Cytoskeleton I Flashcards
Name 8 functions the cytoskeleton serves?
Spindle fibres during mitosis, cell shape, cell migration and motility, intracellular trafficking, supports membranes, mechanically links adjacent cells, muscle contraction
Cytoskeleton supports and protects cell from what?
physical forces
Name the 3 filaments the cytoskeleton is made from, from thinnest to thickestt
Actin (microfilaments)
Intermediate filaments
Microtubules
Filaments are composed of what?
repeating subunits
Describe the structure of actin filaments
Stands in double helix with a positively charged end and negatively charged end
what are the actin filaments used for?
maintain cell shape by resisting tensionn (pull)
describe the structure of intermediate filaments
Fibers wound into thicker cables, multiple protofilaments which bind laterally to each other
describe the structure of microtubules
alpha and beta tubulin dimers form a hollow tube with a positive and negative end
Name 6 types of intermediate filaments and where they are located
Keratin - epidermis Desmin - muscle Lamins - nucelus Vimentin - mesenchymal cells Glial Fibrillary Acidc Protein GFAP - Glial cells Neurofilaments - Neurons
What are the main ways in which intermediate filaments differ from the other 2?
Microtubules and microfilaments are more consistent in terms of diameter and composition in euk cells whereas intermediate filaments vary depending on the location of the cell in terms of subunits..
Intermediate filaments are more permanent fixtures, even after removing the other two filaments, the intermiadte filament will retain its shape.
Is there polarity in intermediate filaments?
No, each end is identical
What allows the intermediate filament to be strong in terms of structure?
The staggered long subunits, which are antiparallel to each other, are twisted into a ropelike structure
Actin filaments play what main roles?
Change cell shape, cell locomtion, movemement of organelles inside the cell
Actin subunits are called what?
G actin
G actin assemble in what way to create what type of filament and give the abbreviation for this
Assemble head to tail creating polar actin filaments, F actin
The G acting has a binding site for what?
ATP Binding site
What does an actin protofilament comprise of?
Two parallel filaments of actin monomers assembled end to end (plus-end and minus end) which make them polarised
Are actin filaments flexible or nah?
Yah
Actin filaments are organised into assemblies via what?
Cross linking proteins
What affects the type of assembly of actin filaments and what are the 3 types of assembly?
The type of cross linking protein affects the type of assembly.
Meshwork vs bundles vs contractile
How are contracticle assemblies formed?
By bundles of F actin forming contractile rings
Describe the assembly and what they allow for the following areas in cell
1) stress fiber
2) cell cortex
3) filopodium
1) Contractile and exert tension
2) Gel like network supports the plasma membrane and allows broadextensions of the cell, lamellipodia
3) Tight bundlesin filopodia allow extension
What is the cortex?
A network of F actin found beneath the plasma membrane of many cells
What regulates the formation of the cortex?
Actin binding proteins
What allows for lamellipodia formation
Filamin
Spectrin regulates actin meshwork in what?
RBC
How do F actins assembly?
Self assembles fromactin subunits so undergoes polymerisation
What happens at critical concentration in terms of factin
The rate of subunits ON = rate of subunits OFF
What end of F actin grows more rapidly?
Plus end of F actin grows and shortens more rapidly than minus end
Actin polymerisation can be used to do what in cells?
Mechanical work in cells
if G actin conc is more than the critical conc then what happens? what happens the other way around?
Filament grows when G actin > Cc
and filament shrinks if less
F actin growth is controlled how?
By regulating local available G actin
What complexes can affect the growth of the f actin and what is the connection between the two
Thymosin and profilin, compete with each other for binding to Gactin
What does the thymosin complex do to factin and how?
Thymosin binds G actin and so prevents addition to either the plus or minus end of F actin so it reduces the local G actin at both ends
What does profilin do and how in terms of factin?
Profilin binds to G actin and the actin can be added to the plus end but not to the minus end. Increases the local at the plus end
F actin growth depends upon balance of what?
Thymosin and Profilin
What is the rate limiting step in F actin formation?
Nucleation is a rate limiting step
Filament stability and formation depends on what
Depends on the number of H bonds between subunits.
Small filament assemblies are unstable
Large assemblies are more stable
Filament formation depends on formation of nuclei of critical size
How can the lag phase/rate limiting step be abolished?
Supply a pre mae nucleus, preformed filament seeds are added to start of with
What complex nucleates F actin growth?
ARP complex
The inactive ARP complex joins with activating factors and then actin monomers are able to join to the complex to form a nucleated actin filament
The ARP complex can promote nucleation from where?
Side of existing actin filament, so its very effective polymerisation results in branching
What moves F actin?
Motor protein, Myosin
myosin 2 forms what ?
Thick filaments in striated muscle cells
What happens during muscle contraction in terms of actin and myosin filaments?
They slide against each other
Myosin generates force by coupling what ?
ATP hydrolysis to conformational changes
Describe how the power stroke is generated by myosin and actin
The actinf ilament is bound to the myosin head. Binding of ATP to themyosin head then causes the myosin head to detach from the actin filament. Hydrolysis of ATP leads to the formation of ADP and Pi which causes weak binding to the actin filament. The release of Pi increases actin binding andthe release of ADP causes the power stroke
