Cytoskeleton Flashcards
what are the 3 types of filament proteins?
- actin (AFs / microfilaments)
- microtubules (MTs)
- intermediate filaments (IFs)
how are filaments constructured?
small subunits of actin and tubulin (both compact, globular proteins) assemble into long, thin filaments
microtubules are linear arrays of tubulin subunits that are held together by weak noncovalent bonds (flexible and dynamic for remodelling)
what accounts for changes in the cytoskeleton (filament)
filaments’ ability to disassemble, diffuse and reassemble rapidly
what is nucleation?
initial step in forming polymers.
clusters of polymers come together to form a nucleus/binding site > once it reaches a critical size, subunits are added to form a polymer chain (elongation process) > once there are enough monomers, it reaches a steady state (equilibrium phase) and is able to interact and adapt with structures in the cell
what is the structure of tubulin?
heterodimer (1 subunit of tubulin is made of 2 different proteins), each subunit binds to GTP to be hydrolyzed at one site (since it has a plus and minus end), 13 protofilaments form a tube-like structure
- –> O
***
- –> O
what is the structure of actin?
is a monomer with a plus and minus end that used ATP to bind into a filament (polymer)
What are dynamic instability?
principles that allow filaments to grow and shrink.
actin and tubulin catalyze hydrolysis of ATP/GTP respectively –> there is a critical concentration (Cc) where subunit addition is equal to subunit loss but caps can form to stabilize a filament and prevent rapid/excessive depolymerization
what happens below critical concentration? above?
below: filament shrinks
above: filament grows
what is treadmilling?
principle of growth in actin and microtubules
when ATP-actin concentration is high: addition occurs on both ends (still higher on the plus end)
if ATP-actin concentration is low: addition is greater at the plus end (but hydrolysis catches up
TREADMILLING: the otherwise stable equilibrium of increasing and decreasing at the same rate, giving the illusion that actin filaments are moving in the plus direction
what is dynamic instability (microtubules)
- tubulin within critical values allows for dynamic growth and shrinkage
- GTP caps at plus end stabilize and set a limit on growth
- transitions continually between growing and shrinking
explain the process of dynamic instability
a GTP cap placed on the plus end promotes growth –> loss of GTP cap (hydrolysis occuring faster than subunit addition) and sudden CATASTROPHE (rapid loss of tubulin) –> tubulin concentration increases (RESCUE), GTP-cap regained, rapid growth occurs again
GTP hydrolysis leads to conformational changes and instability
what changes based on the speed of kinetics (binding and hydrolizing)
the tip of tubulin
- when slow: blunt tip
- when faster: has higher concentration of tubulin and tapered (pointed) tip to promote faster movement of tubulin subunits
what is the point of treadmilling and dynamic instability?
constant ATP consumption which allows for spatial and temporal flexibility, high turnover, rapid growth of filaments without nucleation, allows movement so they can explore the cell and look for structures with attachment cites and remodel
what are intermediate filaments? what’s their structure?
provide structural and mechanical support, often arranged into cross-sections of 16 dimers, with disassembly leading to phosphorylation
staggered long subunits, rope-like appearane, can be formed like a dimer or tetramer antiparallell formation or bound to other tetramers
list examples of IFs (intermediate dilaments) and their uses
- epithelial (keratins): type i and ii, provides strength in hair and nails
- axonal (neurofilaments): found in central and peripheral axons of neurons, growth increases the diameter of axons
what are accessory proteins?
accessory proteins help motify cytoskeletal dynamics (formation of higher order structures)
what does gamma tubulin protein complex do?
intiates nucleation at the - end
what is the process of nucleation by gamma tubulin
Microtubule Organizing Center MTOC (centrosomes in animals, spindle pole in yeast, cytosol in plants) act as a nucleation site, gamma-tubulin forms gamma-tubulin ring complex TURC (a template to make MTs), nucleation occurs at the - end to ensure MTs grows at the +, TURC accelerates and stabilizes structure, after this, existing MTs act as templates for daughter MTs
what is the structure of a centrioles and centrosomes?
centrioles: 9 triplets of microtubules (triplets form like this: ooo)
centromsomes: 2 cylindical centrioles
what do centrioles organize? what is the organizee’s function?
pericentriolar material (PCM), initiates MT assembly
what contains gamma tubulin
PCM and the lumen of cetrioles
how are MTs organized?
- ends are anchored to centrosomes while the + ends radiate outward to enable growth and organization
where are AFs generally nucleated?
- in the cell periphery (cortex) where AF is highly concentrated
- the location defined by function of the AF and is facilitated by actin-binding proteins (ABPs) like actin-related proteins (ARPs)
how are unbranched vs branched AF nucleated
unbranched: nucleated by formin, which aligns actin monomers for polymerization
branched: nucleated by Arp2/3 complex (7 proteins), which bind to the minus end of filaments and creates branches at 70-degree angles (ideal for motility)
how are subunits controlled?
actin and tubulin concentrations are maintained high in the cytosol, where concentration can exceed critical concentration
- sequestering proteins may isolate unused proteins so they are not hydrolyzed, provides control and regulation of filament elongation
what sequesters actin and how
thymosin, it makes polymerization less favourable
what is the role of profillin
profillin competes with thymosin to promote assemly and recruit monomers
what is the role of stathmin? how does it complete this role?
sequester tubulin, stathmin binds to prevent polymerization, reduces the concentration of effective tubulin (available ones for MT growth) and promote dynamic instability (specifically catastrophe)
What are MT-Associated Proteins and what do they do? MAPs
- stabilize MTs and regulate their spacing through MAPs’ multiple binding domains (length of projecting domain = how closely packed MTs are)
what is the role of MAP2 vs tau
MAP2: found in neuronal cell bodies and dendrites
Tau: found in axons, helps stabilize MTs and maintain organization (hyperphosphorylated in Alzheimer’s making it unable to bind or stabilize –> forms insoluable tangles and contributes to neurodegeneration and breakdown of MT networks for function)
list AF binding proteins and their functions
cofilin: destabilizes AFs by binding to the side of proteins, inducing mechanical stress, enhances treadmilling and filament turnover for cell movement
tropomyosin: stabilizes AFs by binding to the side of actin filaments, strengthening them, preventing disassembly, critical for muscle contraction as a regulator
how are AF and MT ends modified
AFs: capping proteins like CapZ bind to the plus end, prevent elongation, allow growth only at the - tromodulin caps in muscle cells at minus end to regulate filament length when contracting
MTs: capping proteins affect dynamic instability by stabilizing and destabilizing ends, influencing growth and shrinkage during mitosis (stablilizing: longer, less dynamic MTs, destabilizing: shorter, more dynamic MTs)
what are the types and functions of cross-linking proteins and AFs
function: cross-linking proteins like AFs to create complex, organized networks
in AFs: distance between these groups on cross-linking proteins determine the type of structure formed with AFs
Types: bundling, gel-forming
what is the function of bundling proteins, what are its different types and what do they do?
align and compact actin into tight, parallel bundles
- contractile bundling with alpha actinin: loose packing, allow mosin-ii to enter (for muscle contraction)
- parallel bundling with fimbrin: tight packing, prevents myosin ii from entering
what is the function of gel-forming proteins?
link actin in more flexible, mesh networks to create gel-like structure
eg. forms spectrin in red blood cell membrane
what leads to changes in cell shape during embryonic development?
neural tube formation in a vertebrate embryo:
MTs: influence cell height to provide structural support
AFs: folds into a tube during development to facilitate cellular movement and shape changes
explain induction by extracellular signals
process through which external cells (chemical cues/stimuli) initiate cellular responses
signals activate intracellular pathways (extracellular signal - activation - nucleation - promotion - elongation - destabilization) ->
cytoskeleton reorg (especially of AF) -> enables cell crawling
What is the basis of motor proteins? how do they work and what are the subtypes?
- use energy derived from hydrolysis of ATP to produce mechanical force
- bind to cytoskeleton (AF, MT)
- produce net movement of protein/cargo
- divided into 3 familes (myosin, kinesin, dynein)
describe myosins
there are many types of myosins, types vary depending on the species they are in, in most eukaryotes, I, II, V, specifically II and V in humans. >— (type II) >—8 (type V)
- conventional myosins (type II) are used in cytokinesis and muscle contraction
- unconventional myosins (type V) are used in organelle transport
what is the structure of myosins?
2 heavy chains of alpha helices (form the head allowing for catalysis and providing force, and tail allowing for dimerization and filament formation)
2 light chains, each made of 1 essential and 1 regulatory chain, which amplify conformational changes
what is the myosin cycle?
- attached: no ATP, locked to actin
- release: ATP binds and creates a conformational change moving myosin from AF
- cocked: ATP is hydrolyzed to ADP and the myosin head cocks forward towards the + end of AF
- force-generating: weakly rebinds to actin after phosphorus is released and power stroke and ADP is lost
how are thick filaments formed?
tail to tail interactions, heads are oriented in the opposite direction (with them pointed towards the center), bundles are formed in sarcomeres
structure of a sarcomere
z-discs: plus end of actin filaments and capZ embedded, on either end of the sarcomere
m line: in the middle of the sarcomere, between minus ends of actin, where the myosin tails align (dark band)
explain the Ca2+ dependence of muscle contraction
when there’s no Ca2+, tropomyosin blocks myosin-AF binding –> Ca2+ is released from the sarcomere and binds to troponin –> conformational changes in troponin i occur –> tropomyosin (bound to troponin t) moves and exposes the binding sites –> myosin binds to actin –> muscle contracts
explain myosin ii at S1 fragments
addition of protease enzyme leads to myosin being cleaved between the neck and tail, since S1 contains a catalytic site, it can be immobilized on glass surfaces and can slide AFs on them by using ATP
explain contractile rings and their steps
important for cytokinesis, composed of myosin ii and actin,
1. myosin ii and actin form a ring at the cells equator
2. they contract to divide the cell into 2 daughter cells
3. they reorganize into ring structures that contract to pinch the membrane and complete cytokinesis
explain the structure of myosin v
has a long neck region and multiple light chains, walks hand-over-hand in steps along actin (continuous movement without detaching), transports cargo within cells
experiment done with an optical trap with feedback control to measure 30-40nm per step, meaning myosin v walks processively with the ability to transport cargo
explain how kinesin works
involved in axonal transport, walk 410 mm/day walking towards the + end
- structure: conserved head like myosin, diverse tail region with a c-terminal that attaches to cargo, arms swing as the linker region interacts with its catalytic core, making it swing its arms
explain the kinesin cycle
to take processive steps along microtubules
1. ATP binds to the leading head, creating a conformational change to kinesin’s linker region, the trailing head advances
2. induces weak binding of the leading head to the MT
3. hydrolysis of the trailing head makes it detach, ADP dissociates from the leading head
how do myosin ii and kinesins differ?
myosin ii: attaches to AFs, contracts muscles with a rapid power stroke, no head coordination, 5% of the time attached, ATP binding causes release, 2-60 micrometer/s, stronger force
kinesins: attaches to MTs, transports organelles with longer attachment, processive steps, 50% of the time attached, ATP hydrolysis causes release, <2 micrometers/s, weaker force
explain how dyneins work (including subtypes)
- move towards the - end of MTs
- 2 divisions: cytoplasmic and axonomel
-> cytoplasmic: used for retrograde vesicular transport within cells, moves larger items towards minus end/cell center
-> axonomel: used to beat cilia and flagella, needed for movement and sensory functions of cells
structure: 8>D
explain and describe cytoplasmic dynein mechanisms
dynactin complex (includes accessory proteins like red actin) helps attach dynein to MT and cargo as it cant bind directly (to cargo but uses as support for MT)
how are dyneins used in axonal transport?
vesicle-bound dyneins walk towards the cell body, with a kinesin attached
