1. The Cytoskeleton and Intermediate Filaments
Lecture 31 BSCI 420, 421 Nov 13,14, 2002
“A hen is an egg’s way of making another egg.”
Samuel Butler
1. Cytoskeleton
2. Actin and Microfilament Structure
3. Tubulin and Microtubule Structure
4. Intermediate Filaments

2. 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

3. 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?

4. Filaments with both end to end and side to side bonds are
more stable.

5. 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)

6. 2. Actin and microfilaments
A globular monomer With a bound ATP
Assembles head to tail

7. Actin
Polymerization
in vitro
(Treadmilling:
a steady state w
net growth at
one end and loss
at the other)

8. Tubulin exists in solution as a dimer w GTP on both
monomers in the polymerizeable state

9. 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
+ -
(Spears, not
arrows.)
+ end binds to
membranes &
Z line of muscle

10. 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)

11. Structure and
assembly of
Intermediate
Filaments

12. 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 muscle
glial fibrillary acidic protein glial cells of CNS; Schwann
cells
Neuronal neurofilament proteins neurons
(L, M, H)

13. Keratin filaments (Figs 18 & 19

14. Neurofilaments and glial filaments (Fig. 20)

15. Steady-state behaviors of MTs and MFs:
Treadmilling and dynamic instability