Cytoskeleton Flashcards
Three major classes of cytoskeletal elements
(1) Actin (2) Microtubules (3) Intermediate filaments
Formation of actin and microtubule polymers is called ___. It is energetically ____.
Nucleation; unfavorable
Critical concentration Cc
The concentration of monomers left over after a steady state is achieved between polymer and monomer forms of actin and MT. Above Cc, polymer will continue to form until Cc is reached. Cc (actin) = 0.1µM; Cc (MT)= 5µM
Actin monomers exist as ___ and actin polymer as ___; tubulin monomers exist as ___. ____ acts as an internal molecular clock that leads to eventual polymer degradation by ____.
Actin monomers exist as actin-ATP and actin polymer as polyactin-ATP; tubulin monomers exist as tubulin-GTP. ATP hydrolysis acts as an internal molecular clock that leads to eventual polymer degradation by ADF/cofilin.
Profilin
Protein that maintains pool of unassembled actin by binding free actin monomers, preventing nucleation. Profilin bound actin adds to barbed end. Profilin promotes change of ADP actin to ATP actin.
Actin is made of _____. It has ___ and ___ ends. It makes ____ (which grows faster?). Monomers bind ___ and the other (___) form falls off the filament. ___ uses short branched filaments and ___ uses long straight filaments.
Actin is made of G-actin monomers. It has a barbed (+) and pointed (-) ends (barbed end grows faster). Monomers bind ATP and ADP form falls off the filament. Motility uses short branched filaments and division uses long straight filaments.
Capping proteins
Prevent elongation of actin filament
Arp 2/3
WASP and Scar
Nucleate short branched actin filaments used for cell motility and pushing. It must be activated by nucleation promoting factors WASP and Scar before it engages in nucleation.
Formin
- Activation requires?
- Domains and what they do
Nucleates long straight actin filaments.
- Activation requires Rho-GTP
- FH2 domain dimerizes around barbed end, preventing capping
- FH1 domain binds profilin-actin and catalyzes its addition to the filament
VASP
Nucleation factor that increases rate of profilin bound actin elongation by 2x
ADF/cofilin
Cleaves polyactin-ADP. Profilin then helps convert the resulting actin-ADP monomers into active actin-ATP monomers
Microtubules are made ____. Only ___ has GTP that is hydrolysable and exchangeable. Tubulin polymers are called ___. # forms single microtubule. MTs have __ and __ ends. Monomers bind ___, (other form) falls off tubule. Gamma tubulin ring complexes cluster where and do what?
What about microtubules is useful for grabbing chromosomes during cell division?
Microtubules are made of alpha and beta tubulin. Only beta tubulin has GTP accessible. Tubulin polymers are called protofilaments. 13-15 of these form a single microtubules. MTs have + and - ends. Monomers bind GTP, GDP form falls off tubule. Gamma tubulin ring complexes act as a template for polymerization and cluster at minus end of MT. MTs undergo dynamic instability (growing and shrinking) – they collapse when their GTP caps are hydrolysed and then rebuild – this is useful for grabbing chromosomes during cell division.
Intermediate filaments are made of
IF dimers
Motor proteins (3)
(1) Myosin: binds actin, walks toward barbed end of actin filament (except Myosin VI). It can bind multiple different cargos. Myosin II (muscle contraction) is nonprocessive; Myosin V (transport) is processive.
(2) Kinesin: moves towards + end of MT (brings things out of cell
(3) Dynein: moves towards - end of MT (bring things into cell).
Cell division involves both ___ and ___. ___ drive the former, and _____ drive the latter.
Cell division involves both mitosis and cytokinesis. MTs drive mitosis, and both MTs and actin drive cytokinesis.
