"cytoskeleton and its Components"-assignment.pdf

1
Assignment of GEB 105
“Cytoskeleton”
Submitted to:
Dr. Afsana Bhuiyan Toma,
Assistant Professor,
Dept of Genetic Engineering & Biotechnology
East West University.
Submitted by: Group 4
1. Md Sultan Salah Uddin
Std id: 2022-1-77-079
2. Shahrin Sultana
Std id:2022-3-77-003
3. Jawad Bin Bashir
Std id:2022-3-77-052
4. Mahtamim Mostafiz Rownak
Std id:2023-1-77-003
5. Aisharja Anjum Tory
Std id:2023-1-77-023
6. Asfar Ahmed
Std id: 2023-1-77-039
7. Nabila Akter
Std id:2023-1-77-064
8. Md. Al Imran Ratul
Std id:2023-2-77-067

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Index
Topic Page no Written by Remarks
Introduction 3-4
Shahrin Sultana
Std id:2022-3-77-
003
Dynamic Architecture:
Construction of
Cytoskeletal Filaments.
5-6
Asfar Ahmed
Std id: 2023-1-77-
039
Accessory Proteins:
Key Players in
Cytoskeletal Function.
7-8
Md. Al Imran Ratul
Std id:2023-2-77-
067
Intermediate Filament 9-10
Jawad Bin Bashir
Std id:2022-3-77-
052
Microtubule 11-12
Nabila Akter
Std id:2023-1-77-
064
Actin Filament 13-14
Md Sultan Salah
Uddin
Std id: 2022-1-77-
079
Cytoskeletal Disorders 15-16
Mahtamim Mostafiz
Rownak
Std id:2023-1-77-
003
Conclusion 17
Aisharja Anjum
Tory
Std id:2023-1-77-
023
References 18

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Shahrin Sultana
Std id:2022-3-77-003
Introduction
All cells have to be able to rearrange their internal components as they grow, divide, and
adapt to changing circumstances. Eucaryotic cells have developed all these spatial and
mechanical functions to a very high degree, and they depend on a remarkable system of
filaments called the cytoskeleton.
Fig: The cytoskeleton. A cell in culture has been fixed and labeled to show two of its major
cytoskeletal systems, the microtubules (green) and the actin filaments (red). The DNA in the nucleus
is labeled in blue. (Courtesy of Albert Tousson)
The cytoskeleton is essential for a cell's structure and function. It pulls chromosomes
apart during mitosis, aids in dividing the cell, and transports organelles and materials
within the cell. The cytoskeleton supports the plasma membrane, helping the cell
withstand environmental changes. It enables movement in various cells, such as sperm
and white blood cells, and provides the machinery for muscle contraction and nerve cell
extension. It also guides plant cell wall growth and influences cell shape diversity. The
cytoskeleton consists of three types of filaments, each with unique properties, yet all
work together to give the cell its strength, shape, and mobility. There are two main

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components to the cytoskeleton including protein fibers and motor proteins. The Three
major Types of Protein Filaments That Form the Cytoskeletons are:
1. Actin filament: Actin filaments (also known as microfilaments) are two-stranded
helical polymers of the protein actin. They appear as flexible structure wit a diameter of
5-9 nm, and they are organized into a variety of linear bundles two-dimensional network
and three-dimensional gel. Although actin filaments are dispersed throughout the cell,
they are most highly concentrated in the cortex, just beneath the plasma membrane.
2.Microtubules: Microtubules are long, hollow cylinders made of the protein tubulin.
With an outer diameter of 25nm, they are much more rigid than actin filaments.
Microtubules are long and straight and typically have one end attached to a single
microtubule-organizing center (MTOC) called a centrosome.
3.Intermediate Filaments: These are Rope-like fibers with a diameter of around 10nm.
These protein filaments are slightly larger than actin fibers and are made of a variety of
proteins. These protein filaments are largely used in cell structure and help anchor
organelles and maintain cell integrity under mechanical stress and the environment.
Some examples of intermediate filaments include: Keratin Laminin, Desmin, Vimentin.
Functions:
➢ Shape and Structure: The cytoskeleton maintains the cell's shape and supports
the plasma membrane.
➢ Cell Movement: It facilitates cell motility through the formation of structures like
flagella, cilia, and pseudopodia.
➢ Intracellular Transport: The cytoskeleton acts as a transport system for moving
vesicles, organelles, and other materials within the cell.
➢ Cell Division: It helps in the formation of the mitotic spindle, ensuring
chromosomes are correctly separated into daughter cells.
➢ Mechanical Support: It resists mechanical stress and helps cells withstand
tension.

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Asfar Ahmed
Std id: 2023-1-77-039
Dynamic Architecture: Construction of Cytoskeletal Filaments
The organization of cytoskeletal systems is dynamic and flexible; they resemble ant
trails rather than interstate highways. Even though a single ant path can stretch for hours
from the ant nest to a delicious picnic spot, the individual ants along the trail are
constantly moving. The dynamic structure quickly rearranges itself to adapt to the
changing circumstances, whether the ant scouts discover a better food source or the
picnickers pack up and depart. Similar to this, large-scale cytoskeletal structures can
alter or endure for periods of time varying from less than a minute to the lifetime of the
cell, depending on what is required. These architectures' constituent macromolecular
elements are always in a state of flux.
Microtubules can rapidly reorganize to form a bipolar mitotic spindle during cell
division. Microtubules are often detected in a star-like cytoplasmic array radiating from
the center of an interphase cell. On the cell surface, they can also form tightly aligned
bundles that act as tracks for the transfer of materials down long neuronal axons, or
motile whips termed cilia and flagella. Numerous forms of cell-surface projections are
formed by actin filaments. Several of these are dynamic structures that cells use for self-
propulsion and territory exploration, like filopodia and lamellipodia. Some are stable
structures, like the inner ear's hair cells' regular bundles of stereocilia, which bend like
stiff rods in reaction to sound.
Fig: Overview of cytoskeletal filaments and their properties. (a) Mechanical and dynamic properties
of the three types of cytoskeletal filaments. (b) Mechanical properties of networks of different filament
types. Adapted from Janmey et al., J. Cell Biol. 113, 155-160 (1991). Copyright 1991 Author(s),
licensed under a Creative Commons Attribution (CC BY) License. 1.

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Many times, cytoskeletal structures extend tens or even hundreds of micrometers from
one end of the cell to the other. However, the individual cytoskeleton protein molecules
are often only a few nanometers in size. Building a skyscraper out of bricks is an
example of how the cell's repetitive assembly of many small subunits allows it to build
huge buildings. Unlike the completed filaments, these subunits can diffuse quickly
throughout the cytoplasm due to their small size. Cells are able to reorganize their
structure quickly in this fashion, breaking down filaments at one location and
reassembling them at a distant location. In general, the linking of protein subunits
together to form a filament can be thought of as a simple association reaction. A free
subunit binds to the end of a filament that contains n subunits to generate a filament of
length n + 1. The addition of each subunit to the end of the polymer creates a new end
to which yet another subunit can bind. However, the robust cytoskeletal filaments in
living cells are not built by simply stringing subunits together in this way in a single
straight file. A thousand tubulin monomers, for example, lined up end to end, would be
enough to span the diameter of a small eukaryotic cell, but a filament formed in this
way would not have enough strength to avoid breakage by ambient thermal energy,
unless each subunit were bound very tightly to its neighbor. Such tight binding would
limit the rate at which the filaments could disassemble, making the cytoskeleton a static
and less useful structure.
Fig: Assembly & disassembly of filaments.

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Md. Al Imran Ratul
Std id:2023-2-77-067
Accessory Proteins: Key Players in Cytoskeletal Function
Accessory proteins associate with cytoskeletal filaments and their monomers, aiding
filament formation and function. They also help in the cross-communication among
cytoskeletal filaments. Cytoskeletal accessory proteins are found in both prokaryotes
and eukaryotes. Accessory proteins can mediate interactions between the two polymers
within the cytoskeleton when associated with microtubules and microfilaments. These
proteins are also known to regulate motor proteins, which associate with microfilaments
and microtubules to facilitate the intracellular transport of cargo such as organelles,
vesicles, and various macromolecules. The motor proteins that move along cytoplasmic
microtubules, such as those in the axon of a nerve cell, belong to two families: the
kinesins generally move toward the plus end of a microtubule and the dynein’s move
toward the minus end. Motor proteins move along microtubules using their globular
heads. Kinesins and cytoplasmic dynein’s are microtubule motor proteins that generally
move in opposite directions along a microtubule. Most kinesins move toward the plus
end of a microtubule, whereas dynein’s move toward the minus end. The transport of
cargo toward the plus end of a microtubule is carried out by different types of kinesin
motors, each of which is thought to transport a specific set of vesicles, organelles, or
molecules.
Figure: Different motor proteins transport different types of cargo along microtubules.

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The head of a myosin-II molecule walks along an actin filament through an ATP-
dependent cycle of conformational changes

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Jawad Bin Bashir
Std id:2022-3-77-052
Intermediate Filament
Intermediate filaments represent a category of cytoskeletal components present in the cells
of both vertebrates and numerous invertebrates. The term "intermediate" is derived from
their diameter, which measures 10 nm, positioning them between microfilaments (actin) and
microtubules in size. Their main role is to confer mechanical strength and support to cells
and tissues, thereby assisting in the preservation of cell shape and structural integsrity.
Fig: Intermediate filament
Functions of intermediate filaments:
The strong bond between protofilaments gives intermediate filaments great tensile strength,
making them the most stable element of the cytoskeleton. They are found in resilient
structures such as hair, scales, and fingernails. The main role of intermediate filaments is to
ensure cell cohesion and prevent epithelial cell sheets from breaking under tension. This is
due to the strong interactions among the protofilaments, which improve their resistance to
compression, twisting, stretching, and bending. These characteristics also assist in
stabilizing the long axons of nerve cells and lining the nuclear envelope, where they protect
the cell's DNA.
Fig:. Intermediate filament structure

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Intermediate Filament protein classes, structure and functions:
❖ Type I and II: Keratins
❖ Type III: Desmin, vimentin
❖ Type IV: Neurofilaments
❖ Type V: Lamins
Keratin proteins are among the largest groups of intermediate filament proteins. They are
categorized as acidic (type I) or basic (type II) based on their amino acid characteristics.
Initially, keratin proteins pair up to form dimers, consisting of one acidic and one basic
chain. These dimers then assemble into protofilaments, which ultimately develop into
intermediate filaments.
The types of acidic and basic keratins can vary depending on the cell type. Keratins are
present in epithelial tissues, and their levels can change throughout a cell's life. They play
an important role in supporting and holding together sheets of epithelial cells. For instance,
the basal layer of epithelial cells, which continuously divide to produce new skin cells
(keratinocytes), becomes filled with keratin filaments as they grow. These filaments help
attach skin cells to the extracellular matrix (ECM) below and to neighboring cells through
structures known as hemidesmosomes and desmosomes. When these skin cells die, they
create a protective layer that prevents water loss. Therefore, changes in keratin genes can
lead to various skin disorders. Keratin is also found in structures outside the epithelial layer,
like hair and nails.

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Nabila Akter
Std id:2023-1-77-064
Microtubule
Microtubules are the most vital and largest filaments in the cytoskeleton. They are strong
and rigid hollow tubes that maintain cell shape by resisting compressive force and direct
intercellular transport, separating chromosomes during cell division, and allowing for
cellular movement.
Fig: Structure of Microtubules
Microtubules are formed from the globular protein, called tubulin, their diameter is 25nm.
Tubulin is created in 3 forms:
1. Alpha tubulin
2. Beta tubulin
3. Gamma tubulin
Each subunit of the microtubules is made of two slightly different units called alpha-tubulin
and beta-tubulin that are bound together to form heterodimer. Tubulin dimers polymerize to
form microtubules, which is a hollow cylindrical structure built from 13 protofilaments.
They are arranged in parallel formations. Due to the unique parts of dimers, microtubules
have two distinct ends, which is called the plus end and minus end. Alpha tubulin is exposed
on the minus end and beta tubulin is exposed on the plus end.
GTP nis bound by both alpha and beta tubulin, which regulates polymerization in the way
similar to that of ATP bound to actin. During after polymerization, the GTP bound to beta-
tubulin is hydrolyzed to GDP.

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In the majority of cells, the minus ends of microtubules have been attached in a microtubule-
organizing centre. The major microtubule- organizing centre is a centrosome in the animal
cells. It can be found close to the centre of interphase, just behind the nucleus. More than
fifty copies of the gamma tubulin ring complex are found in the fibrous centrosome matrix
which makes in a centrosome. Microtubules are nucleated at the centrosome at their minus
ends, so the plus ends point outward and grow to the cell periphery, which continuously
grow and shrink by dynamic mechanism.
Fig: Structure and polarity of microtubules.

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Md Sultan Salah Uddin
Std id: 2022-1-77-079
Actin Filaments
Actin filaments are described as thin, flexible protein filaments that form part of the
cytoskeleton of eukaryotic cells. Within the cell, actin filaments also called microfilaments,
are thin structures 5 to 9 nm in diameter that form around the inner edge of a cell, like fragile
rubber bands. They resist tension, shape the cell's surface, and enable whole-cell movement.
They are relatively flexible threads that can be crosslinked together in different ways to
form very different structures. They are present in most cells but are especially abundant in
muscle cells.
Structure of Actin and Its Constituent Subunits
The major cytoskeletal protein of most cells is actin, which polymerizes to form actin
filaments—thin, flexible fibers approximately 7 nm in diameter. The actin subunit is a single
globular polypeptide chain and a monomer. They are composed primarily of actin ,the
monomer of a globular protein called (G-actin), which polymerizes to form long, helical
chains called filamentous actin (F-actin). Thus, the actin filament also has a plus end (the
growing end) and a minus end (the nucleation or beginning end). F-actin undergoes net
association of ATP–actin monomers at the barbed end (+ end) and dissociation of ADP–
actin monomers from the pointed end (− end), a process termed actin filament treadmilling.
Actin filaments consist of two strands of globular molecules twisted into a helix with a
repeat distance of about 37 nm. Actin filaments are quite adaptable and simple to use, curved
in contrast to the hollow spherical microtubules.
Fig: Actin Structure & subunits

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Branching Patterns in Actin Filaments
In general, the formation of new actin filaments (actin nucleation) depends on a protein
complex containing the actin-related proteins (ARP) complex 2 and 3 complex (known as
the Arp 2/3 complex) nucleates the branching of actin filaments. The ARP2/3 complex,
activated by a factor, binds to an actin filament near its plus end. This initiates actin filament
branching with the help of an activating protein. A tree-like network of actin filaments is
produced by many branches growing at a 70° angle to the originating filament branch.
Fig: Actin filament branching

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Mahtamim Mostafiz Rownak
Std id:2023-1-77-003
Cytoskeletal Disorders: Diseases and Conditions
Defects in the cytoskeleton, a structural framework composed of microtubules, actin
filaments, and intermediate filaments between cells, cause cytoskeletal disorders. These
elements are essential for maintaining cell structure, allowing intracellular transport, and
cell division and motility. A defective cytoskeleton can result in a number of ailments and
illnesses, including:
1. Muscular Dystrophies:
Duchenne muscular dystrophy (DMD) is caused by mutations in the dystrophin gene, which
affects the stability of the cytoskeleton in muscle cells. Without functional dystrophin,
muscle fibers atrophy, leading to progressive muscle weakness and degeneration.
2. Neurodegenerative Diseases:
Amyotrophic lateral sclerosis (ALS) and Alzheimer's disease are associated with
cytoskeletal abnormalities. Atypical tau protein assembly in Alzheimer's disease leads to the
development of neurofibrillary tangles, which damage microtubules and compromise
neuronal function. Defects in the protein peripherin, which stabilizes intermediate
filaments, are associated with motor neuron degeneration in ALS
3. Cancer:
Uncontrolled cell division can be caused by abnormalities of cytoskeletal components such
as microtubules, which can accelerate cancer development. Drugs such as taxanes inhibit
the growth of cancer cells by targeting microtubules.
4. Hereditary Spherocytosis:
Defects in proteins such as spectrin, which help maintain the red blood cell cytoskeleton,
cause this blood condition. Hemolytic anemia is caused by round, fragile affected cells.
Hemolytic anemia is caused by round, fragile affected cells. Cytoskeleton defects disrupt
cellular processes and lead to a wide range of disorders affecting blood, muscle, and nerve
cells, among other tissues.
5. Albinism:
Lack of melanin, a pigment produced by specialized cells called melanocytes, causes this
genetic disorder. The migration and distribution of melanocytes is highly dependent on the

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cytoskeleton, and abnormalities in this structure can disrupt these functions and cause
albinism
Figure : Cytoskeletal diseases and conditions in human body

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Aisharja Anjum Tory
Std id:2023-1-77-023
Conclusion
Microtubules, actin filaments and intermediate filaments form the complex and dynamic
cytoskeleton, which is essential for cellular activity. Each element plays a role in cell
architecture, motility and division. Microtubules form the mitotic spindle during cell
division and serve as intracellular transport pathways, while intermediate filaments provide
mechanical strength for structural integrity. Actin filaments play roles in cytokinesis,
morphology, and cell motility.
By regulating transport along microtubules and actin filaments, accessory proteins such as
motor proteins (dynein, kinesin, and myosin) control cytoskeletal dynamics and allow cells
to rapidly rearrange their architecture in response to environmental changes. These proteins
play an important role in regulating the response to stress or damage and maintaining
cellular homeostasis.
Cytoskeletal disorders are caused by abnormalities in cytoskeletal components or regulatory
proteins. These illnesses include neurological diseases such as Alzheimer's, muscular
dystrophy and some malignancies. These diseases emphasize how important a healthy
cytoskeleton is to preserve cellular integrity and regular physiological functions.
Future Dimension:
In the future, investigations to understand the function of the cytoskeleton in disease and
cellular activity will likely change how we treat cytoskeleton-related conditions.
Technological developments in molecular biology, gene editing, and nanotechnology may
make it possible to target cytoskeletal dysfunctions more precisely, leading to new cellular
treatments.
Further investigation into the relationship between cytoskeletal components and their
regulatory proteins could greatly advance our knowledge of cellular dynamics and structural
conservation.

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References:
❖ Alberts, B., Johnson, A., & Lewis, J. (2002). Chapter 16. In Molecular biology of the
cell (5th ed.). Garland Science.
❖ Goldman RD, Cleland MM, Murthy SNP, Mahammad S, and Kuczmarski ER.
Inroads into the structure and function of intermediate filament networks. J. Struct.
Biol. 2011; 177(1):14-23. [PMID: 22120848]
❖ MW Klymkowsky, JB Bachant, A Domingo ,Cell motility and the cytoskeleton 14
(3), 309-331.
❖ Schweizer J, Bowden PE, Coulombe PA, Langbein L, Lane EB, Magin TM, Maltais
L, Omary MB, Parry DAD, Rogers MA, and Wright MW. New consensus
nomenclature for mammalian keratins. J. Cell Biol. 2006; 174(2):169-74. [PMID:
16831889]
❖ Cooper, G. M. (2000). *The cell: A molecular approach* (2nd ed.). Sinauer
Associates.
❖ https://www.frontiersin.org/journals/cellularneuroscience/articles/10.3389/fncel.202
2.982074/full.
❖ Amberg D.C., Leadsham J.E., Kotiadis V., Gourlay C.W. Cellular Ageing and the
Actin Cytoskeleton. Aging Res. Yeast. 2011;57:331–352. doi: 10.1007/978-94-007-
2561-4_15. [PubMed] [CrossRef] [Google Scholar].
❖ 'Molecular Cell Biology'- Alberts et al. and 'The Cytoskeleton' by Michael W.
Klymkowsky. MICR.

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