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Cellular Motility (Actin assembly and structure (Each actin monomer had…
Cellular Motility
Actin assembly and structure
Each actin monomer had tight binding sites
Head to tail interaction with 2 other actin monomers
Actin monomer polymerise to form filaments
Each monomer rotate 166 degrees in the filaments
Have the appearance of a double stranded helix
Movement involves proteins:
-Internal
crawling cells
actin polymerisation is used.
External
Cells powered by cilia or flagella
Cytoskeleton
Network of protein fibres found in all eukaryotic cells
supports the shape of cell
keeps organelles in fixed locations
-Dynamic system
always forming and disassembling
Roles of Actin Filaments
Abundant in eukaryotic cells
Less in non muscle cells
Made up of identical actin proteins arranged in long spiral chains.
Have polarity, plus and minus ends
More ATP powered growth occurring at plus end
Actin filaments like fingers in a glove
Found beneath cell cortex
Allows cells to hold, move specialised shapes
Role in cytokinesis/movement
Cell movement
Cilia and flagella move as the microtubules sliding along each other
Cytokinesis and crawling produced by actin
Amoebae at leading edge of moving cell actin filaments rapidly polymerise
Rear edge they quickly depolymerise
Large number of proteins participate in actin assembly
Actin molecule folded into 2 domains stabilised by adenine nucleotide
(ATP or ADP with Mg2+)
Cytoskeleton - 3 fibre types
Mircofilaments (actin filaments)
Smallest 6nm
2 protein chains loosely twined together
Movements like contraction, crawling & pinching
Intermediate filaments
Intermediate size 10nm
Very stable - not usually broken down
Strength and support
Microtubules
Largest of the cytoskeletal elements 25nm
Dimers of a & B-tublin subunits
Facilitate movement of cell & material within it
Actin polymerisation is Reversible
Filaments can depolymerise by dissociate of actin subunits
Breakdown when necessary
Apparent equilibrium exists between actin monomers and filaments
Dependent on concentration of free monomers
In cell there is excess actin monomers
Actin filament are able to grow by reversible addition of monomers to both ends
The plus end elongates 5-10 times faster than minus end
Actin monomers bind to ATP
Hydrolysed to ADP following filament assembly
ATP not required for polymerisation
Actin monomers to which ATP is bound polymerise more readily that those ADP bound
Treadmilling, Aper the actin molecule is incorporated into the filament, then ATP is hydrolysed
Cofilin servers the actin filament (filaments are short)
Profolin binds to actin monomers (promotes growth of filament)
Arp2/3 is used to make branches of the filament for remodelling and changes in the cell shape
Force generation by actin polymerisation
As branches grow rapidly they push against the inside of the plasma membrane
Filaments produce a lot of force
Each new filament producing a force of a few picoNewtons
All filaments provide enough force to move plasma membrane forward
Capping protein can terminate the growing filament
Actin, myosin and cell movement
Actin filaments with myosin are responsible for many types of cell movement
Myosin is molecular motor protein that converts chemical energy in form of ATP to mechanical energy
Processes used in muscle cells for contraction
Also sed for contraction by non muscle cells for division
Skeletal muscle cells
Bundles of muscle fibres
Single large cells
50um diameter and few cm long
Formed by fusion of individual cell during development
Most of cytoplasm consists of myofibrils
Cylindrical bundles of 2 types of filaments
thick filaments of myosin 15nm in diameter
thin filaments of actin 7nm diameter
Muscle contraction and locomotion
In skeletal muscle cell
Plasma membrane in sarcolemma
Cytoplasm is the sarcoplasm
Sarcomere is the contractile unit
measured from Z line to Z line
When actin and myosin molecules within sarcomere slide past each other Z lines move toward each other
results in muscle contraction
Binding actin and myosin head and hydrolyse ATP
provide energy to drive sliding filaments
Translation of chemical energy to movement is mediated by changed in shape of myosin resulting from ATP binding
Cytokinesis in animal cells
Divides cell into 2 cells
Contractile ring of actin and myosin filaments pinches the cell into 2 daughter cells
Composed of filamentous actin (F-actin) and the motor protein myosin 2
Along with additional structural and regulatory proteins
Contractile ring in the cell
Cell uses microtubules of the mitotic spindle to perform both physical separation of chromosomes and the specification of the contractile ring
Contractile ring form at cell equator perpendicular to the axis of chromosome separation
Allows contractile ring to pinch the daughter cells apart between separating chromosomes.
Cilia similar to flagella but are usually shorter
9+2 arrangement
Prokaryotic Flagellum
Powered by a proton gradient
2 ring in the cell wall
Motor protein causes entire structure to rotate
Flagellum rotate and create spiral wave down the structure
Flagella often wave like, cilia perform more complicated 3D motion with a power and recovery stroke
Flagell & Cilia
Beating propel cells forward
Some cells ciliary beating sweeps materials across tissues
Flagella and cilia have the same axoneme structure
Including nine doublet microtubules arranged in a circle around 2 central singlet microtubules
Walking of dynein arms extending from one doublet toward the minus end of a neighboring doublet generate a sliding force in the axoneme.
This force is converted into a bend by regions that resist sliding.
Minus ends of the microtubules in the cilium or flagellum are anchored in the basal body are extensions of microtublules located there
Purpose of Cell Crawling
Carry the cell to a specific location
In response to chemotaxis
Movement towards a distant source of diffusing attractant molecules
Concentration of chemoattractant molecules in the environment must modify the distribution of actin in cell cortex so cell moves in specific direction
Earliest cytoskeletal even in chemotaxis is probably a local stimulation of actin polymerisation
Network of signals that regulate the control of the cell machinery for motility
Neutrophils
White blood cells that hunt and kill bacteria
Attracted by peptides produced by bacteria and crawl toward molecules by extending out their actin rich pseudopodia
Redistribution of Arp2/3 into discrete foci on the side of the cell facing the chemoattractant
At foci the actin rich structures that grow out the membrane sites, like rockets of actin filament forming behind the bacteria