Histology Lecture 7: Cytoskeleton Flashcards
Actin Microfilaments
highly conserved among eukaryotes; 7 nm thick; up to 7 micrometers in length; organized into bundles and 3D networks; bind to specific transmembrane proteins either directly or indirectly (via catenins);
What is roughly the length/width of a eukaryotic cell
7 micrometers
First step in actin polymerization
nucleation
the growing end of the actin microfilament
plus or barbed end
actin microfilament; grows 5-10 times faster than the other end
barbed/plus/growing end
where the rate of addition of new G-actin to actin filaments occurs more rapidly and at a lower concentration
barbed/plus/growing end
slower growing end of the microfilament
minus/pointed end
favored by low concentrations of G-actin
disassembly at plus/barbed/growing end
the dynamic equilibrium between the minus end and the plus end that is favored by intermediate concentrations of G-actin
treadmilling
favored by higher concentrations of G-actin
net addition at both ends, and therefore, growth of the actin filament
the dynamic balance between the polymerizing and depolymerizing ends of F-actin
treadmilling
bind to barbed ends of actin to block elongation
cytochalasins
binds to actin filaments and prevents dissociation; prevents depolymerization by binding to actin filaments
phalloidin
responsible for the diversity of function of actin filaments in different regions of the cell
actin-binding proteins
actin binding protein that binds cortical cytoskeleton to the plasma membrane and is found in RBCs
spectrin
actin binding protein that binds cortical cytoskeleton to the plasma membrane and is found in muscle cells; a defective version will lead to muscular dystrophy
dystrophin
actin binding protein that cross-links microvilli; hold together the core of the microvillus [2 answers]
villin and fimbrin
actin binding protein that cross links actin to the plasma membrane in microvilli
Calmodulin and Myosin I
actin binding protein that forms the cap of the fast growing barbed ends of F actin and promotes the elongation of unbranched F-actin
Formin
actin binding protein that regulates the assembly of unbranched actin in cell protrusions, such as the intestinal microvilli, while remaining attached to the end of the barbed end
Formin
actin binding protein that cross links stress fibers and connects actin to protein-plasma membrane complex complexes
alpha-actinin
actin binding protein that cross links actin at wide angles to form screen-like gels
Filamin
actin binding protein that captures actin monomers and prevents actin monomers from being polymerized by sequestering pools of G-actin monomers within cells, thereby regulating treadmilling by keeping monomers out of circulation
Thymosin
actin binding protein that suppresses nucleation of G-actin and promotes F-actin growth at the barbed end
Profilin
actin binding protein that can favor the assembly of monomeric G-actin into filaments by facilitating the exchange of bound ADP for ATP
Profilin
actin binding protein that destabilizes F-actin and caps actin filaments, preventing loss or addition of G-actin
Gelsolin
actin binding protein that, in the presence of Calcium ion, will fragment actin filaments and remain bound to the barbed end, forming a cap that prevents further filament growth
Gelsolin
actin binding protein that triggers depolymerization of ADP-bound actin at the minus end; also known as actin depolymerizing factor
Cofilin
actin binding protein that initiates growth of G-actin from the sides of existing filament and causes branching; actin nucleating complex of 7 proteins; intiates the branch of F-actin
Arp2/3
binds to G-actin and induces F-actin depolymerization
Latrunculins
8-10 nm thick; abundant in cells subject to mechanical stress; provide tensile strength in cells; desmosomes and hemidesmosomes; central alpha-helical rod flanked by head and tail domains
intermediate filaments
dimers associate in a staggered antiparallel fashion to form tetramers and lose polarity in the process; more stable than actin and do not demonstrate dynamic behaviour
intermediate filaments
associated with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell
intermediate filaments
25 micrometers in diameter; alpha and beta tubulin monomers form tubulin dimers; protofilaments are longitudinal rows of tubulin dimers;
microtubules
consist of 13 protofilaments arranged parallel to form a cyclinder with a hollow core; exhibit polarity (fast growing plus end and slow growin minus end)
microtubules
1) two polypeptides form a coiled dimer
2) dimers associate in a staggered antiparallel fashion to form tetramers
3) tetramers assemble end to end to form protofilaments
4) eight protofilaments are wound together to form filaments
Intermediate Filament Assembly
microtubules; grows more rapidly than minus end in the presence of low calcium ion concentrations
plus end
end of a microtubule associated with tubulin dimers with GTP bound to the Beta-tubulin
growing end
condition by which microtubule tubulin dimers are added more rapidly than GTP is hydrolyzed thereby favoring growth of the microtubule
high concentrations of tubulin-GTP
condition by which GTP at the plus end is hydrolyze and dimers are lost, favoring depolymerization
low concentrations of tubulin-GTP
the polymerization-to-depolymerization transition frequency
catastrophe
the depolymerization-to-polymerization transition frequency
rescue
inhibit microtubule polymerization; anti-cancer drug used in the treatment of Hodgkins lymphoma
Vinblastin
stabilizes microtubules; anti-cancer drug used in the treatment of breast cancer
taxol
mediates transport of cargo towards the plus end of a microtubule (anterograde transport)
kinesin
mediates transport of cargo towards the minus end of a microtubule (retrograde transport)
cytoplasmic dynein
motor protein with only one head (other three have 2 heads)
Myosin I
motor protein whose head moves towards the minus end (other three move towards the plus end)
cytoplasmic dynein
