes in humans
- minus end remains close to centrosome
- plus end points towards the cell periphery
cell shape and orientation
IF and microtubules
IF - stabilise shape of axons
MT - stabilise shape of platelets and axons
microtubules - anchoring organelles
microtubules organise the ER of the cell, they hold together ER and their lipid channels
IF - anchoring organelles
IFs form a meshwork around the cell nucleus to hold it into position
actin filaments - anchoring organelles
required to hold synaptic vesicles close to the presynaptic membrane
- they are tethered to the actin cytoskeleton
cytoskeleton - anchoring cells
essential to anchor cells to eachother and to extracellular matrix at cell junctions
cell motility - actin based movements example
e.g. migration of neutrophils to site of infection by chemotaxis
actin based cell movement - four steps
- cell pushes out protrusions at the leading edge. Actin filament polymerisation provides the force for membrane protrusion
- protrusions adhere to the surface on which the cell is moving through focal adhesions. F- actin connects to the focal adhesions to provide a contractile force for the cell
- the rest of the cell pulls against the anchorage points to drag itself forward
- actin depolymerises at the rear
microtubule based movements - cilia and flagellae
- movement is initiated by the microtubule associated protein - dynein - a minus end directed protein
- microtubules slide along one another cause the cilium to bend
movement if intracellular contents and organelles - microtubules
- movement of organelles e.g. synaptic vesicles along axons to synapse
- two motor proteins kinesin and dynein move cargo like vesicles along microtubules
- kinesin moves towards + end (cell periphery)
- dynein moves towards - end (near nucleus)
