Microfilaments (Lecture 18) Flashcards
True or False: Cells differ in internal organization, shape, and motility while the cytoskeleton supports its needs.
True
True or False: Distribution and organization of cytoskeleton filaments structurally polarizes different cells.
True
In epithelial cells, core bundles of actin ______ support the apical microvilli.
Microfilaments
What are the four parts that make up the cytoskeleton?
Microfilaments, microtubules, intermediate filaments, nucleus
In the macrophage cytoskeleton, migrating cells polarized with leading edge oriented in the direction of locomotion.
True
True or False: Cytoskeletons components contain at least 3 of the 4 parts that make up the cytoskeleton.
False, only need to contain one or more
Order microtubules, microfilaments and intermediate filaments from shortest to longest.
Microfilament, Intermediate, microtubule
______ is organized into functional bundles by actin-binding proteins and may serve as tracks for ATP-powered myosin motor proteins
Actin microfilaments
______ extend through cell and provide organizational framework and may serve as tracks for ATP-powered kinesins and dyneins (motor proteins).
Tubulin microtubules
______ provide structural support for nuclear membrane, barrier functions in skin, hair, and nails and are not used as tracks by motor proteins.
True
True or False: Cells sense extracellular signals and interpret them to direct arrangement of the cytoskeleton.
True, Detected by cell-surface receptors that activate signal transduction pathways
______ is a disease that can cause dysregulation or cytoskeletal component defects.
Metastatic cancer, allow cells to break away from the generating tissue.
What are the 9 types of microfilament organizations.
Microvilli, cell cortex, Adherens belt, filopodia, lamellipodium/leading edge, stress fibers, phagocytosis, moving Endocytic vesicles, contractile ring.
The building block of microfilaments is ______ and is molded into different structures by ______ proteins.
Actin, actin binding
True or False: Microfilaments have reverse assembly.
True
True or False: Actin is not an abundant intracellular protein in eukaryotic cells.
False
True or False: Actins from amoebae and animals are 80% identical.
True
How many actin genes do yeast and humans have?
2 and 6
What are the 3 different groups of actin based on charge.
a-actin, b-actin, y-actin
______ are enriched in cell cortex & leading edge of motile cells.
b-actin
______ are found in some stress fibers.
y-actin
______ are associated with contractile .
a-actin
Each actin molecule contains an ______ ion complexed with either ______.
Mg2+, ATP or ADP
True or False: Actin is a ATPase that hydrolyzes ATP to ADP and Pi.
True
What are the two forms of actin?
Globular polymer, G-actin
Filamentous polymer. F-actin
F-actin is a linear chain of G-actin subunits
______ actin is separated into two lobes by a deep cleft, denatures quickly without a bound nucleotide, and addition of cations Mg2+, K+, Na+ to G-actin induces filament formation.
G-actin
______ actin is arranged in helical structures of one another and the ATP binding clefts are all oriented to the - end.
F-actin
When does F actin repeat itself?
72nm or 14 actin subunits`
What end of the F-actin filament is favoured for the addition of actin sub units and what side is favoured for subunit dissociation of ATP.
+, -
True or False: Electron microscopy can’t detect the ATP-binding cleft when looking at microfilaments
True
When myosin binds to actin, the arrow heads are pointing towards the ______ end and the barbed end is pointing towards the ______ end.
-, +
True or False: Actin filaments are dynamic with polymerization and depolymerization.
True
True or False: Actin-binding proteins contribute to the stability and disassembly of filaments.
True
True or False: G-actin cannot be purified at high concentration without forming filaments if the buffer has ATP and low levels of cation.
False, it can
What are the 3 phases of actin polymerization.
Nucleation, elongation, steady state
The Lag period in which G-actin subunits combine into an oligomer of 2-3 subunits is known as the.
Nucleation phase
True or False: In the elongation phase, actin is being added to both the + and - end.
True
What is the critical concentration of actin polymerization?
Below which filaments cannot assemble and above which filament form
What is the steady state of actin polymerization?
- Concentration of monomeric actin remains at the critical concentration\
- Rate of assembly and dissociation are equal
- Mixture of filaments and monomers
True or False: Actin grows 10x faster at + end then negative end.
True
True or False: Dissociation of ATP-G-actin subunits is favoured at the (-) end.
False, similar at both ends
What is the Cc of subunits at the + end?
0.12 µM
What is the Cc of subunits at the - end?
0.60 µM
During actin treadmiling, at the steady state the + and - end adds and dissociated at the same rate.
False, + adds and - dissociates at the same rate.
What is the actin treadmill powered by?
ATP hydrolysis
What are the 3 types of actin binding proteins?
Cofilin, Profilin, Thymosin-B4
______ binds G-actin on the side opposite of the ATP-binding cleft (+ end?) and keeps essentially all free G-actin in ATP-bound state
Profilin
______ specifically binds ADP-F-actin and allows the filament to break into short pieces with more (-) ends that can be disassembled simultaneously.
Cofilin
What two ATP binding proteins enhance treadmilling by 10x.
Profilin and Cofilin
______ is a actin monomer sequestering protein that binds excess ATP-G-actin and will be released when ATP-G-actin becomes low
Thymosin-B4
What is an example of Thymosin-B4
Platelets rich in actin and thymosin-β4; allows burst of actin assembly during blood clotting
______ helps nucleotide exchange so ATP-actin ready for (+) end
______ fragments ADPactin filament regions, enhancing depolymerization
______ provides buffered reservoir of ATP-G-actin for polymerization
Profilin
Cofilin
Thymosin-B4
What is the function of capping proteins?
Prevent continual growth and disassembly in an uncontrolled manner
What are the two classes of capping protein at each end of the filament.
CapZ + end
Tropomodulin - end
The capping protein ______ blocks assembly and ______ blocks disassembly.
CapZ
Tropomodulin
