Cell Structures Flashcards
what are the features of a cytoskeleton?
highly ordered. dynamic network of filaments
what are the roles of the cytoskeleton?
- responsible for maintaining cell shape
- important in movement of cell and internal structures
- cell morphology
- cell migration
- vesicle transport
- cell division
why is the cytoskeleton dynamic?
has proteins that self-assembly in long polymers with repeating subunits
give a brief overview of micro-filaments (actin)
- cellular movements
- muscle contraction
- cell division
give a brief overview of micro-tubules (tubulin)
- scaffolds
- cell shape
- transport tracks
- mitosis, pulls sister chromatids apart
give a brief overview of intermediate filaments
contain various proteins and provides tensile strength
how do these different components of the cytoskeleton interact?
- network that extends through the cell
- some overlap
- reflects cooperation but each have unique functions
- the monomers give the morphological features to set them apart
how is actin dynamic?
it is constantly polymerizing and depolymerization
how do cells move?
through change in cell shape which is driven by the actin cytoskeleton
when is cell movement important?
in healing wounds
what are the different features that help cells move?
- tail
- ruffle
- filopodia
- lamellipodium
- actin bundles
- stress fibres
- leading edge
- focal adhesions
what are ruffle?
assemblies that do not form tight adhesions with the substrate
what is filopodia?
finer cytoplasmic extensions. typical of slower moving cells.
what is lamellipodium?
broad membrane extension that move forward, typical of migrating cells
what are focal adhesions?
structures that form mechanical links between intracellular actin and extracellular substrate
what is needed for a cell to move along a surface?
needs to be able to hold onto something to pull it along
how does the cell move?
- uses focal adhesions to pull it along
- for instance holds onto the extracellular matrix
- link the intracellular actin filaments to the extracellular matrix through integral membrane proteins (integrins)
- as the cell moves focal adhesions assemble and disassemble (push and pull)
how do microfilaments generate force?
they generate force through the assembly of globular monomers (G-actin) into filamentous polymer (F-actin)
why does G-actin form a helical structure?
provides strength
describe F actin
a tight helix
repeating unit = 14 subunits
what are the features of G-actin?
- found in all eukaryotes
- highly conserved
- multiple isoforms
what is the structure of G-actin?
- two lobes separated by a large cleft
- four domains with a hinge between domains I and II
- the hinge allows lobes to move relative to each other forming a nucleotide binding cleft
why is nucleotide binding important in G-actin?
it stabilizes structure
G-actin stability (G-actin is unfolded in the absence of nucleotide)
what are the functions of G-actin?
- nucleotide binding
- can bind ATP and ADP - bound forms are identical apart from domain II
- doesn’t have structure unless ATP/ADP bound
- they have polarity as +ve end (barbed) and a -ve end (pointed)
what happens in a filament?
the -ve ends line up and point the same way. self assembles into F actin
what are the 3 stages of actin assembly?
- nucleation
- elongation
- tread milling
what is nucleation?
- formation of a trimer, 3 actin monomers
- occurs when concentration of action is higher than the critical concentration
why is the critical concentration important in nucleation?
- More likely to interact with each other
- Rate of assembly will increase as concentration increases
what is elongation?
- after nucleation filament extends through addition of actin monomers to each end of the trimer
- creates f actin
- all monomers have the same orientation
- f actin is created in a polarisaed manner
- f actin is polarised
which ends can G actin monomers add to?
both ends, but only when the concentration is high enough
what happens if you put in a solution below the critical concentration?
disassemble
where is there faster assembly?
the positive end
what is treadmilling?
monomers add to the +ve end and dissociate from -ve end
- filaments stay the same length but theres movement
what does treadmilling rely on?
actins atpase activity
- hydorlysis is not required for polymerisation but important in treadmilling
- critical concentration
what has higher ATPase activity?
Factin
describe the process of actin and ATP
- actin with ATP joins at the +ve end of the filament
- when bound it hydrolyses its ATP
- longer its bound the more likely it is to have ADP
- actin at -ve end more likely to have ADP
- actin with ADP has a lower affinity to bind with F-actin
what are the differences between ATP actin and ADP actin?
- conformational differences
- ADP actin has a lower affinity than ATP actin
- Dissociates more rapidly
what is the source of the critical concentration of actin polymerisation?
- more recently polymerised ATP bounds ubunits at the +ve end but more ADp containing subunits at the -ve end
how is the ATP-ADP cycle completed?
- with nucleotide exchange
- at -ve end release and exchanges ADP to ATP
- can rejoin at the positive end
what are the features filopodium?
- parallel bundles
- thin extensions from the leading edge
- positive ends at one end
- negative ends at the other
what are the features of lamellipodium?
- branched and crosslinked netowrks
- plate like projections that go out at the leading edge
- extremely well ordered
- ordered by proteins
what are the features of stress fibres?
- anti parallel contractile structures
- opposite direction from the leading edge
- mixture of -ve and +ve ends
what are the features of the cortex?
branched and cross linked networks.
- underlies the cell surface
- tensile strength
why is actin remodelled?
in response to environmental cues, stimulate cell division, differentiation or locomotion
what are the roles of the cells regulatory mechanisms for actin?
- assembly and disassemble actin
1. inhibition of polymerisation of G to F
2. nucleation of new actin filaments
3. control actin filament length
4. elongation/shortening of pre existing actin filaments
what are the regulatory mechanims for actin carried out by?
- actin binding proteins
what are the roles of actin binding proteins?
regulate actin polymerisation and organisation
associate with monomers or filaments
what are some actin binding proteins involved in polymerisation?
thymosin beta 4
profilin
what is thymosin beta 4
- Found in metazoic cells
- Smaller peptide
- Sequesters monomeric actin
- Binds to the actin to prevent G actin from polymerising
- Get a build-up of actin-ADP
- Unable to add onto actin microfilaments
what is profilin?
- Binds ADP – G-actin
- Binds to monomeric G-actin
- Increases rate of nucleotide exchange
- Binds to opposite ends of nucleotide clefts
- Prevents F-actin binding to the –ve end
- F actin from the +ve end
how is nucleation regulation?
provides a template for the trimer to form
what are the proteins involved in regulating nucleation?
formins and Arp2/3
what is formin?
bind individual actin together
what is Arp2/3
performs a similar function to formins, also initiates branching of microfilaments by binding to an actin filament and initating polymerisation of a new branch
how is actin filament length regulated?
severing and capping proteins
what is severing?
breaks actin filaments into shorter fragments
what is capping?
stop the filament from growing or shrinking, acts to stabilise the filament
what happens if an actin filament is capped at both ends?
it will not lose or gain monomers
what is Gelsolin?
- dissolves actin meshes
what is the function of Gelsolin?
- disrupts subunit organisation by binding to Factin
- severing filament
- capping +ve end
- uncapped -ve end can disassemble
- has 2 different properties
how is Gelsolin stimulated?
- by elavated Ca2+ concentration
- trnaslates extracellular signals
how is Gelsolin inhibited?
- inhibited by P1P2 - different cell signalling molecules
what is Cofilin (ADF-cofilin)?
binds ADP actin in bothe monomeric G actin and in F actin
what is the function of Cofilin?
increases the rate of depolymerisation of ADP actin
- found mainly at the -ve end
- dissociates subunits
- prevents nucleotide exchange (stopping polymerization on the +ve end)
what are some examples of cross linker proteins?
- alpha actinin
- filamin
- fimbrin
what is alpha actinin?
- Contractile bundle
- Stress fibre
- Contains 1 actin binding domain as its part of a bigger protein (dimeric)
- Anti-parallel
- Head to tail structure
what is filamin?
- Gel like network
- Cell cortex
- Contains 1 actin binding domain as its part of a bigger protein (dimeric)
- Coordinating the network in the cortex
- V shape structure
- Binds to F-actin organises the network
what is fimbrin?
- Tight and parallel bundle
- Filopodium
- Contains 2 actin binding domains (monomeric)
what do all cross linker proteins contain?
an actin binding domain (tight parallel)
what is spectrin?
- tetrameric binding
- Organises filaments into a specific network
- Almost exclusively in RBC gives them their shape
what happens if there are mutations in spectrin?
- underlie severe anemia - RBC lose their shape and are not as efficient at carrying oxygen
what is dystrophin?
- monomeric binding
- Carries a single binding domain
- Binds and links a multi-protein complex found in plasma membrane of muscle fibres
- Connecting muscle fibre cytoskeleton to extracellular matrix
- Role in the force generation in muscle
what do mutations in dystrophin cause?
- underlie muscular dystrophies
what role do actin filaments have in the cell?
- serve as tracks for transport of components
- myosins walk vesicle along actin filaments
- sometimes very long distances
what are the key features of myosin?
- multigene family
- 1 or 2 heavy chains and light chains
- heavy chains have a globular head
why is the globular head important in myosin?
- binds to actin (which has ATPase activity)
- allows it to walk a vesicle along the cytoskeleton
what are the different types of myosin?
myosin I, II and V
what is myosin I?
- tail binds to membranes
- cytoskeletal - membrane interaction
- filopodia, microvilli
- link cytoskeleton to plasma membrane
- (monomer)
what is myosin V?
- cytoskeletal - membrane interaction
- vesicle transport
- can link to transport vesicles to power transport
- leave the globular heads to walk along the cytoskeleton
- (dimer)
what is myosin II?
Doesn’t link actin to a membrane but generates contractile force
Muscle contraction, cytokinesis
Tightens contractile ring cell division
Found in muscle fibres instriated muscle they are assembled into higher ordered structures, with tails packed together to form a thick filament where actin-binding globular heads protrude
Organised into thick filaments
All the tails together with the heads sticking out
Regulated by phosphorylation
- (dimer)
what is the sliding filament model in striated muscle contraction?
- sarcomere is the function unit - thick and thin filaments
- actin filaments anchored to the z disk
- controlled by the interaction of myosin heads sticking out from the fick filaments with actin in the thin filaments
what is the first stage of the sliding filament model?
- myosin movement energy
- ATP hydrolysis channeled through changes in myosin heavy chain
what is the second stage of the sliding filament model?
- ATP binding causes conformation change in myosin
- disruption in actin binding site (releases)
what is the third stage of the sliding filament model?
• ATP hydrolysis causes conformational change in myosin, hydrolysis products trapped
- Actin binding site restored (head pivots and binds)
what is the foruth stage of the sliding filament model?
- Conformational changes in head and neck are transmitted and amplified to other parts of the molecule through the light chains bound to the neck (power stroke)
- Structures slide past each other and shorten the sarcomere
what are the key features of intermediate filaments?
- lots of different types - large diveristy in size and sequence
- there are 5 major classes
what are keratins?
- make up cystole intermediate filaments
- extend to the cell membrane in skin epithelial cells
- type 1 and type 2
what are the main features of keratins?
- fibrous proteins
- outer epithelial = skin cells
- prominent in skin, hair and nails
- heterodimers of basic and acidic subunits
what are mutations of keratins involved in?
mutations involved in skin disease
what are type 1 keratins?
acidic keratins
what are type 2 keratins?
basic keratins
what are vimentins?
- type III intermediate filaments
- widely distributed (stromal tissues, lymphocytes, endothelial cells, fibroblasts)
- supports cell membranes and keeps nucleus and organelles in position
what are neuronal IF proteins?
- type IV intermediate filaments
- neurofilaments
- structural role in axons
- determine axon diameter and hence their speed of conduction
what are lamins?
- type V intermediate filaments
- fibrous network supporting the inner nuclear membrane
- may organise different types of chromatin (gene regulation)
what is the process of assembly of intermediate filaments?
- 2 monomers come together and wrap around to form a parallel dimer
- two heads (N terminal) and two tails C terminal)
- 2 dimers go head to tail to form an antiparallel tetramer
- tetramers stack end on ened to form a profilament
- they double up to form a protofibril
- 4 protofibirls wrap around each other to form an intermediate filament
where are the differences mainly found in intermediate filaments?
- the globular heads and tails
- the alpha helical region is highly conserved
what are the different forms of intermediate filaments?
- heterpolymers (keratin type I and type II)
- homopolymers
(vimentins can be either)
what helps form heteropolymers in intermediate filaments?
spacer sequence
what does intermediate filament assembly not require?
doesn’t need ATP or GTP - a spontaneous process
what is the physiological role of intermediate filaments?
- anchor to cell junctions (cell adhesion)
- cell integrity
- position structures within the cell
describe stem cells in the basal epidermal layer
- different and change keratin expression profile as they progress to the outer layer
- the differentiation causes different characteristics of the skin
what are keratin 5/14 heterodimer?
- assemble into IFs in basal keratinocytes
- critical for cell structure
what happens if there are mutations in keratin5/14?
N – or C – terminal mutations unable to form (end to end) protofilaments
Cells at the base of the epidermis are weakened
Epidermis and dermis easily separate
Blistering disease epidermolysis bullosa simples: slight abrasions cause serious wounds. Demonstrates the importance if keratins.
what is hutchinson - gilford progeria syndrome?
- Rare premature ageing disease
- Caused by mutations in the LMNA gene producing an abnormal form of the nuclear intermediate filament Lamin A
- Nuclear envelope becomes unstable and prone to damage and mutations cause phenotypes associated with ageing
- Osteoporosis, hair loss, cardiovascular disorders, diabetes, muscle atrophy
what are microtubules?
- polymers of globular tubulins
- cellular tracks used by microtubule motor proteins to transport
what are some example of motor proteins?
kinesin and dynein
what heterodimers do tubulin form?
alpha tubulin and beta tubulin
what is alpha tubulin?
- binds GTP irreversibly
- GTP is not hydorlysed
- non exchangeable GTP
what is beta tubulin?
- binds GTP reversibly
- GTP hydrolysed to GDP
- exchangeable GTP
how do heterodimers (tubulin) polymerise?
- polymerise head to tail to form polar tubular structure
- they have polarity
how is a tubulin protofilament formed?
- assemble end to end to form a protofilament, this then forms a sheet which curls into a tub
describe protofilament assembly? (tubulin)
- Tubulin dimers exist in cytoplasm
- As concentration increases to above critical concentration for assembly
- Assembly longitudinally to form short profliaments
describe sheet assembly (tubulin)
- protofilaments associate laterally into curved sheets
- increases stability
- can carry on forming longitudinally
how are the tubules formed?
- sheets wrap around to form hollow stable structures
- grows by adding more dimers
- rate of assembly faster at the (+) end
- orientated so that beta tubulin with hydrolysable GTP is at the (+) end
- GTP on beta tubulin hydrolyses on binding
- if rate of addition is greater that GTP hydrolysis, a GTP cap is produced- serves to stabilise the tubule
when are microtubules formed?
when dimer concentration is higher than the critical concentration
- as tubules grow pool of dimers will decrease
what happens to microtubules if the dimers go below Cc?
microtubules start to disassemble
- known as dynamic instability
what are microtubules anchored to?
most are anchored at their -ve end to a microtubule organisation centre (MTOC)
-located near the nucleus which directs assembly and orientation
what is the MTOC in animals?
the centrosome (pair of centrioles)
what is the centrosome?
centrioles are associated with the pericentriolar matrix, through proteins to (-) end of microtubules leaving (+) end free to grow and shrink
what is a microtubules function during mitosis?
- centrioles replicate and go to opposite poles od diving cells
- helps separate chromosome
- use microtubules disassembly
what are some other types of MTOC?
flagella
dendrites
how is a flagella a MTOC?
basal body to organise their microtubules
how is a dendrite a MTOC?
show mixed polarity
important for transport
what is the microtubules role in transport?
- cellular tracks
- traffic vesicles, organelles and chromosomes
- used by motor proteins
- work in oppsitie directions
what direction do kinesins work?
anterograde
what direction do dyneins work?
retrograde
how do vesicles cope when the cell is MT poor but MF rich?
a single vesicle may have both kinesin and myosin motors attached
describe the structure of kinesins
similar to myosin, have globular heads which binds to MT
describe the function of kinesins
- direction of movement is towards to positive end
- ATP dependent, use energy to walk the cargo
describle cysotolic kinesins in vesicle transport
- Kinesin head domains (ATPase activity) dock with microtubules
- MT remains stationary and vesicle transported towards (+) end (anterograde)
- Kinesins ‘walk’ cargo along microtubule tracks
