Cell Structure Lecture 3 Cytokeleton Sep 5 Flashcards
What is the purpose of a cytoskeleton?
It helps organize the cytoplasm and the organelles within the cytoplasm.
It’s also used for structure, motility, and division
What are the three basic levels of the cytoskeleton?
- Microfilaments
- Microtubules
- Intermediate filaments
What is the main purpose of microfilament?
Where are they primarily located within a cell?
It’s the muscle of the cell.
They are the primary filament used for motility within the cell and what give the cell its shape.
They are concentrated around the plasma membrane with offshoots into the cytoplasm
What is the main purpose of the microtubules?
Where are they located in the cell?
Microtubules are more the orgnizers of the cytoplasma and organelles–they’re very important for intracellular transport–like railroad tracks.
They’re also critical for cell division and they construct cilia and flagella (as in sperm) and thus provide the cell with motility.
Microtubules are connected to the centrosomes of chromosomes in the nucleus, and branch out from the centrosome in an array
What are the primary purposes of intermediate filaments?
Where are they located?
Intermediate filaments are the most “skeletal” like.
They strengthen the sytoplasm and tissues.
They support the nucleus by making up the nuclear lamina
They are large ocmponents in epidermal appendages such as nail and hair. (this is keratin).
THey strengthen cell-cell junctions
They are located throughout the cytoplasm
What cytoskeleton component supports microvilli?
Microfilaments
What protein makes up microfilaments?
How are microfilaments assembled?
ACTIN
It’s a medium sized globular protein which is very well conserved and less variable. Present in high concentrations in almost all cells.
Actin is polar protein and assembles into filaments based on a nucleotide binding protein that can bind and hydrolyze ATP to ADP.
When the actin proteins are associated with ATP, polymerization is favored, and they join to make a filament.
THey always add to the plus end.
Over time, this nucleotide binding protein will hydrolyze the ATP to ADP.
Actin associated with ADP is destabilized because there is less negative charge, so if the hydrolysis reaches up to the growing plus end, the Actin-ADP molecules will fall off the plus end, and depolymerization will occur.
What protein makes up microtubules?
How are microtubules made?
TUBULIN
It’s a medium sized globular protein that comes in three subtypes: alpha, beta, and gamma.
THey are prominently expressed in almost all cells, expecially ncurons
Polymerization is controlled by a nucldotide binding protien that can bind and hydrolyze GTP to GDP.
THere is a plus end and a minus end. Tubulin molecules add to the plus end.
The alpha and beta subtypes of tubulin dimerize almost immediately after synthesis. When the dimer is associated with GTP, it they will combine wiht other dimers to make long strings called protofilaments
These protofilaments bind together laterally to form a sheet, which will wrap back on itself when it has 13 protofilaments attached, creating a tube.
Eventually the nucleotide binding protein will hydrolyze the GTP to GDP, and if the hydrolyzation reaches the plus end, the dimers will fly off and depolymerization occurs.
The gama tubulin is the tubulin type that conects to the centrosomes, after that it’s all alpha and beta
What are intermediate filaments made of?
How are they constructed?
They use intermediate filament proteins
These vary in size and makeup. They are present in most but not all cells and are expressed in a tissue specific manner. All have a high alpha helical content
Polymerization is regulated directly by phosphorylation
They polymerize (wrap around each other to form coiled coils, which laterally bind to make sheets) when they are UNPHOSPHORYLATED and depolymerase when they are PHOSPHORYLTED, because the addition of negative charges cause mutually repulsive interactions between the filament proteins and they separate.
These filaments do NOT have polarity.
In terms of microtubules, what is catastrophe? What is rescue? What does dynamic instability mean?
Catastrophy is depolymerization.
Rescue is when tubulin molecules fly in and reestablish the GTP cap and stop the catatrophe.
The dynamic instability refers to the constant oscillation between rapid depolymerization and growth
Nucleotide binding protein hydrolysis has a larger affect on microtubules or microfilaments?
Microtubules.
In microfilaments that are actin associated proteins that will actually superseded the ATP-ADP hydrolysis and keep the filament together.
In microtubules, the tubulin is also associated with other proteins, but the hydrolysis state of GTP/GDP has a much more profound effect than any of these proteins.
What are some examples of actin associated proteins that influence microfilament polymerization?
Actin polymerization often forms 3d meshworks.
Signals activate ARP (actin related protein), which serve as seeds to initiate actin polymerization from the sides of existing microfilaments.
THe new microfilaments then elongate, creating a meshlike network.
Capping proteins arrest elongation
ADF/Cofilin severs microfilaments, creating more free ends and promotin actin depolymerization
Profilin binds to actin monomers and catalyes eschange of ADP for ATP, promoting polymerization.
How do microfiliments provide the cell with motility?
They form in 3D meshworks that push out underneath the plasma membrane and pull the rest of the cell in one direction.
What drug will bind to actin and stabilize the filaments (too much) resulting in cell death?
What drug will bind to tubulin and stabilize the microtubules (too much) resultin in cell death?
Phalloidin
Taxol
ALthough actin microfilament orgnization is ARP-mediated to give rise to 3D networks, there are areas of the cell where microfilaments are organized into parallel and contractile bundles of microfilaments.
What proteins mediate this?
The Rho family proteins including Rho, Rac, and CDC42
These proteins are turned on and off depending on GTP and GDP binding.
Rho will convert the meshwork into parallel bundles associated with myosin
Rac promotes the meshwork in lamellipodia, stabilized by filamin
Cdc42 will promote parallel bundles that push out under the plasmsm membrane as filopodia, stabilized with cross-linking proteins
