Lecture 7 Learning Objectives Flashcards
a. List and differentiate the three types of cytoskeleton components
i. Microfilaments (actin) (7nm thick)
ii. Intermediate filaments (8-10 nm)
iii. Microtubules (25nm)
characteristics of actin monomers and filaments
i. Actin filaments: highly conserved among eukaryotes
ii. Up to 7 micrometers in length, 7 nm thick
iii. Organized in bundles and 3D networks
iv. Bind to specific transmembrane proteins either directly or indirectly
v. Exist as monomers (G-actin) and long chains (F-actin)
actin polymerization
i. Each monomer (G-actin) has a binding site for ATP.
ii. Monomer binds to three others forming Trimer – displays polarity
iii. ATP-actin associates with growing (plus or barbed) end
iv. ATP is hydrolyzed to ADP following polymerization
Treadmilling
Intermediate concentrations of G-actin favor a dynamic equilibrium between minus end and the plus end
Results in zero net growth
High concentrations of G actin = growth
Cytochalasins
drugs that affect polymerization
Bind to barbed ends
Block elongation
Can inhibit movements (cell division)
Phalloidin
drugs that affect polymerization
Binds to actin filaments and prevents dissociation
Can label with fluorescence
Spectrin
Actin-binding molecule
Found in RBCs
Binds cortical cytoskeleton to the plasma membrane
Dystrophin
Actin-binding molecule
Binds cortical cytoskeleton to the plasma membrane
Villin and Fimbrin
Actin-binding molecule
Cross-links in microvilli
Calmodulin and Myosin I
Actin-binding molecule
Cross-links actin to plasma membrane in microvilli
Alpha-actinin
Actin-binding molecule
Cross-links stress fibers and connects actin to protein-plasma membrane complex complexes
Filamin
Actin-binding molecule
Cross-links actin at wide angles to form screen-like gels
Actin-binding molecules that control treadmilling
Thymosin Profilin Gelsolin Cofilin Arp 2/3 Phalloidin Latrunculins
Thymosin
Actin-binding molecules that control treadmilling
Captures actin monomers: prevents monomers from being polymerized
Profilin
Actin-binding molecules that control treadmilling
Binds to actin monomers and prevents monomers from being polymerized
Facilitates exchange of bound ADP for ATP which favors polymerization
Only ATP-actin monomers can be assembled into F-actin
Gelsolin
Actin-binding molecules that control treadmilling
Destabilizes F-actin and caps actin filaments, preventing loss and addition of G-actin
In presence of Calcium ion, fragments actin filament and remains bound to plus end
Cofilin
Actin-binding molecules that control treadmilling
Triggers depolymeriztion of ADP-bound actin at the minus end
Arp2/3:
Actin-binding molecules that control treadmilling
Initiates growth of F-actin from sides of existing filament- causes branching
Phalloidin
Actin-binding molecules that control treadmilling
Prevents depolymerization by binding to actin filaments
Latrunculins
Actin-binding molecules that control treadmilling
Binds to G-actin and induces F-actin depolymerization
basic structure of intermediate filaments
8-10nm thick – abundant in cells subject to mechanical stress
Provide tensile strength
Strengthen epithelial cells Head and tail domains impart specific function
Form a cytoplasmic network in most cells
Associate with other cytoskeletal elements to form a scaffolding that organizes the internal structure of the cell
Formation of intermediate filaments
Two monomers form a dimer, two dimers align antiparallel to form tetramer.
Tetramers align end to end to form protofilament
Pairs of protofilaments associate laterally to form protofibril
Four protofibrils wind up to form rope – final product
structure of a microtubule
Composed of tubulin dimers – alpha and beta dimers
Protofilaments are longitudinal rows of tubulin dimers
Microtubules consist of thirteen protofilaments arranged parallel to form a cylinder with a hollow core
Fast growing plus end and slow growing minus end
functions of the cytoskeleton
Cell movement Support and strength for the cell Phagocytosis Mitotic spidle formation Cytokinesis Cell-to-cell and cell-to-extracellular matrix adherence Changes in cell shape
