PREPARED BY :-

PRIYANKA YADAV

M.Sc. Life sciences Ist

sem

INTRODUCTION

HISTORY TIMELINE

STRUCTURE

FUNCTIONS

CYTOPLASMIC FILAMENTS

INTERMEDIATE FILAMENTS

MICROTUBULES

MICROFILAMENTS

DIFFERENCES

SUMMARY

CONTENTS :

TYPICAL EUKARYOTIC CELL

INTRODUCTION

This is a scan of the first electron micrograph taken intact of eukaryotic cell.

CYTOSKELETON

The cytoskeleton is the structure consisting of fibrous proteins that occur

in the cytoplasm and maintain the shape of the cell.

History Timeline• 1942 Discovery of actomyosin

• 1954 Sliding filament model for muscle contraction

• 1965 Dynein, the first microtubule-dependent motor

• 1968–1978 Identification of intermediate filaments

• 1972–1977 Actomyosin contractile ring in cytokinesis

• 1973 Isolation of the first non-conventional myosin

• 1984 Microtubule dynamic instability

• 1989 & 1995 g-Tubulin and microtubule-organizing centres

• 1973 Isolation of the first non-conventional myosin

• 1984 Microtubule dynamic instability

• 1989 & 1995 g-Tubulin and microtubule-organizing centres

STRUCTURE

• Network of filamentous proteins

• filaments formed from a few proteins

• monomer protein forms polymer filaments

• located in nucleus and cytoplasmic compartments

• not within organelles

• location based upon cellular function

• named on basis of physical size

FUNCTIONS :

functions based upon the filaments physical properties

integral strength

cell shape

motility

1. inside the cell

2. whole cell

3. motor proteins associated with 2 filament systems

signal transduction

Note - the Extracellular Matrix has a similar structural role

outside of the cell.

Cytoskeletal filaments:

1. Microfilaments

2. Microtubules

3. Intermediate

filaments

INTERMEDIATE FILAMENTS

different cell types, different intermediate filaments

all eukaryotes nuclear cytoskeleton the same

resist stresses applied externally to the cell cytoplasm

10-nanometer diameter

cross-linking proteins allow interactions with other cytoskeletal

networks

intermediate filament associated proteins (IFAPs)

coordinate interactions between intermediate filaments

and other cytoskeletal elements and organelles,

human disorders

mutations weaken structural framework

increase the risk of cell rupture

Intermediate filaments provide mechanical strength and

resistance to shear stress.

There are several types of intermediate filaments, each

constructed from one or more proteins characteristic of it.

Some functions of Intermediate Filaments :

MICROTUBULES

25 nm diameter, 14 nm internal channel tubulin cytoplasmic

All cells contain

Same core structure

Same motors Dynein (-) and Kinesin (+)

Different associated proteins

Dynamic

Continuous remodelling

Movement

Intracellular > cellular

Cell division mitotic spindle

Specialized structures

centrosome,Spindle pole

Cell processes - cilia (9+2)

MICROTUBULE STRUCTURE

STRUCTURE OF MICROTUBULE

If the rate of GTP hydrolyses is

faster then the rate of polymerization

, the microtubule will disassemble

(GTP cap is lost)

If the rate of polymerization is

faster than the rate of GTP

hydrolysis, the microtubule will

grow (It contains a GTP cap)

Microtubules participate in a wide variety of cell activities.

Most involve motion that is provided by protein “motors”

that use ATP.

They determine the positions of membrane-enclosed

organelles and direct intracellular transport.

The migration of chromosomes during mitosis and meiosis

takes place on microtubules that make up the spindle fibers.

Some functions of Microtubules :

MICROFILAMENTS

Twisted chain 7 nm diameter

most abundant protein in cells (5% of all cell protein)

Motility

Adhesion

Actin binding proteins

myosin motors

Muscle actins

CELL CRAWLING DEPENDS ON ACTIN

FORMINS – promotes the addition of G actin to the end of an unbranched filament

ACTIN BINDING PROTEINS

Actin filaments are anchored to the Z-disc at their + ends

The actin filaments face each other in the sarcomere

Upon contraction, the myosin heads moves towards the + end

Pulls the actin filaments towards each other.

MUSCLE CONTRACTION

Some functions of actin filaments are:

- To provide mechanical strength to the cell by forming a

band under the plasma membrane

- Link transmembrane proteins to cytoplasmic proteins

- Form contractile ring during cytokinesis in animal cells

- Cytoplasmic streaming

- Generate locomotion in cells such as white blood cells and

amoeba

- Interact with myosin to provide force of muscular

contraction

DIFFERENCES

• The cytoskeleton organizes the structures and activities of the cell.

• The cytoskeleton interacts with motor proteins.

• The cytoskeleton also plays a major role in cell motility.

• There are three main types of fibers in the cytoskeleton: microtubules,

microfilaments, and intermediate filaments.

• Microtubules, the thickest fibers, are hollow rods about 25 microns in

diameter.

SUMMARY

• Microfilaments, the thinnest class of the cytoskeletal fibers, are solid rods of

the globular protein actin.

• Microfilaments are designed to resist tension.

• Intermediate filaments, intermediate in size at 7nanometers, are specialized

for bearing tension.

• Intermediate filaments are more permanent fixtures of the cytoskeleton than

are the other two classes.