7.2 Microfilaments I Flashcards
What are cytoskeletons required for?
Cell shape
Internal organization
Cell behavior
How are cytoskeleton destroyed and what are the effects of destroying it?
Drugs/chemical and Injury can destroy cytoskeleton like ischemia and when cytoskeleton is destroyed cells become spherical and they lose their attachments to each other
What is the form of actin when polymerized (assembled into filaments) ?
A single chain of monomers that has a shallow twist to it. It is called F-actin for filamentous actin.
What is the form of actin when not assembled into filaments? Where is it found?
Found on monomers in cells It is called G-actin for globular actin
What is the structure of actin?
Small globular protein (round), about 40kilodaltons in molecular weight. Small.
What happens when G-actin is bound to ATP? What about ADP?
G-actin bound to ATP can easily assemble into actin filaments
G-actin bound to ADP can’t assemble into filaments very well.
How do we know that prokaryotes also have cytoskeletal filaments?
Because prokaryotes contain proteins that may be evolutionary actin precursors and they resemble those found in eukaryotic cells. Mutation in these proteins affects the shape of bacteria
In a test tube, G-actin assembly is dependent on what?
salt concentration that is sufficient to promote assembly
What happens during the lag phase (nucleation)? AKA why don’t G-actin assemble into filaments immediately after adding sufficient salt concentration?
Lag phase period is a period in which no filaments are formed even though conditions for assembly are correct. Lag phase occurs during the period in which there are very few nucleation sites
What happens during a lag phase is that monomers interact w/each other, but for the most part, G-actin monomers form dimers. Dimers are unstable and fall apart.
When a dimer is able to add one more G-actin subunit, making a trimer, the resulting trimer is stable and can act as a nucleation site for additional growth and of the actin filament
What does the rapid growth phase (elongation) in G-actin assembly represent?
The addition of G-actin subunits to nucleation sites that have already been formed. (trimers formed in lag phase)
addition of g-actin subunits to trimers formed in lag phase
How can the role of nucleation be demonstrated?
By adding nucleation sites to a solution of G-actin at the start of the experiment. In this case, actin assembly begins as salt concentration is adjusted, and there is no lag phase.
Why is it good for a cell if actin assembly doesn’t occur even when all of the conditions for assembly are right?
b/c cells have control over where actin filaments assemble because of this, and they can provide nucleation sites only in specific areas of the cell where they want actin assembly to occur.
For example, leukocyte cells produces nucleation sites for actin assembly only at the front of the cell in the direction that it wants to move in
In G-actin assembly, why does assembly reach a steady state (equilibrium phase)?
This happens when the concentration of free G-actin falls to some critical value known as critical concentration (Cc). If the concentration of available G-actin is above the Cc, more growth occurs. If the concentration of G-actin falls below the Cc than existing filaments will shrink.
What is a critical concentration?
It’s the concentration of actin monomer at which a filament end exactly balances growth and de-polymerization. Cc is different value in different situations.
Aka where the rate of assembly is the same as the rate of hydrolysis for whatever end of the filament you’re looking at, and if the rate of addition is slower than the rate of ATP hydrolysis, the filament end will disassemble
In actin filament what is the pointed end? What is the name of the other end?
The direction that the myosin arrows are pointing (- end)
The barbed end of the filament (+ end)
