Lecture 9: The cytoskeleton 1 Flashcards
What is the structure of microtubules?
Cylinders 25nm diameter
Made from tubulin heterodimers
What are stable dimers as part of microtubules?
Alpha and beta tubulin form a stable dimer when they are synthesised
Unseparable
What are the features of the plus end of microtubules?
Grows quickly
B tubulin is exposed
What are the features of the minus end of microtubules?
Grows slowly
What is nucleation in microtubules?
Tubulin concentration is too low for polymerisation to be spontaneous
Cells use a template of gamma tubulin to speed up polymerisation
This is nucleation
What is the orientation of microtubules within the cell?
Plus ends are at the perimeter of cell
Minus ends are at the centre
Why are microtubules considered dynamic?
Each one grows and shrinks independently of the others
They can switch between growing and shrinking which is dynamic instability
What is the role of GTP and ATP binding in microtubules?
Can control the shape activity and function of proteins
What is tubulins role as a GTPase?
GDP tubulin cannot polymerise
GDP is exchanged for GTP
GTP tubulin can now polymerise
What is the role of the GTP cap in microtubules?
If the cap is present the microtubule continues growing
What happens if the GTP cap is lost on a microtubule?
The microtubule will depolymerise
If a new cap forms it continues growing again
Why do GTP microtubules grow and GDP shrink
GTP is more tightly bound therefore more stable than GDP
How are growing tubules marked?
Protein EB1 binds preferentially to GTP tubulin
GTP tubulin is growing
How does stabilisation occur from tubulin dimers to microtubules?
Binding of microtubule-associated proteins along the microtubule OR binding of taxol
What are two ways in which microtubules can be depolymerised experimentally?
By putting cells on ice
Using drugs that prevent assembly
How does putting cells on ice depolymerise microtubules?
Microtubules can depolymerise but not grow
How does using drugs depolymerise microtubules?
Nocodazole binds dimers and stops them assembling
Colcemid and colchicine bind to ends of microtubules and stop further assembly
What are the two subunits of microtubules?
Alpha and beta tubulin dimers
What do the subunits of microtubules bind?
GTP
What is the structure of the filament in microtubules?
A hollow rigid tube of 25nm
What is the stabilising and destabilising drugs of microtubules?
Taxol is stabilising
Nocodazole is destabilising
What are the motor proteins of microtubules?
Dyneins
Kinesins
What is the structure of actin filaments?
Made from monomeric actin
Thin flexible helical filaments
What is the role of actin filaments with ATP?
They hydrolyse ATP after assembly because actin is in ATPase
What is the role of capping proteins on actin filaments?
The bind to the minus end of actin filaments and prevent depolymerisation
What is the difference between actin filament and microtubule polymerisation?
Disassembly happens from different ends
Actin polymerisation is altered by natural small molecules
What are some natural small molecules that alter actin polymerisation?
Phalloidin stabilises actin filaments
Cytochalasin caps filament ends and stops actin polymerisation
Latrunculin binds to actin monomers and prevents actin polymerisation
Why is only 50% of actin polymerised in the cell compared to 100% in a test tube?
Nucleating proteins promote polymerisation
In the cell they are used to control where polymerisation happens and maintain a balance between polymer and monomer
What are the subunits of actin filaments?
Actin
What do the actin filaments subunits bind?
ATP
What are some features of the actin filament?
2 stranded flexible helix
7nm diameter
What are the stabilising and destabilising drugs of the actin filaments?
Phalloidin is stabilising
Cytochalasin and Latrunculin are destabilising
What are the motor proteins of actin filaments?
Myosins
