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Actin. Highly conserved Most abundant intracellular protein (in eukaryotes). Fig 18-4. Structure of actin. Polarity of molecule ‘-” and ‘+’ ends T form and D form G- actin vs F- actin. Fig 18-2. Structure of actin. View from ‘-’end. Fig 18-2. Bundles and Networks.

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Highly conserved

Most abundant intracellular

protein (in eukaryotes)

Fig 18-4

structure of actin
Structure of actin

Polarity of molecule

‘-” and ‘+’ ends

T form and D form

G- actin vs F- actin

Fig 18-2

structure of actin1
Structure of actin

View from ‘-’end

Fig 18-2

bundles and networks
Bundles and Networks

What is the benefit of these kinds of associations?

stability of actin
Stability of actin

Stability depends on environment– ion and g-actin concentration

Provides support through DYNAMIC arrangements

including both structure and gel-like qualities of cytosol

actin filaments dynamics
Actin filaments dynamics

Filaments utilizes 3 steps: lag period, elongation, steady state

ATP hydrolysis NOT required for polymerization

ATP hydrolysis changes kinetics of polymerization

+ vs - end?

Illustration of treadmilling CBI 25.2



Polymer of

a /b tubulin heterodimers

13 protofilaments= microtubule

Stable and unstable populations

Exhibit dynamic instability

Fig 19-5

Fig 19-1

microtubule dynamics
Microtubule dynamics

+ vs - end

Steps in formation

protofilament formation

microtubule assembly

microtubule elongation

Microtubules treadmill AND undergo dynamic instability

factors effecting polymerization depolymerization
Factors effecting polymerization/ depolymerization

Critical Concentration: Cc

Actin: ends have different Cc

(+) 0.1mM

(-) 0.8mM

cellular concentration 0.5mM


Microtubules: One end ‘in’ MTOC


Associated proteins


and why are we discussing this
And why are we discussing this?

Shmoos form from reorganization of actin cytoskeleton

Shmoo tip ‘extends’ due to vesicles specifically delivered

via actin bound motor protein

Microtubules are main ‘highway’ vesicular traffic

why important for neuronal function
Why important for neuronal function?

Fig 23-30

Actin in development CBI21.5

Microtubules in axon core and protein localization

Intermediate filaments in mechanical stability of neurons


Effecting Actin

Cytocholasin D



Bind + ends

Bind G-actin

Bind between subunits on F-actin

Effecting Microtubules





Block formation by binding MT + end

(favors depolymerization)

Same as colchicine but reversible

Stabilizes by binding sides

Lo concentrations act like taxol: hi

promotes crystalline arrays

Net effect on cell-- death

Research and Medicinal purposes for some

associated proteins cellular

Actin associated

Microtubule associated

Associated proteins: cellular

Roles of associated proteins

could this relate to bipolar disorder
Could this relate to Bipolar Disorder?

next time
Next time

Introduction to molecular motors