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The Cytoskeleton

By: Chris Xu
by

Christopher Xu

on 5 July 2013

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Transcript of The Cytoskeleton

The Cytoskeleton
Characteristics
Cellular Functions:
Chromosome Separation
Cellular Infrastructure
Cell Motility
Cell
Division
Axon Growth
Cytoskeleton Systems:
Microtubules
Microfilaments
Intermediate Filaments
Polymers of smaller protein subunits
Consists of Multiple "Protofilaments"
Dynamic Remodeling -
Rapid reorganization of the cytoskeleton
Polar filaments
Assembly dependent on concentration
Utilizes a nucleotide for assembly and disassembly
Nucleation is a slow process
The cytoskeleton distributes the endoplasmic reticulum
Membrane traffic occurs along the cytoskeleton
Microtubules
Made

from:
Alpha-Tubulin can only bind GTP
Beta-Tubulin can both bind and hydrolyze GTP
The tubulin dimer is 8 nm long
Minus End
Plus End
Thickness
13 tubulin protofilaments form a microtubule
The alpha-tubulin on the 1st protofilament becomes adjacent to a beta-tubulin on the 13th protofilament.
+
Assembly
Addition and loss is faster at the plus end
Proportional to the concentration of dimers present
GTP
Microtubule asssembly and diassembly is impacted by:
Dependent on beta-tubulin:
Microtubules with GTP - straight protofilament
GTP Hydrolysis causes weakened bonds and a curved protofilament
Hydrolysis of GTP is not coupled to assembly!
DYNAMIC INSTABILITY
GROWING
AND SHRINKING
LAB TECHNIQUES
Dynamic instability allows the cell to "explore" its surroundings by using microtubule filaments as a way to feel the environment.
Microtubules
Cytoplasm
Microtubules
Microtubules
Microtubules
The concentration of GTP tubulin dimers in equilibrium with the polymerized microtubule
Lag phase: nucleation of the microtubule is very slow
For assembly to take place, the concentration of GTP-dimers must be higher than the critical concentration
Elongation Phase
Eventually, the microtubule reaches an equilibrium phase, or steady state
Photobleaching
Fluorescent Speckle Microscopy
Both illustrate dynamic instability
Centrosome
The microtubule organizing center of the cell
Gamma-tubulin forms a ring complex that initiates the nucleation of microtubules
Gamma-tubulin binds to alpha-tubulin
A pair of centrioles are often within the centrosome
At 90 degree
Angles
The MTOC can find the center of the Cell
TUBULIN
On the surface of the centrosome
Associated Proteins
Stathmin binds to free
tubulin subunits to control
assembly
Motor Proteins:
Kinesin
Dyneins
Microfilaments
Actin
Made of:
Generates cell motility - the change of shape that causes cell movement
Unlike microtubules, actin filaments lack an organizing center. However, they organize into:
However, they can be organized in:
Networks
Cell cortex
Cables
Sarcomeres
In muscles
Two actin protofilaments create a microfilament
G-Actin
F-Actin
POLAR due to asymmetric interactions of actin monomers
Polymerization
of actin occurs preferentially at the plus end
+
Similarities
between:
Both filaments have polarity,
Use a nucleotide for assembly and disassembly,
Dependent on concentration,
And have a rate-limiting nucleation step in polymerization
ATP
Actin filament assembly is ATP dependent
Filament has higher affinity for ATP-Actin than ADP-Actin
Hydrolysis of ATP is
NOT
coupled to polymerization
olymerization
is preferential
at the + end
critical concentration
The critical concentration at the plus end is lower than the critical concentration at the minus end
Treadmilling:
Actin monomers simultaneously polymerize from the plus end and lose monomers from the minus end
Critical concentration
of both ends
Nucleation is the rate limiting step
NUCLEATION
Arp2/3
Arp 2 and Arp 3 create a complex with other proteins to cap the minus end of an actin filament
Both Arp 2 and Arp 3 resemble the shape of the plus end of an actin filament
The ARP complex is activated by an activating factor, which then recruits actin monomers to polymerize onto the ARP complex
ARP - Actin related protein
Arp2/3 complex binds to the side of a preexisting filament
New ARP complexes bind at 70 degree angles
Arp2/3 induces branch formation
Actin filaments tend to be oriented with the plus ends toward the membrane
Formin
Bind to the growing plus end of the actin filament
Formin does not crosslink actin filaments
Formin contains two domains
The formin ring binds to the plus end, while its "whiskers" capture G-actin by its association with the protein, profilin
Profilin is an ADP/ATP exchange factor
Associated Proteins
Myosin
An actin based motor that is normally plus end directed
Can also form bipolar filaments
Cofilin
promotes actin filament depolymerization

Profilin
binds subunits and speeds elongation
Sarcomere related proteins:
Tropomodulin - caps minus end
Nebulin - serves as a rule for length of thin filaments
Cap Z - caps plus ends and anchors them to Z-disc
Titin - connects thick filaments to Z-disc
Troponin & Tropomyosin:
Troponin - binds calcium to shift the position of tropomyosin
Tropomyosin - blocks access to the actin filament
Other associated proteins:
Intermediate Filaments
properties
TYPES AND COMPONENTS
Made of long, filamentous proteins
Intermediate filaments fold into long alpha-helix segments
These alpha-helix segments form a parallel dimer, forming a coiled-coil
The parallel dimers associate with each other to form an anti-parallel tetramer
Tetramers bind to each other in head to tail arrangements to form a protofilament
Intermediate filament tetramers are symmetric, which means they do not have polarity
Composed of 8 protofilaments
The stagger of intermediate filament proteins make it a strong filament
Lateral contacts cause the stagger of intermediate filaments, which allows the filaments to withstand large shear forces
Nuclear Intermediate Filaments:
Epithelial Intermediate Filaments:
Axonal Intermediate Filaments:
Neurofilament proteins
Keratin
Lamin
Citations:
Made by: Chris Xu
Stabilization and Destabilization Factors:
Alberts, Bruce. Molecular Biology of the Cell. New York: Garland Science, 2008. Print.
http://ghr.nlm.nih.gov/handbook/illustrations/actin.jpg
TEXT
IMAGES
http://static.mechanobio.info/Home/list-of-figures/image-cabinet/adf_cofilin_severs_actin_filaments.jpg
http://www.nature.com/ncb/journal/v6/n12/images/ncb1204-1158-F1.gif
http://tools.invitrogen.com/content/sfs/gallery/low/g001270.jpg
http://origin-ars.els-cdn.com/content/image/1-s2.0-S1046202302002827-gr1.jpg
http://micro.magnet.fsu.edu/cells/microfilaments/images/microfilamentsfigure1.jpg
http://meiosis-and-mitosis.pbworks.com/f/cell.jpg
http://plantphys.info/plant_physiology/images/filamenttubule.gif
http://www.washington.edu/news/archive/images/20101124_pid61307_aid61303_anaphase_w400.jpg
http://people.bu.edu/hman/Images/axon%20growth.jpg
http://www.ijbs.com/v03/p0303/ijbsv03p0303g01.jpg
http://www.msg.ucsf.edu/agard/images/gammaTURC_model.jpg
Full transcript