Cytoskeleton Flashcards
What are the 2 main types of actin filaments how do they form actin fibers?
Globular actin (g-actin) and fibrullar actin (f-actin). F-actin is composed of ATP bound g-actin, which automatically assemble with the addition of ATP. F-actin is polarized, with a + and a - end. Only the + end grows, and only the - end comes apart.
Describe F-actin disassembly.
F-actin is composed of g-actin bound to ATP. When ATP is hydrolyzed to ADP and P, the g-actin disassociates. This happens at the - end.
What factors mediate whether G-actin forms F- actin?
The presence of ATP, Mg, and K.
List 6 functions of actin filaments.
- Terminal web
- intracellular movement
- microvilli
- filipodia,lamellapodia
- structural attachment (zonula adherens)
- contractile rings (during cytokinesis)
Describe the distribution of actin throughout the cell.
Actin is much more prevalent towards the borders of the cell, forming the terminal web.
How does actin facilitate cellular movement?
Actin forms the terminal web. When the actin filaments in the terminal web grow, they push the plasma membrane out and form extensions extracellularly.
List 7 actin binding proteins.
myosin, tropomyosin, gelsolin, villin, alpha-actinin, fibrinin, formin
What does myosin do?
A motor protein that moves actin. It can move vesicles/organelles or, it bound to the membrane, it will move the actin filament itself.
What does tropomyosin do?
Important in muscular contraction, it binds to and stabilizes actin. Certain subtypes may act to shield the actin binding sites from myosin.
What does gelsolin do?
Cuts actin filaments in the middle.
What does villin do?
It is an actin bundling protein.
What does alpha-actinin do?
It is an actin bundling protein.
What does fimbrin do?
It is an actin bundling protein.
What does formin do?
It is an actin branching protein.
List 3 substances that disrupt the normal functioning of actin filaments and describe how they do this.
Cytochalisin B and C- prevent the polymerization of g-actin into f-actin.
Palloidin- found in mushrooms- prevents the depolymerization of f-actin into g-actin
How thick is f-actin?
7 nm
What does MTOC stand for and why is it important?
Microtubule organizing center- MTs are organized in such a way that they all begin at a MTOC (usually a basal body;cilia or a centrosome) and radiate outwards
Describe how a MT is made and its structural composition.
each strand is comprised of alternating alpha and beta tubulin monomers. They rely on the presence of GTP and will spontaneously polymerize. 13 of these alpha/beta strands will H-bond together to form a MT. Growth occurs at the + end.
Describe the disassembly of a MT.
Disassembly occurs with the conversion of GTP to GDP and P. MTs also disassociate from the + end. The - end, comprised of g-tubulin, does not dissociate from GTP and therefore is quite stable.
Describe the relative functions of a/b/g-tubulin
a and b tubulin alternate to create single strands of tubulin polymer, 13 of which will go on to form a MT.
g-tubulin is found near the MTOC and forms the base for the entire MT
List 5 MT functions
- Intracellular Transport- via dyneins and kinesins
- Cilia
- Mitosis- chromosome movement
- Organelle/vesicle movement
- shape/structure of the cell- via extended MTs w/ capping proteins
What are dyneins and kinesins?
Motor proteins on MTs that are important in cell transport. Kinesins move towards the + end and dyneins move towards the - end.
Describe the structure of a motile cilia.
Have a 9+2 MT organization and powered by dyneins that move along the inside 2 MTs. Dyneins cause the MT to bend and then passively “snap” back. Cilia MTOCs are basal bodies and can often be seen as a dark layer underneath the cell membrane on slides.
What does vinblastine do?
drug disrupting MT polymerization
What does taxol do?
drug disrupting MT depolymerization
was does cholchicine do?
disrupts MT polymerization
How thick are MTs?
25 nm
Describe the structure of an Intermediate filament.
composed of staggered tetramers of coiled dimers. two short chains will coil around each other to form a dimer (has polarity). The two dimers will then associate in an antiparallel manner to form a tetramer, destroying the polarity. These tetramers will associate in a staggered way, and 8 strands of staggered tetramers will coil to form an IF.
What is a major distinguishing factor between IF, actin, and MTs in terms of their dynamic nature?
Actin and MTs are dynamic and remodel constantly. IFs do not dynamic, making them strong and stable.
The primary function of IFs is what?
Strength/stability/structure of the cell
Why can IFs be important in cancer cases?
Every type of cell has a different type of IF. If you can identify the IF in the cancerous cell, you can identify where the cell came from.
What is the IF found in skin cells?
keratin
What is the IF found in connective tissue/mesenchyme?
Vimentin
What is the IF found in muscle?
Desmin
What is the IF found at the nucleus?
Lamins
What is the IF found in neural cells?
neurofilament
What is the IF found in glial cells?
GFAP
Where do different types of IFs differentiate themselves?
proteins at the end. middles are conserved usually
What determines the transport speed of kinesins and dyneins?
The affinity for the cargo. If it is dropped more often, it will take longer to move.
What are the 3 main components of the cytoskeleton in ascending order of size?
Actin filaments, intermediate filaments, microtubules
