Cytoskeleton Flashcards
What is the cytoskeleton
Allows cells to form specific shapes-useful in nerve cells, skin cells, blood cells
Allows cells to change their shape- useful in immune cells or brain neurons
Allows cells to move- useful in pathogen cells or phagocytic cells
Gives cell strength while also allowing it to be dynamic
Core components of the cytoskeleton
Microfilaments- made of actin, versatile, dynamic, strong, polar, important in cell shape and movement, diameter 7nm
Microtubules- made of alpha and beta tubulin, highway of the cell, dynamic, strong, polar, used for intracellular transport, 25nm diameter
Intermediate filaments- for mechanical strength, less dynamic, not polar, diameter 10nm
How do structure of components relate to their function
Made up of multiple protofilaments as more thermally stable as breaking in two requires breaking more bonds but removal from one end breaks a small number of bonds
Made up of subunits- easier to break down and rebuild, allows cytoskeleton to grow and shrink which allows cell to be responsive, small subunits can rapidly diffuse more easily to move around the cell
Structure and importance of intermediate filaments
No polarity
Rope like strength
Elongated fibrous subunits each composed of 8 tetramers
Lateral hydrophobic interactions
Less dynamic than other filaments
No nucleotide binding site
Not found in all eukaryotes
More diverse than microfilaments and microtubules
Alpha helical monomer > coiled coil dimer > staggered tetramer ( to prevent a weak point where all the ends meet) > lateral association of 8 tetramers
E.g. keratin and laminins
Compare eukaryotic cytoskeleton and bacterial cytoskeleton
Bacteria have homologues of eukaryotic cytoskeletal proteins
FtsZ-tubulin homologue (forms z ring for cell division)
MreB- actin homologue (contributes to cell shape and acts as a scaffold for directing synthesis of peptidoglycan cell wall)
Homologues have more diverse structure and function
Structure of actin microfilaments
5-9nm
Globular subunits (g-actin) make up filaments (f-actin)
+ end and - end
Square molecules
Sort of double helix
All subunits have same orientation
Held together by lateral contacts
Relate microfilament structure to function
Polymerisation needs energy
Actin hydrolyses ATP to ADP
Each unit is polar, giving filament polarity
Held together by non-covalent forces
Filaments have fast (+) and slow (-) growing ends
Explain actin dynamics including treadmilling
Treadmilling- when rates of monomer gain and loss are equal
Critical concentration - concentration at which the end will remain in equilibrium with no net growth or shrinkage
Critical concentration can be different at + end from - end
Faster rate of addition for ATP actin at + end
Rate of loss of ADP actin higher then rate of loss of ATP actin at both ends
ATP actin added, ADP actin lost
ATP actin added to both ends but faster at +
Critical concentration = rate of loss of ATP actin divided by rate of addition of ATP actin
Describe nucleation and why you need it
When actin subunits come together
Can take some time, rate limiting step in formation of filaments
Needed for subunits to assemble before they can start to elongate
Spontaneous nucleation is too slow and random for cell to rely on as it relies on diffusion
Nucleators in controlling actin filament dynamics
Facilitate localisation and timing of filaments formation
More likely to get filaments forming
Nucleator ARP2/3 in actin microfilaments
Related to actin and looks like actin
Binds to side of actin filaments
Creates branches where polymerisation can occur
Remains associated with - end
Nucleator formins in actin microfilaments
FH2 is a part of the formin that binds to actin subunits to create a filament
Remains associated with + end
Formins are dimeric, each subunit has a binding site for monomeric actin and the dimer nucleates by capturing two monomers
Filament elongates as actin-profilin complexes bind to multiple sites of FH1 (another part of formin) and then transfer rapidly to barbed end (+ end)
Actin binding proteins in actin microfilaments
Regulate actin states beyond nucleation
Some proteins to limit the growth of filaments as soluble conc of monomeric actin is often more than 100microM but critical conc is much lower
Thymosin- binds ATP actin (prevents filaments forming)
Profilin- converts ADP to ATP and delivers to + end, speeds elongation
Cofilin- binds ADP-actin and contributes to depolymerisation, accelerates disassembly
There are also cap proteins that can bind to both ends to help stabilise the filaments
Filament severing proteins- multifunctional and can bind to the filament, twisting it and causing it to sever, more ends created so disassembled faster, can be regulated by phosphorylation
Filament binding proteins- filaments can be linked in bundles or gel-like networks, fimbrin causes tight packing, alpha actinin cases loose packing and space for myosin II
Roles of microtubules
Meiotic and mitotic spindle, MTs disassemble which pulls chromosome pairs apart
Internal structures of cilia and flagella
Intracellular transport
Movement of vesicles and organelles
Plant cell wall formation
Structure of microtubule
Alpha and beta tubulin heterodimer
Binds to GTP
Multiple heterodimers form protofilament
Usually 13 protofilaments make up microtubule
Beta subunit sits on top of alpha
Tube like with varying length
Polar
Multiple contacts between subunits make MTs stiff and difficult to bend
Explain microtubule nucleation
MTs nucleation from complex called microtubule organising centre (MTOC)
Nucleate filament from - end
Nucleation complex composed of a gamma tubulin ring complex
Gamma tubulin ring complex is a template for 13 protofilaments
Most mammalian cells have a single MTOC, the centrosome which is a pair of centrioles
Fungi and plants dont have centrioles
Explain MT dynamic instability
Alpha tubulin is GTP bound, doesnt change
Beta subunit is GTP or GDP bound, can change
Dynamic instability is dependent on rate of addition and rate of GTP to GDP hydrolysis
When conc of GTP tubulin is high it is added on to the end
Catastrophe is rapid shrinkage, GDP induced disassembly because GDP end depolymerises 100x faster than GTP
Catastrophe happens if GTP tubulin conc decreases and GTP cap is lost
Rescue is a switch back to polymerising as GTP cap regained
Describe the role of MAPs in controlling MT dynamics
Microtubule associated proteins (MAPs)
Allow MTs to connect and interact with other proteins
MAP2 causes bundles
TAU makes much tighter bundles
Both have an MT associated part and an arm that contributes to distance between MTs
Stabilising proteins capture and protect the growing + end and decrease catastrophe
Katanin severs MTs, has ATPase activity, causes conformational change that puts strain on MTs, causing them to sever, more ends so more likely to be depolymerised
Describe how drugs can be used to impact the cytoskeleton
Colchicine- binds to beta tubulin and inhibits assembly
Vinblastine- aggregates tubulin heterodimers, can be used in cancer treatment
Taxol- stabilises microtubules, can be used in cancer treatment- MTs can’t disassemble
What is the requirement for motor proteins in a cell
Diffusion alone in cells is not enough
Motor proteins used in flagella
Muscle contraction (myosin)
How to motor proteins generate movement
Energy from ATP hydrolysis used to stretch an elastic element in the motor, causing conformational change
If motor is anchored and elastic force exceeds resistance, the track will move
If the track (filament) is anchored, the motor and any attached cargo will move
Kinesins
MT motor
Takes large cargo e,g, vesicles
Move towards + end of MT
2 heavy chains, 2 heads, coiled coil tail with light chain bound to the ends of each heavy chain
Cargo binds to tail
Core catalytic domain folded
Bind and hydrolyse ATP, bind to MT
2 heads work together to take alternative steps along MT
Binding of ATP and loss of ADP pulls the rear head forward in a step motion
Dynein
MT motor
Moves to - end
2 or 3 heavy chains plus several chains associated with tail
Cargo binds to tail in cooperation with dynactin complex
ATPase ring
Largest and fastest molecular motors
Diverse- move cargo and beat cilia
ATP hydrolysis causes motor to bind MT
Myosin
Globular head and coiled coil tail domains
Head generates power stroke
Light chains associated with neck to stabilise head for power stroke
Mostly move towards + end
