Cytoskeletal Dynamics David Stephans L1-2 Flashcards
(27 cards)
alpha and beta tubulin
capable of binding guanine NTs
Microtubules
Predominantly involved in separating the chromosomes
Contact cortex of the cell where actin is
Actin
Generate cleavage furrow for separating 2 daughter cells
Dynamic instability of MTs
Reversible process
Filaments grow and shrink in an energy dependent manner
The property of dynamic instability means that the cell can respond to its environment very rapidly
GTP capped end promotes growth of MT
Promotes association of another a/b dimer
Accidental loss of GTP cap-CATASTROPHE, subunits are no longer added DEPOLYMERISATION
Rapid shrinkage
Regain of GTP cap, RESCUE-factors promote more association of tubulin to the ends of MTs (stabilisation) promotes association of GTP
Rapid growth of GTP-capped end
Microtubules-GTP binding and conformational strain
GTP promotes a conf change of the whole protofilament that is straight
GTP hydrolysis changes subunit conformation and weakens bond in the polymer-curved protofilament
Depolymerisation
Protofilaments don’t stick together well in GDP bound form as forces weaken
Decreases tensile strength within a MT
Microtubule binding proteins MAPs
Lots exist in cells
MT associated proteins-any protein that binds to the MT
eg all the motor proteins
MAPs bind to GTP cap(+end) and bridge the subunit-stabilisation, scaffolds the straight ends
RESULT:longer less dynamic microtubules
Catastrophe factor (KINESIN 13) eg motors-binds to the GTP cap and uses its energy from ATP hydrolysis to pull apart the protofilaments (even if GTP is bound)
Promotes GTP hydrolysis and disassembly of the filaments
RESULT:shorter, more dynamic MTs
Important in mitosis when dynamic reorganisation is required
Gamma tubulin ring complex
Promotes nucleation of MTs
The centrosome (-end of MTs) contains gamma ring tubulin complexes
GCP proteins
gamma tubulin-encoded by separate gene-distinct in structure
Sits at base of MT onto of accessory proteins
CENTROSOME=
Big balls of protein (matrix) that assemble around a pair of centrioles-these are always 90degrees to each other in a normal cell
Made up of several 100s of proteins that associate with the centrioles in a very ordered manner
PERICENTRIN-major proteins in centesimal matrix
Centrioles themselves are made up of short specialised MTs
Made up of TRIPLETS of protofilaments
2 Centrioles are different
The ability to find the middle of the cell is an intrinsic property of the centrisome
Approx 0.5micro-m across
Quite a big structure within a cell
Mother (more elaborate centriole with all the functions) and daughter centrioles-this difference is important in asymmetrical cell division
The centriole matrix itself has capacity to regulate signalling NEK2-kinase PP1-protein phosphatase
Mother centriole
Distal appendages
Subdistal appendages-specialist factors associate here -used to detect only mother centriole
Intercentriolar link -connects mother and daughter centrioles
Primary cilia formation-axonemal MTs extending from the mother centriole
Nucleation
a/b dimer more likely to bind adjacent to one that is already there-more interactions
A template increases the numbers of interactions
A nucleator gets rid of the lag phase and promotes process from the outset
What proteins are in the gamma TuSC complex (Tubulin small complex) and how is it formed?
GCP2/3 with 2 gamma subunits on top
7 copies form a helical structure for the nucleator/template in a “lock washer” spiral
gamma tubulins define the protofilaments in your MTs
Adjacent complexes overlap by about 1/2 -lose one from your 14
13 fold symmetry of MTs-beta subunits overlay directly onto the gamma-tubulins
Microtubule “seam”
Intergral for how MTs depolymerise, the helical arrangement zippers it up well yet since the subunits are so offset from each other you end up with a structural defect along it
The MT is discontinuous
Proteins and structure of gamma-TuRC
GCP 456 and gamma-TRSC
The GCP attachment factors can also change the behaviour of the complex to make it more or less likely to polymerise its MT
It can also modulate behaviour since it can localise the centrosome
These factors can also specify binding of the gamma-TuRC to the side of an existing spindle MT. Increases density of meshwork
Attachment factors can alter MT stability, anchoring and even direct branching (mitosis spindle)
What protein dictates the NINE FOLD SYMMETRY of the centriole hub?
Sas-6 assembly
Sas-6 dimer sticks out 9 times.
MT triplets are interlinked by a hub
hub defines the spatial organisation of the rest of it
hub is made up of Sas-6 DIMER-forms a base and a rod which sticks out
Non-cycling cells
Cant send off cilium in a cycling cell
In non-cycling cells(means cell cycle) (FORMATION OF PRIMARY CILIA), the mother centriole matures to FORM the basal body
The basal body directs the formation of primary cilia
Centrosome function underpins both the cell cycle and cilia function
Mutations in genes encoding centrosomal proteins are linked to many developmental diseases and cancer
Non cycling-forms primary cilia
Cycling-need both centrioles to duplicate so that the 2 daughter cells each have a mother and daughter centriole pair
Centrosome during cell polarisation
1) moves to the front of a migrating fibroblast- MTs concentrated at front of cell
2)T-target cell interaction, centrosome migrates to the contact point on the p.memb and delivers the granules for the active cytotoxic killing
3)axon specification-centrosome specifies the fates of one of the neurites in an immature neutron to become an axon-localises to neurite.
Stabilsing MTs within a neurite makes it become an axon
Position of centrosome acts to determine the fate of other parts of the cell
TAXOL
Drug that stabilises MTs-used in treatment of breast cancer to stop the continual assembly and disassembly of MTs in mitosis which promotes cell division/proliferation
Wounds and cell migration
Centrosome nucleus and golgi all polarise.
Polarisation of the centrosome and regrowth of the MTs towards the direction you want to migrate it
MTs interact with the p.memb (+ end)
Plus-end complexes capture MTs at the cell cortex
GTPase effector
GTP-active
GDP-inactive and MT detaches from the membrane
More GTP more tight interactions with cell cortex
+ end tracking proteins that selectively bind to the + end of MTs e.g. EB1 (end binding protein 1acts as a scaffold for GTPase effectors)
Does the centrosome move?
NO
The centrosome is maintained yet the nucleus moves to the “back” of the cell
MTs maintain the position of the centrosome in the middle of the cell since they pull from all directions
The dynein is not pulling on the centrosome
Dynein maintains the position of the centrosome
Myosin II drives the nucleus to the back of the cell.
Dynactin
Dynein/dynactin mediates VTC movement
Vesicular tubular cluster
ER….COPII….VTCs….Golgi
Links dynein to cargo
Increases PROCESSIVITY of dynein
Dynactin has its own MT binding domain and gives dyne a third leg-there are always 2 attachment sites to the MT where-ever it is-therefore its more likely to stay associated with its MT
Which motors are regulated by unfolding?
Myosin V
Kinesin I
They unfold upon cargo binding
Induced by binding of activators(KINESIN1)/adaptors(BOTH)
Which motors are regulated by dimerisation?
Myosin VI
Exists in inactive monomeric state
Binding of cargo or cargo adapter triggers dimerisation and membrane association
This dimeric form is the active motor form
Melanocytes-pigmented cells
Model for BIDIRECTIONAL motility
Kinesin-2
Dynein (drives things into middle of cell, -end directed)
Myosin Va
ALL ASSOCIATE WITH MELANOSOMES (pigment granules)
Kinesin 2 and dynein are both coupled to melanosomes by dynactin (control by same activator)
Movement regulated by hormones and neuronal activity
Dispersion vs aggregation controlled by signalling
Positioning of melanosomes are defined by crosstalk b/w actin and MT networks
Actin filaments distribute randomly through the cytoplasm
