Question

Cell Biology Tutorial - Microfilaments

Answer 1

[This is a tutorial, not a question. Please save all comments or questions until the end. I will be handing out warnings and possibly suspensions for annoying or irrelevant spam comments.]

Answer 2

\({\bf{microfilaments}}\) are components of the cytoskeleton responsible for organization, structure, and movement. There are three types: microfilaments, microtubules, and intermediate filaments.

\({\bf{microfilaments}}\)
- 7-9nm thick
- polymers of actin
- can be tracks for myosin motor proteins

\({\bf{microtubules}}\)
- 25 nm thick (so the widest of the three)
- polymers of tubulin
- can be tracks for myosin motor proteins
- framework for organelles + cilia/flagella
- make up he mitotic spindle (involved in chromosome separation)
- kinesins: transport along microtubules, powered by ATP


\({\bf{intermediate filaments}}\)
- 10 nm
- many functions, ex: structure of the nucleaer membrane/cells, structure/barrier functions in hair/skin/nails
- *not* used as tracks

Answer 3

\({\bf{actin~structures}}\)

- cell cortex: less ordered network beneath plasma membrane
- adherens belt: band around the cell
- leading edge: filopodia protrusions, mobile cells
- endocytic vesicles: short, dynamic actin bundles
- contractile ring; cytokinesis, constricts the two new cells

Answer 4

\({\bf{important~facts~about~actin}}\)

- can reversibly assemble into polarized filament
- microfilament = actin in its polymerized form
- actin gene in eukaryotes is related to Mreb in prokaryotes
- three types, based on charge, alpha beta and gamma
- subunits are arranged in helical structure

Answer 5

\({\bf{G-actin~vs~F-actin}}\)

G-actin: (g)lobular monomer
- separated into lobes
- ATPase fold, where Mg+ and ATP bind
- needs a nucleotide bound or it won't stay stable
F-actin: (f)ilamentous polymer made of G subunits

Answer 6

\({\bf{actin~polymerization}}\)

1. nucleation phase: lag period, G-actin units combine
2. elongation: addition of actin monomers to both ends until equilibrium achieved
3. steady-state: G-actin monomers exchange with the units at the filament ends but no net change in filament length

Answer 7

\({\bf{critical~condition}}\) concentration of G-actin required to start forming filaments

\({\bf{filament~growth}}\) occurs faster and has lower critical conc. at the + end than - end. Difference in growth rates -> treadmilling effect. Disassembly rates are equal at both ends.

Answer 8

\({\bf{profilin~and~cofilin~cycles}}\)

profilin - binds to actin, facilitates adp -> atp exchange, cannot bind to the - end
cofilin - binds to F-actin in the - end, breaking the filament and generating new - ends, enhances disassembly

Answer 9

\({\bf{thyomyosin}}\) inhibits addition of actin subunit; "sequestering protein"

\({\bf{capZ}}\) blocks synthesis at + end
\({\bf{tropomodulin}}\) blocks synthesis at - end
\({\bf{gelsolin}}\) can cleave/break filaments

Answer 10

\({\bf{actin~nucleation}}\)

two types of proteins: formin and Arp2/3 complex

two types of domains: FH1 and FH2

two FH2 domains make "donut" shape that binds two actin subunits and binds actin in a "rocking" motion where it repeatedly transfers the actin between two end subunits to free up space for the addition of another subunit

Answer 11

FH1 is proline rich and serves as "landing site" for profilin-ATP-G-actin complexes

Answer 12

\({\bf{the~ARP2/3~complex}}\) nucleates the filament, must be activated by nucleation promoting factor (NPF). NPFs have a region WCA (WH2 + connector+ acid), where actin binds and activates the arp2/3 complex.

The complex is regulated by WASp; normally this is inactive but is activated when it binds the regulatory phospholipid through B + the Rho-Domain, allowing the W domain to bind to actin

Answer 13

Movement of Listeria and endocytosis is powered by "short bursts" of Arp2/3-dependent actin assembly, actin-comet tails for propulsion

four proteins necessary: atp-g-actin, arp2/3 complex, capZ, cofillin