MICROTUBULES AND FILAMENTS

1. Microtubules &
Filaments
By:- Arvind Tewatia

2. Cytoskeleton
•The major structural elements of the
cytosketeton are:-
•microtubules,
•microfilaments

4. •The range of techniques to understand the
cytosketeton include fluorescence microscopy,
digital video microscopy and electron
microscopy.

5. Microtubules
•Microtubules were first discovered by De
Roberties and Franchi (1953)
•In the axons of medullated nerve fibres and were
named neurotubules. Porter(1955)
•Term “Microtubules” was given by
Slautterback (1963)

6. •Microtubules are electron-microscopic structure.
•Found only in Eukaryotic cellular structures.
•But absent in mature mammalian erythrocytes,slim moulds
and prokaryotes
•Microtubules (MTs) are largest element of the cytoskeleton.
•Cytoplasmic microtubules:- 1) maintain axons; 2) maintain
shape; 3) orient cellulose microfibrils (in plants); 4) mitotic
and meiotic spindles for chromosome movements; and 5)
vesicle movement.
•Axonemal microtubules:- highly organized, stable
microtubules in specific movement associated subcellular
structures (cilia, flagella and attachment basal bodies). The
central shaft (axoneme) consists of a highly ordered bundle of
axonemal MTs (see sperm tail axoneme).

7. Microtubule Structure
Microtubule Structure:- is made up
of heterodimers of alpha-tubulin and
beta-tubulin (form an alpha-beta-
heterodimer).
•An a,b-tubulin heterodimer is the
basic structural unit of microtubules.
•The heterodimer does not come
apart, once formed.
ab
GTP GTP
tubulin
heterodimer

8. •The alpha-tubulin and beta-tubulin molecules are 4-5
nm in diamater and 55 kDa, have almost identical
shapes but only share 40% amino acid sequence
identity.
•A microtubule is a hollow cylinder of 13
protofilaments around a lumen with an outer
diameter (25 nm), inner diameter (15 nm) and dimer
width (8 nm).
•Singlet microtubules are 13 protofilaments;
doublets are 13 plus 10 (or 11)
[in cilica or flagella] and
•Triplets are 13 plus 10 (or 11)
plus 10 (or 11)
•[in basal bodies or centrioles].

9. •A microtubule is a hollow cylinder,
about 24 nm in diameter.
•Along the microtubule axis, tubulin
heterodimers join end-to-end to form
protofilaments, with alternating a & b
subunits.
•Staggered assembly of 13 protofilaments
yields a helical arrangement of tubulin
heterodimers in the cylinder wall.
seam
microtubule3-starthelix
b-GDP
a-GTP
b-GTP
a-GTP
+

Note :-Electron microscopy of microtubules
decorated with motor protein heads indicate a "3-
start helix.“
Each turn of the helix spans 3 tubulin monomers
(e.g., a, b, a).

10. •Microtubules form by addition of tubulin dimers at the ends.
New microtubules form nucleation centres (oligomers) and
grow by addition of subunits on either end: elongation.
Critical concentration is when disassemby and assemby is
exactly balanaced: treadmilling.
The plus end: rapidly growing end; the minus end: the slower
growing end.
•Microtubule-organizing centre (MTOC)
•serves as a site for initiation and
as an anchor.
•A centrosome (animals cells) has two
centrioles surrounded by pericentriolar
material.
•The microtubule polarity can vary with cell function.
•Microtubule-associated proteins (MAPs) act as stabilizing
and bundling proteins.
Centrioles at right angles,
as in the centrosome.
cartwheel structure
at one end

11. Microfilaments
Microfilaments were given by PALEVIZ et.
al.(1974)

12. •Microfilaments (MFs) are smallest element of the
cytoskeleton.
•Thread-like protein fibers
•Found only in the cytoplasm of all eukaryotic cells
•This are present in microvilli, muscle fibres. In muscle
fibres these are called myofilaments
•They are 3-7nm in Diameter
•This are long, narrow, cylindrical,
contractile & proteinous structure
•These are absent in prokaryotes

15. Functions
1) As cytoskeleton:- 1) Actin filaments are interlinked by
actin-binding proteins like Fimbrin & Villin while these are
linked to the micrevillin membrane by another actin-binding
protein calmodulin
2) The arrangements of proteins make the microfilaments
make a rigid structure which provide mechanical support to
maintain the form of microvilli
2)Movement of cell membrane:- MF act as a cell
muscles & help in the cell membrane movement which also
help in endocytosis through the cell membrane

16. 3)In movement & locomotion :- Microfilaments of the
myofibers of the muscles fibres help in movement &
locomotion during Actomyosin complexes.
These are formed bw the heads of thick myosin and actin
sites of thin actin filaments in the presence of energy released
during hydrolysis of ATP by myosin & ATPase enzyme in
presence of calcium ion released form yhe depolarized
sarcoplasmic recticulam
The head of myosin filaments undergo swivelling(rotaion)
which pull the actin filament inward which inward over the
myosin filament. This is called sliding-filament theory

18. Difference BW
Microtubules
• These are non-contractile
• A microtubule contain 13
protofilaments
• They are hallow tubules
• Microtubules are formed of
a&btubulin proteins.
• The diameter of
microtubule is 25 nm
Microfilaments
• These are contractile.
• These do not posses
longitudnal sub-units.
• Microfilaments are solid
structures.
• Microfilaments are mainly
made up of actin.
• The diameter of a
miocrofilaments is 5-6 nm.
•