Cytoskeleton-MT Flashcards
Cytoskeleton role
1- structural support
2- movement of material
3- force generation for movements
4- framework for organelles
5- cell division
Structurally moch of cell polarity is due to
Cytoskeleton
Cytoskeleton is —— structure
Capable of reorganizing quickly
Dynamic
MT are —— component of cytoskeletal cell
Largest
MT two types
1- cytoplasmic MT: pervade cytosol
— maintaining axons
—formation mitotic spindles
—movement vesicles
—maintain/alter cell shape
2- axonemal MT
— structures specialized for movement: cilia/flagella/basal bodies
MT protein buildings are
Tubulin heterodimers:
One alpha tubulin
One betta tubulin
Bind non covalently to form ab-heterodimer
Protofilaments
MT are straight, hollow cylinders
Having 13 longitudinal arrays of polymers (ab-heterodimer)
Alpha and betta subunits have —— structure but —— identity
Similar 3D structures
40% amino acid
Each subunit has
N-terminal GTP binding domain
Central domain
C-terminal domain interacting with MAP
All dimers in MT oriented in ——
Same way
Because of dimer orientation
Protofilaments have an inherent polarity
Two end differ chemically and structurally
+ end
- end
Cytoplasmic MT are —— tubes
Simple (single tubes)
Axonemal MT are ——
Doublet or triplet MTs
One 13-protofilament tubule (A)
Additional incomplete rings (B/C) of 10-11-Protofilaments
Doublet MT can be found in —
Cilia and flagella
Triplets can be found in
Basal bodies and centrioles
Reversible polymerization of tubulin dimer can only ocuur in presence of
GTP ang Mg
Nucleation
Dimers aggregate into oligomers which serve as nuclei
MT grow is slow
Elongation
Addition of more subunits at either end
Much faster phase
Plateau phase
When concentration of tubulin becomes limiting. Assembly is balanced by disassembly
MT assembly depends on
Concentration of tubulin dimers
Concentration that MT assembly is balanced by disassembly is
Critical concentration
Higher than C(c)
MT grow
Lower than C(c)
MT disassemble
Addition of tubulin dimers happen
More quickly at plus end
Treadmilling
Addition of subunits at plus end
Removal from the minus end
Higher tubulin conc. Than C(c) in plus end and lowe tubulin conc. than C(c) in minus end
Treadmilling occurs if
Tubulin conc. Is higher than plus end C(c) but lower than minus end C(c)
If tubulin conc. Is higher than both C(c)
MT will grow from both side
If MT conc. Is less than both C(c)
MT will shrink from both side
Both alpha and betta tubulin in tubulin heterodimer binds ——
GTP
(Doesn’t let them to shrink)
Betta subunit will ——— after heterodimer is added to MT
Hydrolyzed GTP to GDP
—— is needed to promote heterodimer
GTP
—— is not required for MT assembly
Hydrolysis
(Happen after assembly)
Dynamic instability model
Polymerization at the plus end and shrinking in minus end
Growing MT have —— and shrinking MT have
GTP at plus end
GDP at minus end
—— prevent subunit removal
GTP cap at plus end
When tubukin conc. Is high
GTP-tubulin is added to MT faster than GTP can be hydrolyzed, GTP cap stabilize the MT tip and promote further growth
Low tubulin conc.
Rate of growth decrease allowing GtP hydrolysis to catch up
Hydrolysis of GTP will
Change the conformation of subunit
Force protofilament in curved shape that is less able to pack into MT wall
Gtp-bound betta tubulin —— MT tip
Stabilize
Gdp-bound betta tubulin —— mt tip
Destabilize
MT originate from
MTOC ( micro tubule organizing center)
Mtoc serve as
Site of MT assembly and anchor for minus end
During interphase many cell have and MTOC called
Centrosome
In animal cells, centrosome is associated with
Two centrioles
Centriole structure
-Centriole wall: 9 sets of triplet MT
-Oriented at right angle
-Involved in basal body formation
-Cell without centrioles have poorly organized mitotic spindles
How MTOC speed up the nucleation?
Centrosome have large ring shaped y-tubulin ring complexes (y-TuRC) which nucleate the assembly of MT away from centrosome
By y-TuRC effect ——
Only plus end extends but C(c) is still important
MT polarity in non-diving cell
MT orientation may vary with cell function
MT polarity in dividing cell
High MT nucleating activity during prophase and metaphase (spindle forming)
——— regulate the assembly and structure of MT
Microtubule-binding proteins
MT-binding proteins:
1- some MT-binding proteins use ATP to drive vesicle or organelle transport
2- MT-stabilizing proteins
3- MT-Destabilizing proteins
4- Plus end tubulin interacting proteins (+-TIP)
MT-stabilizing proteins
Microtubule associated proteins (MAP)
Bind at regular intervals along MT wall
Tau cause MT to form tight bundles to axon
MAP2 promote formation of looser bundles in dendrite
MAP binding
One region bind MT wall and another part extend at right angle to MT to allow interaction with other proteins
Length of extended arm will
Contols spacing of MT in bundle
MT are unstable to-remain intact for long periods and will depolarize unless ——
They stabilized in some way
+-TIP can stabilize by capturing the plus end and protecting from catastrophic subunit loss
+-TIP kind
EB1 association with GTP-tubulin stabilize MT
MT-destabilizing Protein
Promote depolarization
Stathmin/Op18 bind to tubulin heterodimer and prevent polymerization
catastrophins(kinesins) act at end of MT, promoting peeling of subunits from emd
katasins severe MTs
