Cytoskeleton Flashcards
Describe the structure of an actin filament
Made up of monomeric actin protein subunits
Assembled into a twisted, two stranded polymer
Describe the structure of a microfilament
Made up of monomeric actin protein subunits
Assembled into a twisted, two stranded polymer
What are the general purposes of actin filaments?
Provide structural support (particularly to plasma membrane)
Important roles in certain types of cell mobility
Describe the structure of a microtubule
Composed of alpha and Beta-tublin heterodimers.
Assembled into a hollow, tubelike cylinder.
What are the general purposes of microtubules?
Provide structural support
Involved in certain types of cell motility
Help to generate cell polarity
Describe the structure of intermediate filaments
Formed from a family of related proteins (such as keratin or lamin)
Subunits assembled into strong, ropelike polymer
What are the general purposes of intermediate filaments?
(Depending on specific protein)
Provide support for nuclear membrane
Provide support for cell adhesion
List microfilaments, microtubules and intermediate filaments in order of width size, smallest to largest
Microfilaments, intermediate filaments, microtubules
What is the width of a microfilament?
7-9nm
What is the width of a microtubule?
25nm
What is the width of an intermediate filament?
10nm
What is the subunit of a microfilament?
Actin
What is the subunit of a microtubule?
alpha-beta Tubulin dimer
What is the subunit of an intermediate filament?
Various
Formed from a family of related proteins such as keratin or lamin
Where are microfilaments usually located in cells?
Around cell membrane, especially in microvilli and filopodia
Cytoplasm
Where are microtubules usually located in cells?
Around edges of cell (sides and bottom)
Cytoplasm
Where are intermediate filaments usually found in cells?
Around edges of cells at junctions to other cells
Surrounding nucleus
What are the general functions the cytoskeleton performs?
Cell shape
Cell movement / migration
Cell contraction
Organisation and movement of organelles
Briefly describe how external cell signalling can lead to cell migration, eg; after a cut in the skin
Other cells (eg; those adjacent to cut) release growth factor which attracts more cells (eg; epithelial cells)
Growth factor interacts with receptors on plasma membrane of other cells
Through signal transduction pathways, production of actin stimulated, cells migrate to target area (eg; a cut in the skin)
What does G-actin stand for?
Globular actin
What does F-actin stand for?
Filamentous actin
Which end of F-actin is the ATP binding site?
The negative end
How does G-actin assemble into F-actin?
ATP binds to G-actin, allowing assembly into F-actin
If the ATP on G-actin is hydrolysed what happens?
G-actin does not assemble into F-actin / F-actin disassembles
Is energy required for G-actin to assemble into F-actin?
No.
ATP doesn’t need to be hydrolysed for the G-actin to assemble into F-actin
Describe the process of G-actin assembling into F-actin.
Include what causes the microfilament to disassemble / continue growing
G-actin binds to ATP. This allows it to assemble into F-actin - no energy is required.
After ATP-G-actin assembles onto the actin filament, the ATP is hydrolysed to ADP and phosphate. The molecule becomes ADP-F-actin.
If ADP-F-actin is left exposed at the ends, it is unstable and the filament will disassemble from the ends.
If there is lots of G-ATP continuing assembly, the filament is stable and keeps growing.
What will happen if there is a mutation preventing G-actin from binding ATP?
G-actin won’t assemble into F-actin, microfilaments will not form
What will happen if there is a mutation preventing the ATP of ATP-G-actin from being hydrolysed?
The actin microfilament will be made up of ATP-F-actin / ATP-G-actin which is more stable than ADP-F-actin, and so won’t be able to disassemble normally once G-ATP stops being added
In a graph of the mass of microfilaments against time, describe the usual curve
Slow rise during “nucleation” phase (“lag phase”), then a faster increase during “elongation”, then a plateau after reaching the “steady state” phase.
In a graph of the mass of microfilaments against time, describe the curve if nuclei are added at time = 0
Immediate fast increase during “elongation” phase, then a plateau after reaching the “steady state” phase.
No “nucleation” / “lag phase” at beginning.
Which end of a microfilament grows more rapidly?
The positive end grows more rapidly
Define the term critical concentration (Cc) when referring to microfilaments
Critical concentration (Cc) is the concentration of free ATP-G-actin at which assembly / disassembly are equal at one of the ends of the microfilament.
(Means that end is not growing or decreasing, it is stable)
(Not at both ends, as they have different Ccs)
When the critical concentration is exceeded at an end of a microfilament, the microfilament will _______ at that end.
When the critical concentration is exceeded at an end of a microfilament, the microfilament will GROW at that end.
When the critical concentration is not reached at an end of a microfilament, the microfilament will _______ at that end.
When the critical concentration not reached at an end of a microfilament, the microfilament will SHRINK at that end.
When the critical concentration is exceeded for one end of a microfilament but is not reached for the other, the microfilament will ____________________ and ____________________. This is called ___________.
When the critical concentration is exceeded for one end of a microfilament but is not reached for the other, the microfilament will grow at one end and shrink at the other end. This is called treadmilling.
What is treadmilling?
When actin subunit concentration exceeds critical concentration for positive end (0.12microM) of microfilament but is less than critical concentration for negative end (0.6microM).
Causes microfilament to grow at positive end and shrink at negative end at same rate, so filament length remains constant as subunits flow through filament.
What is the critical concentration for the positive end of a microfilament
0.12 microM
What is the critical concentration for the negative end of a microfilament
0.60 microM
At what concentration of actin subunits does treadmilling occur?
Any concentration between 0.12 microM and 0.6 microM
What happens to the length of the microfilament when treadmilling occurs?
It stays approximately the same length (but the subunits themselves are changing)
List four types of actin-binding proteins
Profilin
Cofilin
Thymosin beta4
(Various different) Capping Proteins
What does profilin do?
It is an actin-binding protein that enhances the exchange of ADP for ATP on G-actin
What does cofilin do?
It is an actin-binding protein that enhances the loss of ADP-actin from the negative end of the microfilament
What does thymosin beta4 do?
It is an actin-binding protein that binds G-actin to provide reserve actin when needed
What is the general function of capping proteins (actin-binding)
Bind to microfilament ends to stabilise them, preventing assembly and disassembly
What capping protein binds to the positive end of a microfilament to prevent assembly and disassembly?
CapZ
What capping protein binds to the negative end of a microfilament to prevent assembly and disassembly?
Tropomodulin
What end of the microfilament does the capping protein CapZ bind to?
The positive end
What end of the microfilament does the capping protein Tropomodulin bind to?
The negative end