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3. The Cytoskeleton
Terms in this set (65)
Gain an understanding of the functions of cytoskeletal systems
What comprises the cytoskeleton?
The skeleton and muscles of cells
A system of protein polymers that provide for the architecture, shape, and motility of cells and for the directed movement of organelles and molecules within the cell
Components of the cytoskeleton
Microtubules, microfilaments (aka Actin filaments), intermediate filaments, and a host of accessory and regulatory proteins
made up alpha-tubulin and beta-tubulin
Tubulin is a
GTPase (hydrolyzes GTP)
Hollow with an outside diameter of 24 nm
Accessory proteins called microtubule-associated proteins (MAPs) stabilize and space the polymers and regulate interactions between cytoskeletal elements
Highly dynamic if not stabilized; can undergo rapid bouts of assembly and disassembly
Act as substrate for microtubule-based motor proteins to transport cargo (e.g., organelles)
with a plus-end and a minus-end
"Plus end is where the action happens--growing and shrinking"
Plus end is dynamic (lengthens and shortens)
Motor proteins read the polarity and move to one end or the other of the microtubule
Make up the mitotic spindle
Provide railways for organelle transport in most interphase cells
Key determinant of cell shape
Very important in neurons for cell shape and axonal transport
Provide the backbone for cilia and flagella
Microtubules are Composed of ____________________?
13 (+) Protofilaments
Protofilaments -- 'stacks' of tubulin dimers
The plus end is where the dynamic instability occurs
The tip of a growing microtubule usually has a GTP loaded in the end--GDP along the shaft
Rapid Shrinking of a microtubule
Period where microtubule begins polymerizing (growing) again
+ and - ends
Microtubules: The Highways of the Cell
What a motor protein looks like
Microtubules Associated Motor Proteins
Both have head domains--contain microtubule binding domains. Both motors are ATPases. Associated polypeptide chains can have control over how fast motor is moving, OR what the motor is going to attach to.
KINESIN: (+ end directed) -- move cargo toward the plus end.
DYNEIN: (- end directed) -- major retrograde motor protein.
Dynein carries things toward the center of the cell
Microtubules Associated Proteins
(structural non-motor proteins)
Don't confuse structural with motor
Functions: organize MTs, regulate MTs stability,
Regulate MT dynamics.
In microtubules, nucleation occurs at the _______________?
The genesis of a cytoskeletal polymer
A gamma tubulin mediated event
Often occurs at the centrosome
The elongation of a cytoskeletal polymer following the initial nucleation
Cell has 2 small microtubules that are perpendicular at the center
Gamma tubulin is found almost exclusively in centrosome
Required for the initiation of nucleation. Required for the assembly of alpha/beta dimers into micro tubular like structure
Microtubules in Cell Division
Centrosome Microtubules, DNA Microtubules
Make spindle fibers
Functions: segregation of Chromosones, reorganization of cytoplasm
Required components: motor proteins, MT dynamics
Which microtubule based structures are used for movement?
Cilia and Sperm Tails (flagella)
The Basal Body
A microtubule organizing center for cilia and flagella
The basal body is at the negative end of the microtubule in cilia
9+2 microtubule structure of cilia and flagella
Have 9 "doublets" (2 microtubules fused together)
Dynein drives axonemal motility
Diseases Associated with Malfunction of Microtubule Based Cellular Structures
Immotile cilia syndrome
Immotile cilia syndrome
A body-wide defect in axonemal structure that result in obstructive lung disease and sterile males.
no mucalarry escalator system
A combination of Situs inversus (reversal of normal body asymmetry) and Immotile cilia syndrome
Heart on wrong side, etc.
Target for therapeutics, e.g. disruption of MT dynamics to block cell division (e.g., Taxol)
Mutations in the microtubule proteins LIS1 and doublecortin.
The brain does not develop typical gyri and sulki --
"smooth brain syndrome"
has been reporteddisease type 2A in one case (other families have mutations in mitofusion2, a protein of mitochondria).
Abnormalities/mutations in tau, dynein, kinesin, disorders spastin
Not properly cut
__________________ exploit the neuron's microtubule based transport system to reach cell bodies
Viruses can use microtubules to attack the cells. Can be incorporated into an endosome, or incorporated on the motor proteins.
Gain and understanding of the differences between cytoskeletal systems
Microfilaments (aka Actin filaments): Composition
Differences between actin and microtubules
Non-hollow polymers of the globular protein
Actin is an
(as opposed to GTPase)
Helical in structure (two chains wrapping around each other)
Roughly 7 nm in diameter
Microfilaments (aka Actin filaments): Properties
if not stabilized; undergo rapid bouts of assembly and disassembly
Huge array of configurations regulated by accessory proteins (examples: bundles and meshworks)
Act as substrate for members of the myosin family of motors to move along and carry cargo
Unlike microtubules actin filaments do not have specific organizing centers like the centrosome and can be nucleated almost anywhere in the cell
Used to move things locally (short distances)
Where are actin filaments nucleated?
Unlike microtubules, actin filaments do not have specific organizing centers like the centrosome and can be nucleated almost anywhere in the cell
Microfilaments (aka Actin filaments): Structure
Polarized filament with barbed end (+) and pointed end (-)
Barbed end favored for assembly over pointed end
Myosin motors read the polarity and move toward one end or the other (most myosins move toward plus end)
Barbed end is more dynamic
Microfilaments (aka Actin filaments): Functions
Concentrated in cell cortex for various functions
Cleavage furrow for pinching off cells in final stages of mitosis
Short-range organelle transport
Contractility (in both muscle and non-muscle)
3 Actin isoforms:
Alpha -- expressed almost exclusively by cells of
similar to Gamma
Actin Filament Dynamics
Polymerization is ATP hydrolysis dependent
barbed end is dynamic
Must be ATP loaded
pointed end has less going on
Actin Filament Nucleation
Start out with soluble actin, and start the filament
Gain an understanding of the basic rules governing the assembly of cytoskeletal systems
2 general systems of nucleation mechanisms:
1. Arp2/3 complex -- generates specific type of actin filament organization. Requires an existing filament to begin.
Arp2/3 binds to the side of an existing filament.
Begins to nucleate another filament that grows as a branch at approximately 70 degrees
types of actin filaments
2. Foramins / Spire -- do not need an existing filament
Start by grabbing two actin monomers
Organize them and bind them to each other
Nucleate the filament
Functions of Actin Associated Proteins
Regulation of actin--proteins will bind to sequester the actin molecules
Or load with ATP to promote regulation
Can cap the filaments
Can organize the filaments
Fibroblasts stained to reveal F-actin
F-actin amd myosin II at the Contractile Ring During Cell Division
Most myosins move toward the _______ end of filaments
Myosins: F-actin Associated Force Generating Mechanoenzymes with Roles in Contractility and Intracellular Transport
Contractility of muscles
2 types of myosin molecule
monomeric -- one molecule
dimerics -- 2
In all cases there are the long chains
Essential Chains--required for basic function
Regulatory Chains--subjected to phosphorylation --> when phosphorylated, allow motor to be active
Actin based epithelial projections
Small intestine: increase surface area for absorption during digestion
Ear cells: detect sound waves using stereocilia
Cytoskeleton and Cell Attachment to the Extracellular Matrix (ECM)
There is an indirect physical link made by multiple proteins made directly to the actin filaments inside the cell
Forces can be communicated from inside to outside or from outside to inside
Erythrocyte (red blood cell) cytoskeleton
F-actin serves as scaffold for spectrin web
The structure of the spectrin web is determined by the ability of the spectrums to form nodes within the web
Actin is present in these nodes
Diseases Associated with Malfunction of Actin Based Cellular Structures
Familial hypertrophic cardiomyopathy
Deforms red blood cells to fragile spherocytes because of weakened binding affinity of spectrin to Band 4.1
Deforms red cells to fragile elliptocytes because of incomplete formation of spectrin
Some forms the actin associated protein Tensin, which links integrin receptors to the actin cytoskeleton, is disrupted promoting metastatic migration of cancerous cells
Familial hypertrophic cardiomyopathy
Mutations in a specific cardiac actin.
Mutations/alterations in myosin underlie diseases.
Myosin VI mutations cause deafness.
Myosin VII mutations associated with deafness, neurological disorder, and blindness (Usher syndrome Type 1)
Attacks actin cytoskeleton. Binds to and stabilizes actin filaments
Roughly 10nm in diameter
Comparatively NON-dynamic (much more stable)
Give cells tensile strength
Specialized functions depending on cell type
Important at cell junctions
: Important point
Much less conserved across cell types than microtubules or microfilaments
Different cell types have different kinds of intermediate filaments (can be used as cell-specific "markers")
General Scheme of Intermediate Filament Formation
Diseases Associated with Malfunction ofIntermediate Filament Based Cellular Structures
Progeria 'fast aging disease'
Epidermolysis bullosa simplex mutation in keratin genes expressed in basal cell layer of epidermis, result in a skin that is very sensitive to mechanical injury.
Progeria 'fast aging disease' associated with a mutation in nuclear lamin protein
Actin -> actin filaments
Tubulin -> microtubules
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