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B. Sc (Honors) Zoology 2
Structure and Functions: Microtubules, Microfilaments and Intermediate filaments
B. Sc (Honors) Zoology 2nd^ Semester [CBCS]
CORE COURSE IV
Cell Biology [CREDIT 4]
Theory
Unit 6: Cytoskeleton
Structure and Functions: Microtubules, Microfilaments and Intermediate filaments
By:
Dr. M.R. Ngasainao
Dept. of Zoology
Deshbandhu College, University of Delhi
New Delhi- 110019
Structure and Functions: Microtubules, Microfilaments and Intermediate filaments
Cytoskeleton:
Structure and Function
Cytoskeleton can be defined as โ structural frame/ support of cells โ in simple terms ( Cyto = Cell + Skeleton = structural support/ frame). Just like
skeleton of vertebrates, so is cytoskeleton for Eukaryotic cells. These cytoskeletons are filamentous in nature made up of protein subunits that
are held together by weak non-covalent bond.
They are categorized as:
1. Microtubules: hollow, long, unbranched and stiff. They are composed of protein tubulin.
2. Microfilaments: solid, thin, branched and stiff. They are composed of protein actin.
3. Intermediate filaments: rough, unbranched and robe like - flexible. They are composed of variety of proteins.
Irrespective of the types, cytoskeletons are polymers of protein subunits that elongate (increase in length) by polymerization. The process of
polymerization is the addition of protein subunits to the existing subunits/ structure. Therefore, the increase in length occurs from one end - this
end is termed โ+โ end and the opposite end as โโโ end. The filament shortens by shedding of their subunits from the โโโ end by the process called
de-polymerization. The cytoskeletons are in a state of constant flux of polymerization (addition of subunits) in โ+โ end and de-polymerization
(Shredding of subunits) in the โโโend. This phenomenon is often termed as โstate of dynamic instabilityโ.
[**Tough it is important to know how they form and how they disintegrate through various process, we shall limit our understanding to the
syllabus prescribed and discuss further in the future. So, for now we shall deal with them in brief, and, one can refer the suggested readings].
The Key functions of cytoskeleton are:
1. To provide structural support in maintaining shape of the cells and resilience to tension and stress.
2. Intracellular transport of vesicle and movement of mRNA (refer to vesicular transport: from ER to Golgi apparatus to Plasma membrane)
and translocation of organelles (to position various organelles within the cell).
3. The cytoskeletons also functions as apparatus for cell motility by crawling movement (filopodia, lamellipodia) on substratum or
swimming in aqueous medium through cilia or flagellar movement (microtubules) in single cell animals.
4. Motility: In multi-cellular organism, the contraction of muscles, movement of sperms, neurons, WBC and phagocytes are some mentions.
5. It forms the most essential component of cell division machinery. Cytoskeletons are responsible for the alignment and separation of
Chromatids and subsequent cytokinesis to form daughter cells.
Function of Microtubule:
- end Microtubule filament +end
Vesicle
Lysosome
e
Adaptor
protein
Dynectin
complex Towards Cell
Membrane
Towards Golgi App, ER and Nucleus
Fig 6 : Transport of vesicles/ organelles to and fro Endoplasmic Reticulum-Golgi Apparatus- Plasma Membrane. Note;- kinesin moves from โ-โ to โ+โ end; In dynein from โ+โ to โ-โ end
Fig 2. Structural Support Fig 3.^ Separation of chromatids
1 2
3
4 (^65)
7
8
9
Fig 4 : Structure of cilia or flagellar axenome cross section consist of i) 2 Central microtubule (Complete), ii) 9 peripheral microtubule (aka outer doublet: A tubule (complete), B tubule (incomplete), iii) 9 radial spokes from the incomplete microtubule, iv) outer and inner dynein arms from each A tubule. [Complete tubule = 13 tubulin molecules, incomplete = 10]. Note 9 microtubules in pairs + 2 microtubules in the centre.
A tubuledoublet B tubule
Radial spoke
Central Sheath
Outer Dynein arm Inner Dynein arm
Centralmicrotubule
Interdoublet(nexin) bridge
Outerdoublet
Fig 5A) Centriole (note: 9 microtubule in triplets + 0 at the center). Fig 5B) Centrosome with pair of centrioles.
Centriole pair
Pericentriolar material (PCM)
C tubuledoublet B tubule A tubule
Fig 5A Fig 5B
2. Microfilaments:
Microtubules are also known as actin filaments. They flexible branched and inextensible helical filaments found in all eukaryotes.
Basically its function is for motility and contractility of the cell. The diameter of the microfilament or actin filament is 8 nm with ATP-
Actin molecules as protein subunits (monomers). The actin molecules are incorporated on the + end. They are associated with Myosins
and actin binding proteins.
FilamentousActin (ADP-Actin)
Globular Actin
ADP-Actin (-) end
(-) end
(+) end
(+) end
ATP- Actin
Fig 7. Structure and Actin Polymerization Nucleation: initial phase, ATP- bound Actin monomers aggregate into short oligomer called NUCLEI. The nuclei in vitro : It occurs in 3 stages: maturation occurs after the hydrolysis of bounded GTP to GDP (colour change from brown with red to blue with green). The actin in nucleation stage with GTP is also called G-actin. Elongation : the nuclei elongates rapidly by adding ATP-Actin monomers from both ends. Steady state: after the formation of nuclei, the ATP bounded in the actin molecule hydrolyze to form ADP-Actin and elongates to become stable (F-actin). The de-polymerization in the โ-โ end is stabilized by protein capping (tropomodulin). In the โ+โend cap protein CapZ prevents addition of loss of actin molecules in a steady state.
Nucleation
Elongation
Steady State
iP
Tropomodulin
1 ATP hydrolysis
associated with TropomyosinMicrofilaments are (TM) and Troponin (TN)
Cap Z TN
TM (^) Actin-TM-TN-Ca2+
Actin-TM-TN
Ca2+ Ca2+
Muscle Contraction
Muscle relaxation
Functions of Actin filaments/ microfilaments:
- Membrane endocytosis during phagocytosis
- Vesicle transport along ER - GA โ PM axis
- Locomotion for single cell organism: endoplasmic streaming
- Muscle Contraction: filament sliding
- Cytokinesis during cell division
MTOC
Fig 8 : Distribution of cytoskeleton in a eukaryotic animal cell. Plakin type intermediate filament (IF) Protein of outer membrane Protein of inner membrane Adheren Junctions Microtubule Organizing centre (MTOC) Microtubule (MT) Nuclear Pore Nuclear Lamin Desmosomes Intermediate anchoring plaques Actin-anchoring plaques Actin filaments(Microfilaments , MF) Hemidesmosomes Focal adhesion
IF
MF
Cytoskeleton network in a Eukaryotic cell: Microtubule โ Intermediate filaments-
Suggested readings :
1. Karp, G. (2010). Cell and Molecular Biology: Concepts and Experiments. VI Edition. John Wiley and Sons. Inc.
2. Becker, W.M., Kleinsmith, L.J., Hardin. J. and Bertoni, G. P. (2009). The World of theCell. VII Edition. Pearson Benjamin Cummings
Publishing, San Francisco.
3. Lodish, H. Molecular Cell Biology 5th^ ed, freeman, 2003. (ISBN 0716743663/c/967/s)
