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Cytoskeleton I


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What is def. of cytoskeleton?
Complex netwrok of interconnected filaments & tubules extending thruout cytosol, from nucleus to inner surface of plasma mem.
What are some traits of cytoskeleton?
􀂄 Architectural framework for the cytoplasm
􀂄 Dynamic and changeable
􀂄 Critical for many cellular processes
What are components of cytoskeleton?
􀂄 Microtubules
􀂄 Microfilaments (actin)
􀂄 Intermediate filaments
Describe microfilaments.
􀂉 Actin protein
􀂉 7 nm diameter
Describe intermediate filaments.
􀂉 Different proteins
depending on cell type
􀂉 8-12 nm diameter
What are accessory proteins?
􀂄 For all cytoskeletal elements
􀂄 Give wide structural and functional diversity
to the cytoskeleton
What are techniques for studying cytoskeleton structure?
-Immunofluorescent microscopy
-Computer enhanced digital videomicroscopy
-Fluorescence techniques for living cells
-Electron microscopy
What are techniques for studying cytoskeleton function?
􀂄 Videomicroscopy
􀂄 Drugs that disrupt specific cytoskeletal
􀂄 Mutations in cytoskeletal proteins using a
molecular genetic approach
What are the 2 anti-cytoskeletal drugs?
􀂄 Anti-microtubule drugs
􀂄 Anti-actin drugs
What are the anti-microtubule drugs and how do they work?
􀂉 Colchicine
-Alkaloid from the Autumn crocus, Colchicum autumnale
-Binds to tubulin monomers
-Prevents formation of microtubules
􀂉 Taxol
-From the Pacific Yew, Taxus brevifolis
-Binds to microtubules
-Stabilizes microtubules
What are the anti-actin drugs and how do they work?
􀂉 Cytochalasin D
-Fungal metabolite
􀂉 Latrunculin
-Red Sea sponge Latrunculia magnifica
-Both inhibit polymerization of actin monomers into
􀂉 Phalloidin
-Toxin from Amanita phalloides
-Blocks depolymerization of microfilaments
What are microtubules?
􀂄 Largest cytoskeletal elements
􀂉 Straight, hollow cylinders
􀂉 15 nm inside diameter
􀂉 25 nm outside diameter
What are the 2 types of microtubules?
􀂄 Two types
􀂉 Differ in degree of organization and structural
􀂉 Axonemal microtubules
􀂉 Cytoplasmic microtubules
What are traits of axonemal microtubules?
􀂄 Highly organized
􀂄 Stable
􀂄 Found in specific
􀂉 Cilia, flagella and
basal bodies
What are traits of cytoplasmic microtubules?
􀂄 Loosely organized
􀂄 Dynamic network
􀂄 Observed initially by
electron microscopy,
now more commonly
by fluorescence
What are functions of cytoplasmic microtubules?
􀂄 Various functions
including maintenance of:
􀂉 Axons
􀂉 Polarized shape
􀂉 Cellulose fibril
organization (plants)
􀂉 Mitotic and meiotic
􀂉 Spatial distribution and
directional movement of
vesicles and organelles
Describe the microtubule structure.
􀂄 MT wall composed
of longitudinal arrays
of linear polymers or
􀂄 13 protofilaments
side-by-side form
the wall
What is tubulin?
􀂄 The protein that
􀂄 Basic subunit of a
protofilament is a
heterodimer of α-
tubulin and β-tubulin
Describe the alpha/beta-tubulin molecules.
􀂉 4-5 nm diameter
􀂉 55 kDa
􀂉 40% amino acid sequence identity
􀂉 Almost identical 3-dimensional structure
􀂉 Three domains
􀂄 GTP-binding domain at N-terminus
􀂄 Middle domain (colchicine binding)
􀂄 MT-associated protein (MAP) domain at C-terminus
What are tubulin isoforms?
􀂄 Different forms of α-tubulin and β-tubulin
􀂄 Differ in the C-terminal, MAP-binding domain
How does microtubule polymerizatoin occur & what is required?
􀂄 Formation by reversible polymerization of
tubulin dimers
􀂄 In vitro formation requires tubulin dimers,
GTP, and Mg2+ warmed from 0oC to 37oC
What are steps in microtubule polymerization?
Dimers → oligomers (nucleation) →
protofilaments→ sheets of protofilaments →
closing microtubule → elongating microtubule
What are the terms/steps associated with the formation of microtubules?
􀂄 Slow start or lag phase
􀂄 Active growth of MTs is the elongation phase
􀂄 Limit in free tubulin gives a plateau phase
􀂉 MT assembly balanced by disassembly
What conditions must be met for microtubule formation?
􀂄 High free tubulin → MT polymerization
􀂄 Low free tubulin → MT depolymerization
􀂄 Critical concentration – amount of tubulin
available to balance MT assembly and
Why is MT growth faster at the plus end? What is this called?
􀂄 Polarity of MTs both
structurally and chemically
􀂄 Plus end grows and
shrinks faster
􀂄 Critical concentration
lower for plus end
􀂄 Simultaneous assembly at
plus end and disassembly
at minus end
􀂄 Treadmilling

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