Cell movement

CELL

1. CELL MOVEMENT By: Gopal Kumbhani M.Sc. Biotechnology

2. contents What is cell movement? Introduction of cytoskeleton Actin & its role in cell movement Microtubule & its role in cell movement Intermediate filament & its role in cell movement References

3. Cell movement Cell movement is necessary function in organisms. Without the ability to move, cell could not grow and divide or migrate to areas where they needed. The cytoskeleton is the component of the cell that makes cell movement possible. What is involved in cell movement? In addition to playing this structural role, the cytoskeleton is responsible for cell movements. This include not only the movement of entire cells, but also the transport of organells & other structures through cytoplasm.

4. Cell movement is a complex process. Cell motility  is accomplished through the activity of  cytoskeleton fibers .   The cytoskeleton is a network which is composed of 3 distinct biopolymer: Actin Microtubules Intermediate filament They are differentiated by their rigidity.

5. actin Actin filaments are semiflexible polymer(Lp~17µm). They are 7nm in diameter. They are built from dimer pairs of globular actin monomers & are polar in nature i.e they have two distinct ends: one is fast growing end & other is slow growing end. Fast growing end denotes as plus(+) end & slow end denotes as negative(-) end. The minus end has a critical actin monomers concentration i.e ~6 times higher than at plus end.

6. when the end of an actin filament is exposed to a concentration of monomeric actin that is above it’s critical concentration, the monomers bind at filament end & grows by polymerization & viceversa . So, due to different critical concentration value at both end AFs grows assymetrically . When the monomers concentration lies between two value only the + end will grows & - end will shrinks. So, length of filament remain constant but monomer elongates forward due to asymmetric + end polymerization which is known as treadmilling.

7. ACTIN & THEIR MOTOR PROTEINS(MYOSINE) Actin filaments also generate motility forces by interaction with myosin motors. motors are made up of head, neck & tail region. The head region is responsible for attachment & force production. While tail region is used for connecting cargo such as vesicle, filament or other myosines . Myosin motors work on actin filaments through 3 step process: binding, power stroke & unbinding .

8. FUNCTIONS OF ACTIN FILAMENTS 1) Actin filament plays a critical role in cell-cell contact called adherens junctions. In sheets of epithelial cells, these junctions form a continuous belt like structure (called an adhesion belt) around each cell in which an underlying contractile bundle of actin filaments is linked to plasma membrane.

9. contacts at adherens junctions are mediated by cadherins. which serve as site of attachment of actin bundles. In sheets of epithelial cell. These junctions form a continuous belt like structure. The cadherins are transmembrane proteins that bind β -catenin to their cytoplasmic domains. β -catenin interact with α -catenin, which serves as a link to actin filaments.

10. 2: Cell crowling Many cells moves by cell crowling for e.g migration of embryonic cells during development; invasion of tissue by WBCs to fight infection; migration of cells involved wound healing. Spread of cancer cells called metastasis, phagocytosis. All these movement is based on the actin cytoskeleton. This cell movement is divide into 3 phases:- In the first phase: The cell detaches from the extracellular matrix at its foremost position and extends forward.

11. 2 ) In the second phase: the detached portion of the cell moves forward and re-attaches at a new forward position. The rear portion of the cell also detaches from the extracellular matrix. 3) In the third phase: the cell is pulled forward to a new position by the motor protein myosin. Myosin utilizes the energy derived from ATP to move along actin filaments. Each of these steps requires physical forces which is produced by actin filaments.

12. 3: Actin myosin contraction in non muscle cells It is useful for cytokinesis (the division of cell into two daughter cells at the end of mitosis.) Toward the end of mitosis in animal cells, a contractile ring is assembles which contain actin filaments & myosin2 just underneath the plasmamembrane . Its contraction pulls the plasmamembrane inward side & pinching in two separate daughter cell. As cell division completed ring totally gets disappear. Actin filaments also play an important role in muscle contraction also.

13. 2) MICROTUBULES Microtubules are the stiffest biopolymers with Lp ranging from 100-5000µm. It is an rod like polymers with an outer diameter of ~25nm. It is made up of tubulin protein/monomer. Tubulin monomers associate assemble into protofilament & 13 of these protofilaments align to form a hollow tube. They are also polar, treadmill & can impart a force through polymerization.

14. MTs & their motorproteins (kinesins & dynein) MTs are responsible for a variety of cell movements such as intracellular transport of membrane vesicle, the separation of chromosomes at mitosis, movement of cilia and flagella. This action of MTs is based on motor proteins which is utilize the energy from ATP hydrolysis to produce force & movement. Motor proteins are kinesins & dyneins which are responsible for movement in which microtubules are participate. The kinesin move toward the plus end & dynein moves towards the minus end when they are bind to MTs The first motor protein is identified is dynein.

15. Kinesin is similar as myosin. In structure the tail portion of the kinesin molecule consists of the light chain & a heavy chain. This portion of the kinesin is responsible for binding to other organelles that are transported along MTs. The amino terminal globular head domains of heavy chains which bind to both microtubules & ATP. This ATP hydrolysis provides energy for movement. Dynein is also contain head & tail portion. Head binds with MTs & ATP & tail binds to cargo membrane enclosed organelles such as the ER, Golgi apparatus, Mitochondria within the cell. For e.g the ER is extend to the periphery of the cell in association with microtubules.

16. Drug(Demecolcine & Nocodazole) that depolymerizes the MTs cause ER extends to retracts toward the cell center, indicating that association with MTs is required to maintain the ER in its extended state. This positioning of ER occur due to involvement of action of kinesin which pulls the ER along microtubules in the plus end direction, towards the cell periphery. Same as dynein is play a role in positioning of Golgi apparatus. The Golgi vesicles transported to the center of the cell (towards the minus nd of MTs) by dynein.

17. Cilia & flagella Cilia &flagella are microtubule based projection of plasma membrane which are responsible for movement of eukaryotic cell. Many bacteria also have flagella, but these prokaryotic Flagella are made up of protein filaments which is projecting from the cell surface, rather than plasma membrane supported by MTs in eukaryotes. In short the cilia and flagella are made up of MTs & their associated motor proteins. Cilia are 10µm in length. They move in a coordinate back & forth motion. which moves fluid over the surface of the cell. It act as a wiper which is wipe the fluid from the surface of the cell.

18. Flagella is more in length(~20nm). They moves in a wave like pattern. Cells usually have only one or two flagella which are responsible for locomotion of a cell such as sperm cell.

19. 3) INTERMEDIATE FILAMENTS They are much more flexible than AFs & MTs(Lp~0.3-1.0µm) The diameter is 8-12 nm. IFs has different classes which are present in different cells. e.g vimetin , desmin , keratin(hair & nails), lamin etc. IFs are not polarized, not treadmill. They generally do not depolymerized under physiological condition. Once get polymerized it can not undergo in depolymerization under normal physiological condition.

20. They are more static in nature than AFs and MTs. These three kinds of biopolymers build an internal cellular scaffold k/a cytoskeleton. These network also have some proteins. This network work together for cell function such as shape, division & organelle transport & cell motility.

21. FUNCTIONS OF INTERMEDIATE FILAMENTS The main function of IFs is to provide structural support & to organize cells into tissue. The supportive role of IFs is carried to an extreme in claws & hair which are composed of IF. It provides mechanical support to the plasma membrane. IFs are not participate in cell motility.

22. REFERENCES The journal of cell biology NCBI Book shelf The cell: A Molecular approach. 2 nd edition by Cooper Wikipedia

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