Cytoskeleton - Microfilaments, Intermediate filaments and Microtubules

We are all familiar with the film ‘Jurassic Park’. In the film, different types of dinosaurs are roaming around the jungle, some are like birds, some are gigantic, and some are like small lizards. Dinosaurs were wiped out from the earth millions of years back possibly by the hit of a meteorite that reshaped the earth’s topography. Then how could we know the shape, the structure and morphology of those giants? It was the work of palaeontologists that helped us to revisit the good old days of dinosaurs “the Jurassic period” in the film Jurassic park. Similar to that of a jigsaw puzzle, bones and remains of dinosaurs (fossils) on excavation were arranged in the best possible way to reshape these wonderful creatures that once rule the world. The structure and shape of a vertebrate is determined by the skeleton that protects and keeps the soft tissue of the body in shape. Likewise, Eukaryotic Cell has a skeletal internal framework and is called the cytoskeleton as it is distributed in the cytoplasm. 

Cytoskeleton that consists of three main polymers: microtubules (green), intermediate filaments (purple) and actin filaments (red). {Neurons (A&B), (C) neurofilaments (a class of intermediate filament )(D) actin filament. (E) Microtubules (F) Neurofilaments (G) Actin filaments}

Definition: These are extremely minute, complex interactive network of three well defined filamentous structures microfilaments, intermediate filaments and microtubules. They are concerned with structural framework, orientation and distribution of cell organelle, movement by or within cells and maintenance of shape of the cells. It is also the site for anchoring mRNA and facilitating their translation into proteins. It is also referred as cytomusculature”. Cytoskeleton is absent in bacteria.

History: In 1928 Koltzoff, postulated the existence of cytoskeletal structure in the protoplasm. Robertis and Franchi (1953) observed the structure microtubules in the myelinated nerve fibres and called it as neurotubules.

Later electron microscopic studies after gluteraldehyde fixation by Sabatini and Barnett 1963 described more about the structure of microtubules. The presence of microtubules in plant cells were first described by Ledbetter and Porter (1963).

The major protein present in the cytoskeleton are tubulin in microtubules, actin myosin and tropomyosin in microfilaments and keratins, vimentin, desmin,lamin in intermediate filaments.

Location: These structures are found in cytoplasm of all Eukaryotic cells.

Microfilaments

Microfilaments

Discovered by Paleviz and his co-workers (1974).They are long, thin, very fine protein filaments which are about 5-10nm in diameter and show beaded appearance due to the close helical arrangement of actin molecules. They are abundant in eukaryotic cells and made up of actin. They occur as sheets or bundles just below the plasma membrane and form a network in the cytoplasm which extend up to the core of microvilli. They take part in cytoplasmic streaming in plant cells (Chara and Nitella), membrane undulations, cleavage, contraction of muscles, movement of microvilli to absorb food and endocytosis. 

 Functions:

• They form a part of cytoskeleton to support the relatively fluid matrix.
• They form mitotic spindle in some primitive organisms.
• Microfilaments are sensitive to Cytochalasin-B, an alkaloid 

Intermediate filaments

 Intermediate filaments

They are supportive elements in the cytoplasm of the eukaryotic cells, except the plant cells and are about 10 nm thick. They are composed of non-contractile proteins. Intermediate fibers are of four types-keratin filaments, neurofibrils, gilal filaments and heterogeneous filaments (Desmin, vimentin, synemin, lamin and keratin filaments). They provide rigidity to the cell and maintain the cell structure. 

Which are the different types of intermediate filaments?

Type I: acidic and basic keratins present in hair and nail.

Type II: Vimetin, desmin and synemin, present in muscle cells and glial cells

Type III: neurofilament proteins, present in nuerons

Type IV: Nuclear lamins A, B and C, present in nuclear lamina of all cells.

Functions: 

• They form major structural proteins of skin and hair.
• They provide strength to the axons and keep nucleus and other organelle in place.

Microtubules

These are found in almost all eukaryotic cells. These are elongated unbranched cylindrical tubules of about 25mμ in diameter. Each tube is formed of 13 subunits celled protofilaments. Protofilaments are polymer chains of globular protein units called tubulin.

Microtubules Structure and Function

Comparison Chart: Microfilaments, Intermediate filaments and Microtubules

Properties  of Microfilaments, Intermediate filaments and Microtubules
Property	Microfilaments	Intermediate filaments	Microtubules
Distribution	All eukaryotes	Animals	All eukaryotes
Structure	Solid made up of polymerized actin (F-actin)	Hollow with walls made up of 4 to 5 protofilaments	Hollow with walls made up of 13 protofilaments
Diameter (nm)	7-9	10	24-25
Monomer Units	G-actin	Five types of protein defining major classes	Alpha and beta tubulin
Enzyme activity	ATP ase	none	GTPase
Motor Proteins	Myosin	none	Knesins, Dynelins
Major group of associated proteins	Actin binding proteins	Plakins	MAPs
Functions	Muscle contraction Cell shape changes Protoplasmic streaming cytokinensis	Integrate contractile units in muscle Cytoskeletal structural function in cytoplasm	Motility of eukaryotes Chromosome movement Movement of intracellular materials Contribute toward maintaining cell shape