The%20Cytoskeleton%20and%20Intermediate%20Filaments

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The Cytoskeleton and Intermediate Filaments
Lecture 31 BSCI 420, 421 Nov 13,14, 2002 A hen
is an eggs way of making another egg. Samuel
Butler 1. Cytoskeleton 2. Actin and
Microfilament Structure 3. Tubulin and
Microtubule Structure 4. Intermediate Filaments
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1. The Cytoskeleton Cells have the ability to
create a shape characteristic of their cell type,
dynamically change that shape, move around in
their environments, and move organelles
and other structures within them. The
cytoskeleton is the system of filaments and
associated proteins that give cells these
properties. The 3 major types of cytoskeletal
filaments are a) Actin (micro)filaments b)
Microtubules c) Intermediate filaments
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Why filaments? The principle of subunit
assembly Elongated polymers are required for
movement in one direction. (trains needs
tracks) All of these filaments are built from
subunits. Why subunits? Why not build cell
structures out of very long, single molecules?
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Filaments with both end to end and side to side
bonds are more stable.
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Stronger filaments can be formed from elongated
fibrous subunits with more lateral contacts.
E.g. intermed. fils (but at the expense of easy
disassembly and reassembly)
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2. Actin and microfilaments A globular monomer
With a bound ATP
Assembles head to tail
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Actin Polymerization in vitro
(Treadmilling a steady state w net growth at
one end and loss at the other)
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Tubulin exists in solution as a dimer w GTP on
both monomers in the polymerizeable state
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The 2 ends of MTs and MFs are different and
cause polarity of growth. - or fast and slow
ends E.g. flagellar axoneme or
Myosin-head labeled MF plus tubulin plus actin
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(Spears, not arrows.) end binds to membranes
Z line of muscle
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4. Intermediate filaments are strong, rope-like
filaments Found inside nuclear
envelope as nuclear lamins in all eukaryotic
cells, but only as cytoskeletal filaments in
animal cells. (lamins are presumably their
ancestors)
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Structure and assembly of Intermediate Filaments
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Families of Intermediate Filaments (Table
16-1) Type Protein Location Nuclear lamins
A,B,C Nuclear lamina Epithelial keratins (I
II) Epithelial cells, hair, nails Vimentin-like
vimentin cells of mesenchymal origin desmin mu
scle glial fibrillary acidic protein glial cells
of CNS Schwann cells Neuronal neurofilame
nt proteins neurons (L, M, H)
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Keratin filaments (Figs 18 19
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Neurofilaments and glial filaments (Fig. 20)
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Steady-state behaviors of MTs and
MFs Treadmilling and dynamic instability