Topic 28: The cytoskeleton & Actin Self-Assembly

Assembly of cytoskeletal filaments from small subunits allows for what?

• rapid remodeling

what underlie cell motility, alternating cell contact and cell division?

• dynamic cytoskeletal rearrangements

why is rapid reorganization possible?

• because large cytoskeletal filaments are built up from small aoluble subunits

what are able to create stable, long-lasting strucutures?

• cytoskeletal polymes

stable structures that persist for the lifetime of the cell can be assebled, but what happens to the individual subunits within them?

• they continue to flux in and out of the structures

what are sturdy, but easily altered by polymerization and depolymerization?

• cytoskeletal filaments

how are cytoskeletal filaments altered?

• depolymerization and polymerization

What is nucleation?

the rate limiting step for the formation of actin filaments in a test tube

what are the starting conditions of nucleation?

• tube with high concentration of free actin subunits, but concentration of salt is too low for polymerization.

• then: add salt

Lag Phase in actin polymerization

• small oligomers form, fall apart, & form again

• “nucleation”

growth phase in actin polymerization

-filaments grow long enough to become stable, rapid elongation occurs

• “elongation”

equilibrium phase in actin polymerization

• free soluble actin concentration has dripped to the point where addition of new subunits is just balanced by loss of old subunits

• “steady state”

Through the 3 phases (lag, growth, equilibrium) what are actin filaments able to do in vitro?

• self assemble

what does its mean at C_c (critical concentration)?

• at C_c rate of subunit addition = rate of loss

How can the lag phase in actin polymerization be eliminated?

• by adding preformed filament seeds

what is a key feature of the actin filament structure?

• actin monomers within a filament all “point” in the same direction - actin filaments have polarity

which end of the actin filament is more dynamic?

• the (+) end is ore dynamic than the (-) end

what end is new actin filament added to?

• added to (+) end of the seed

when do actin subunits hydrolyze their ATP nucleotides?

• after polymerization

T-form

• ATP-actin

D-form

• ADP-actin

what does the hydrolysis of T form to D form cause?

• causes a conformational change that favors depolymerization

which rate of subunit addition is favored more? Form D or T?

• form D is favored

A lower C_c at the (+) end than the (-) end creates the possibility for what?

• treadmilling

what is treadmilling?

• net addition to plus end with net loss at minus end

what is C_ss and what is happening?

• C_ss = concentration at steady state, where subunit addition at the T (+) end is just balanced by subunit loss at the D (-) end