lec 16 Flashcards
(25 cards)
treadmilling
1 end of the cytoskeletal filament grows while the other shrinks
- rates of subunit addition and loss are the same at each end
actin monomer (g-actin)
ATP bound to a monomer
slowly hydrolyzed to ADP post-polymerization
actin filaments (f-actin)
- subunits assemble head-to-tail to generate helix structure w/ 2 parallel protofilaments
- has structural polarity
- short to long chains of actin monomers
- nucleate near the PM
- highest density near cell periphery - in eukaryotic cells
- highly conserved
what are the main isoforms in actin filament?
- alpha
- muscle actin - beta and gamma
- in all cells
nucleation
- first step in filament formation
- first actin monomer combine to begin forming new filament
- slowest step
actin cellular function
- cell shape
- microvilli structure
- contractile ring
- cell movement
actin filaments are dynamic and can undergo “treadmilling”
dynamic: can form, crosslink, breakdown and reform rapidly
“treadmilling”: rate of subunit addition on one end is the same as the loss on the other end
key structural elements of actin filaments
- filaments = polymers
- polymers of g-actin as a helix - structural polarity
- kinetic rate constants for association and dissociation are greater at 1 end than the other - microfilament-associated proteins or centrosome
- help actin filaments form and determine structure and function of actin filaments
microtubules
- long, hollow cylinders made out of tubulin
- genes encoded in tubulin = highly conserved - larger and more rigid than actin filaments
- long and straight and have 1 end (minus end) attached to 1 centromere
- nucleate at centromere and emanate out toward cell periphery
- polymerization involves GTP hydrolysis
- GTP in beta-tubulin only is hydrolyzed to GDP once polymerized - rapidly falls apart
- dynamic instability
microtubules func
- move chromosomes apart during cell division
- form mitotic spindle
- structural component of cilia and flagella
- direct transport of the intracellular components through molecular motors
what are the isoforms of the microtubules?
- alpha and beta
- form tubulin dimers that make up the microtubule
> each dimer is heterodimer of alpha and beta tubulin - gamma
- in centrosome
> nucleation site of microtubules
intermediate filaments
- made of intermediate filament proteins
- constitute large and heterogenous family
- not dynamic and less common
- no structural polarity or molecular motors
- does not aid in intracellular transport
what are some examples of intermediate filaments?
- lamin
- forms the nuclear laminate beneath the inner nuclear membrane - keratin
- in epithelial layer
- spans the cytoplasm from 1 cell-cell junction to another
> strengthens the entire epithelial
> Provide mechanical strength
molecular motors
- proteins that move along the microtubules or actin filament
- ATP dependent
- important for intracellular transport and cell movement
what are the basic types of molecular motors?
- myosins
- all except one moves along the actin filaments towards the plus end
- there is one that moves towards the minus end - kinesins
- moves along the microtubules towards the plus end
> away from the centromere - dyneins
- moves along the microtubules towards the minus end
> away from the centromere
molecular motors = ATPases that couple hydrolysis w/ movement
globular heads on fibrous tails
1. heads have ATPase activity, dictate filament track, direction and speed
2. tails have “cargo” binding
globular heads bind to actin filaments/microtubules
ATP binding and hydrolysis on globular heads lead to
conformational changes that cause “walking”
how are actin filaments a part of the formation on he centromere
- assembled and disassembled rapidly
- help regulate cellular activity and movement
formation dependent on centromere’s that mediate different aspects of assembly
- nucleation: formin and ARP 2/3 complexes nucleate f-actin
- rate-limiting step for filament formation - kinetics: thymosin, profilin and capping protein
- determine speed of subunit addition - shape: filament binding proteins
- determine final structural shape - durability: stabilizing and disassembly of proteins
- determine permanence
formin (nucleation)
- nucleates actin polymerization
- captures 2 actin monomers
- once nucleated
> Actin polymer rapidly extends - during polymerization
- formin stays associated w/ the plus end - forms straight, unbranched filaments
ARP (actin-related proteins) complexes
- nucleates growth and stays attached to the minus ends
- binds to existing actin fibers at an angle
- to form a tree-like web of actin - to make dense actin meshwork/gel
- cell must express and activate ARP complexes - activating factor = req to activate ARP complexes
- cells can manipulate act in polymerization by manipulating the expression of activator factor
thymosin and profilin
- regulate the speed of actin polymerization (kinetics)
- compete for g-actin binding
thymosin
- inhibits polymerization by keeping the g-actin in a “locked” state
profilin
- promotes polymerization
> facilitates ADP to ATP nucleotide exchange
capping proteins (kinetics)
- helps form long-term stable structures
- prevents polymerization and depolymerization at the plus end - cap = slow growth and stabilized fibers
filament binding proteins (shape)
- bundling proteins (e.g. actinin and fimbrin) cause lateral association of filaments
- actin cross-linking proteins (e.g. filamin) that have a bent connection between 2 binding domains
- stabilize actin webs/gels
